Category 6™

Looking back at Hurricane Gustav's record 211 mph wind gust

By: JeffMasters, 4:28 AM GMT on December 30, 2008

As we look back at the weather events of 2008, perhaps the most impressive record set during the year occurred during Hurricane Gustav, which pounded Cuba as a Category 4 hurricane in August. Gustav set a new world record for highest wind gust ever measured in a hurricane. As Gustav passed over the Paso Real de San Diego meteorological station in the western Cuban province of Pinar del Rio, Cuba, on the afternoon of August 30, 2008, a wind gust of 211 mph (94.4 m/s) was recorded (it was originally pegged at 212 mph, but has been "downgraded" to 211 mph after an official review by the World Meteorological Organization). The powerful winds blew down the anemometer, and it is possible that higher gusts occurred after the instrument failed. Not only is this the highest wind speed ever measured in a hurricane, it is the second highest wind gust ever measured at a non-mountain location on Earth, and is the third highest wind gust ever measured on the surface of the planet. The highest wind gust in recorded history is the amazing 253 mph reading recorded on Barrow Island, Australia, during Tropical Cyclone Olivia in 1996. The second highest wind speed ever measured was 231 mph (370 km/hr) on the top of Mt. Washington, New Hampshire, on April 12, 1934, during passage of an extratropical storm. The fourth highest wind gust on record was the 207 mph gust measured in Greenland at Thule Air Force Base on March 6, 1972. The previous highest wind gust measured in a hurricane was 186 mph at Blue Hill Observatory, Massachusetts, during the notorious 1938 "Long Island Express" hurricane.

Figure 1. Anemometer used to measure the record 211 mph gust in Hurricane Gustav. Gustav's powerful winds flattened the instrument against the roof of the observing station. Image credit: Jose M. Rubiera Torres, Instituto de Meteorologia of Cuba.

Is this a believable record?
The instrument used for the measurement in Gustav was a Dines pressure tube anemometer mounted on the roof of the weather office. According to Jose M. Rubiera Torres of Cuba's Instituto de Meteorologia, "The graph is neat and the instrument was in perfect technical working condition. The wind peaked up to 340 km/h and then the anemometer mast fell over the concrete roof of the station's building, sharply interrupting the measurement. The graph [Figure 2], shows that wind gusts were increasing at a regular pace with time, until the instrument broke down when it got to the 340 km/h mark." Dines anemometers have a proven track record of reliability, and have been used in Cuba for over 60 years. A formal committee under the auspices of the World Meteorological Organization (WMO) certified the record in 2009.

Figure 2. Trace of the Dines anemometer used to measure the record 211 mph gust in Hurricane Gustav. Image credit: Jose M. Rubiera Torres, Instituto de Meteorologia of Cuba.

How did such a strong gust occur?
At the time Hurricane Gustav moved over the Paso Real de San Diego meteorological station, the storm was rated a Category 4 hurricane with sustained winds of 150 mph, gusting to 185 mph. When the peak wind gust of 211 mph was measured at 22:35 GMT, the western eyewall of Gustav was over the anemometer site, as seen on Cuban radar (Figure 3). The town of Paso Real de San Diego is at an elevation of about 40 meters, and lies 25 km inland, about 12 km south of a rugged line of mountains up to 700 meters high. The counter-clockwise flow of air around Gustav's eyewall meant that the winds arriving at Paso Real de San Diego were forced to pass over these mountains first. The mountains probably focused and accelerated the winds through gaps between the peaks, and the air accelerated further as it rushed downhill under the force of gravity. Strong downbursts due to collapsing precipitation cores inside Gustav's eyewall probably contributed to the extreme gusts. When hurricanes make landfall, the intense thunderstorm cells that comprise the eyewall sometimes collapse suddenly, sending a downward cascade of intense winds to the surface. When this rush of wind hits the ground, it spreads out in all directions, forming a strong surface wind event known as a downburst. It has been theorized that some of the extreme damage noted in Florida during Hurricane Ivan in 2004 and Hurricane Andrew in 1992 may have been associated with downbursts from collapsing eyewall thunderstorm cells. This behavior may also be responsible for some of the extreme damage in Mississippi from Hurricane Katrina. Animations of infrared satellite imagery available from the University of Wisconsin CIMSS Satellite Blog show that the eyewall of Gustav collapsed during passage over the high mountains to the north of Paso Real de San Diego, but this occurred after the world record wind gust was measured.

Figure 3. Radar image of Hurricane Gustav (top) at 22:25 GMT on August 30 2008, five minutes before the world record 211 mph hurricane wind gust was measured. The site of the Paso Real de San Diego meteorological station where the record was set is marked with a red dot. A topographic map (bottom) shows the line of mountains up to 1200 meters high that lies just north of the town. The counter-clockwise flow of air around the eye of Gustav brought the strongest winds of Gustav across the mountain range then downhill to Paso Real de San Diego. Radar image credit: Instituto de Meteorologia of Cuba. Topographic map image credit: Wikipedia.

Note: this post was updated in 2010 to reflect the official WMO review of Gustav's wind gust, plus the addition of the new World Record wind gust set in TC Olivia in 1996.

Jeff Masters

A Christmas wish--more data!!

By: JeffMasters, 7:48 PM GMT on December 25, 2008

Well, it's been another crazy weather year in 2008 here on planet Earth. As we look back on the year, I want to thank all of you for participating in the unique community we've built here at Weather Underground to help document, understand, mourn, and celebrate the ways weather impacts our lives. Special thanks go to all of you who helped out those affected by this year's destructive hurricanes. As we look ahead towards 2009, I'm sure you're wondering what's on my Christmas wish list for the coming year. Hmmm, let's see--let's start with money to fund improved hurricane intensity forecasts, a new QuikSCAT satellite...and more data!

More data, more data,
Right now and not later.
Our storms are distressing,
Our problems are pressing.
We can brook no delay
For theorists to play.
Let us repair
To the principle sublime:
Measure everything, everywhere,
All the time.

For data are solid,
Though dull and though stolid;
Consider their aptness,
Their matter-of-factness.
Theory is confusion,
A snare and delusion,
A dastardly dare,
A culpable crime.
Measure everything, everywhere,
All the time.

No need to be weary
Of the mysteries of theory.
We only must look
At the data we took.
Immediately inspired,
Grasp the answers required.
What are so rare,
As reason and rhyme?
Measure everything, everywhere,
All the time.

More data, more data,
From pole to equator;
We'll gain our salvation
Through mass mensuration.
Thence flows our might,
Our sweetness, our light.
Our spirits full fair, our souls sublime:
Measure everything, everywhere,
All the time.

It shall come to pass, even in our days,
That ignorance shall vanish and doubt disappear.
Then shall men survey with tranquil gaze
The ordered elements shorn of all fear.

Thus to omniscience shall we climb,
Measuring everything, everywhere, all the time.

Poem credit: A. Fleisher. Originally published in 1957 in the Proc. Sixth Weather Radar Conf., American Meteorology Society, Boston, MA, P. 59. Slightly modified by Peter Black, NOAA's Hurricane Research Division.

Happy Holidays, everyone!

Jeff Masters


The 2008 Flying Reindeer Forecast

By: JeffMasters, 6:04 AM GMT on December 23, 2008

Santa's reindeer will be hitting the skies tomorrow, and the official Weather Underground Flying Reindeer Forecast (FRF) calls for some tricky turbulence and tough sledding for Santa in some regions of the world. As Santa begins his adventure, temperatures will be a crisp -20 degrees at the North Pole Workshop. Foggy conditions will require Rudolph's red nose to light the way, since the light winds blowing simultaneously from the south and north at the Pole (think about that for a second, it's true!) will not be strong enough to disperse the fog. As Santa heads south through Siberia into China, he'll be riding a strong jet of polar air that could bring some tricky turbulence. Santa's sleigh will be heavily loaded at this point, and he'll have to negotiate the turbulent skies with great caution to avoid dumping any of his precious cargo. Fortunately, heavy snows cover much of China and eastern Asia, allowing smooth rooftop landings for the sleigh.

As Santa heads south to the Australian continent, the sledding gets much tougher. He'll have to dodge Tropical Cyclone Billy, along Australia's northwest coast. With hot summer temperatures prevailing across Australia, Santa's team will have to execute pinpoint rooftop landings to avoid showering sparks on the dry, fire-prone vegetation. As Santa heads west across Indonesia into southern Asia, he'll need to call upon the thunderstorm avoidance skills of his reindeer, Donner and Blitzen. These storm-savvy critters, named after the German words for thunder and lightning, are experts at sniffing out turbulence-free routes through seemingly impenetrable areas of thunderstorms.

Approaching Turkey, Santa will have to avoid the razor-sharp blue triangles of a cold front. Once through the front, though, he'll find ideal conditions over Turkey and Eastern Europe, where cold temperatures and snowy rooftops abound. However, the sledding quickly degrades over Western Europe, where only Scandinavia and the Alps can expect a white Christmas.

The Western Hemisphere portion of the Flying Reindeer Forecast calls for smooth flying over South America and the Caribbean. But the best conditions of Santa's entire journey await over the United States and Canada, where a series of major snow storms have left excellent rooftop snows. In the Northeast U.S., though, it'll be slow slogging for Santa, with heavy rains expected in an advance of a strong cold front expected to track through on Christmas Eve. The cold front will bring thunderstorms to the Southeast, and Donner and Blitzen will be called upon to safely guide Santa's sleigh through the front to the Southern U.S., where smooth sailing and clear skies will prevail. Excellent sledding awaits Santa in the Midwest and Northwest, where fresh snow from the latest in a series of significant winter storms will delight Santa's reindeer. However, significant rains over Southern California could make for some dangerous mudslide conditions. Santa may be forced to break into his stash to find boogie boards intended for California kids, to help him ride out the mudflows. Santa's final leg northwards across Alaska to the North Pole should be mostly uneventful, and we can expect that clear skies over the Pole upon Santa's return will make for a beautiful Christmas scene as Rudolph guides the sleigh to a perfect landing.

Merry Christmas, everyone!

Jeff Masters


One hundred days which changed the lives of so many

By: JeffMasters, 3:30 PM GMT on December 22, 2008

The following is a guest blog from Paul Timmons, Jr. (Presslord), leader of the charity. Thanks to the efforts and donations of the Weather Underground community, the charity has made a major difference in the lives of hundreds of under-served victims of Hurricane Ike:

Sunday September 14, 2008, was supposed to be, for me, an intentionally uneventful day. I had even made a point of going to mass the previous evening, so that I'd have no obligations on Sunday. For a variety of reasons, I needed a day of rest and relaxation. So there I sat Sunday morning, in my living room recliner, having walked the dog and fed the cat, ready for a day of nothing. Some cable news program was on the television as background noise, reporting the aftermath of Hurricane Ike and my laptop was lap, as I read Dr. Masters blog and the comments on the same subject. Just your average, garden variety lazy Sunday morning.

Then a fellow by the name of Patrick Pearson (y'all know him better as Patrap) posted a comment which would profoundly and positively transform my life...and the lives of hundreds and hundreds of people.

It's a story about the fundamental goodness of the human spirit and the power of community...and the Internet as a tool to facilitate great works.

And it goes something like this.

Pat posted that he was putting together a truck load of relief supplies to take to Texas, as repayment for the generosity shown by Texans toward his beloved New Orleans in the wake of Hurricane Katrina. I, and several others, e-mailed him to ask where we might send donations to help defray expenses.

Then, it occurred to me that I was on the board of a small non-profit, tax exempt 501c3 organization called Portlight Strategies, Inc., which might be able to help. This group was founded in 1997 to facilitate a variety of projects involving people with disabilities, including post-disaster relief projects. I suggested to Pat that we run donations through this group, as it would enable people to deduct their contributions. He agreed...and set up a PayPal account to accept donations. Portlight Strategies, Inc., would then reimburse his expenses. It was a win/win situation for everyone involved. I expected we'd raise about $1500 from a handful of folks, Pat would make a run to Texas...and that would be the end of it.

There is simply no way I could have been any more wrong.

What ensued throughout the rest of that day...and the following days and weeks...can best be described as a nuclear explosion of human generosity. The blog filled quickly with posts from virtually all of you wanting to help the Ike survivors. My e-mail and WU mail boxes filled up with notes from people wanting to help. Ideas were flying around like (I know this is lame, but it kinda fits)...debris in a hurricane. And the PayPal button was on fire.

Volunteers, money, and ideas began rolling in. Crashing in. It was astonishing...and a bit frightening...and overwhelmingly gratifying. Within a day or two we had collected over ten times what I expected in contributions.

At some point on that Sunday, I called my friend John Wilbanks (Stormjunkie) here in Charleston (it's in the Carolinas), and told him what was happening...and he agreed to meet me at my office the next morning to help sort through things and come up with a plan. As Dr. Masters has noted, hurricanes cause chaos. And in my conference room that Monday morning I handed John a big old bag full of chaos...and asked him to begin sorting it out.

(By the way...this is a good time for me to address a very important point. Literally hundreds of you are integral to this wonderful story. It has truly been a grassroots phenomenon...and we all share equally in it. To mention everyone would likely shut down the WU servers. But three people have been critical: Patrick Pearson, John Wilbanks and Jeff Masters. Without the vision, focused effort and support--in that order--of these three, we couldn't have accomplished a fraction of what we've accomplished.)

So...John got busy. By the end of the first week he had made contact with people in the affected areas outside of Houston and we began to get a sense of what their needs were. Meanwhile, members of the Weather Underground community were continuing to communicate their thoughts and ideas with us, and plans were coming into focus. Dr. Masters got wind of all this and began mentioning it in his blog entries. Thanks largely to input from a variety of WU faithful, we realized that the most pressing needs were in the rural and small town areas away from Houston; additionally, people with disabilities throughout the area needed help. Thus we began to plan our efforts to assist the un-served, under served and forgotten people.

Supplies were secured to fill specific requests. Trucks were arranged. People were recruited. Funds were budgeted. And by the next week we were rolling. Into Houston...into Anahuac...into Winnie...into Bridge City...onto the Bolivar Peninsula.

The stories of the devastation we heard from our crews were unimaginable...and heart wrenching. The stories of the gratitude expressed to our crews by the Ike victims they met were heart felt...and heart warming.

"We thought the world had forgotten us," said one tearful Bridge City official.

"I don't have the vocabulary to adequately thank you all", said a Houston official.

A poor mother at a shelter in Winnie broke into sobs when handed a box of diapers. A box of diapers.

Other truck loads of supplies were delivered to the area in a second wave of relief about a month later. Thanks to some brilliant WU minds, much of our effort can now be seen and heard in real time via webcam at Many of you "rode in the back seat" and saw first hand the destruction of the Bolivar Peninsula on one of these trips.

Additionally, a significant amount of specifically requested supplies--from clothing to medical equipment--have been freighted into the area to contacts we made.

Also, we've facilitated ramp building for disability service organizations, arranged a scholarship for a college student with a disability, repatriated WU blogger BillyBadBird to his home on the Bolivar Peninsula, hosted a pizza party on the Bolivar Peninsula for survivors and relief workers. And we put on a big Christmas party with gifts for children in Bridge City on December 20th which was attended by WU members from around the country...and which was webcast at

And our work isn't done. Sometime early next year we are going to begin making small financial grants to groups in the Ike affected area.

At this point, we've raised about $40,000...and delivered over $500,000 in goods and services.

So, we now turn our attention to the future. Ike will hardly be the last hurricane to create chaos on our shores. But out of the chaos of Ike came some great revelations.

There are certain populations which are simply not well served by the large institutional and government relief infrastructure. It's a niche which Portlight Strategies, Inc., has filled in relatively small ways in past disasters. And which this community has filled in a big way since Ike. We are deeply committed to filling this niche in the future.

We are working now to establish several permanent staging areas along the East and Gulf coasts, so that we can respond to the needs of un-served, under served and forgotten people in an efficient manner. A series of grants we are scheduled to receive will begin to fund this. We are also working through budgeting and planning for the future. We are committed to having a reserve fund of $100,000 by June 1 of next year.

John is planning a Valentine's online auction to help raise funds. And I am working on a multi-city Walk A Thon which will raise awareness and funds. Shortly after the Holidays, we will unroll details on all of this. We're gonna need y'alls' help with all this. In the meantime, if you have any money laying around that you don.t know what to do with.the PayPal button at still works nicely. Or checks can be mailed to: Portlight Strategies, Inc. 2043 Maybank Hwy, Charleston, SC 29412.

OK...I got the shameless plug for money out of the I'll wrap this up.

Each of you owns an equal share of all this. You...I mean YOU...made this happen. Hundreds of people have come together in a spirit of community to help people in desperate need. And next year, any one of us could find ourselves in the exact same situation.

This can be one of two things:

1) A one shot deal that did a lot of good for a lot of people.


2) The beginning of an ongoing transformational process that makes us all the better for it for the rest of our lives.

I'm casting my lot with the second choice. And, based on what I've seen in the last 100 days, I'm pretty sure you will, too. We should all be proud of ourselves..and grateful for the opportunity to serve.

Paul Timmons, Jr.

Geoengineering: a bad idea whose time may come

By: JeffMasters, 3:46 PM GMT on December 19, 2008

Yesterday, at the annual meeting of the American Geophysical Union (AGU), climate change scientists discussed the risks and benefits of deliberately altering Earth's climate through "geoengineering". One measure of the concern scientists have about Earth's climate could be gauged by the standing-room only crowd of 200 that packed the presentation room. The eleven speakers at the session laid out some radical and dangerous ideas for deliberately altering Earth's climate. They uniformly cautioned that the uncertainties and dangers of implementing any of these schemes was high, but that geoengineering may be necessary if efforts to control greenhouse gases fail and the climate begins to undergo rapid and destructive changes.

David Keith presented the results of a week-long workshop held earlier this year that brought together ten of the world's experts on geoengineering. He emphasized that even if we stopped emitting CO2 today, the possibility of dangerous climate change capable of causing a "climate emergency" may still be higher than 1%, thanks to the tremendous inertia of the heat stored in the oceans. Of course, we're not going to stop emitting CO2 today. Dealing with a future climate emergency is technically feasible, if we inject large quantities of sulfur into the tropical stratosphere via aircraft, artillery, or tethered balloons with hoses. Sulfur injection into the stratosphere is considered to be the leading candidate for geoengineering, since nature has done this many times via volcanic eruptions, and we have some idea of what to expect. As I reported in a blog post earlier this year, the idea is being championed by Nobel prize-winning atmospheric chemist Paul Crutzen.

One problem with injecting sulfur into the stratosphere is that it tends to settle back to the surface in about ten months. A. V. Eliseev explained that in order to keep global temperatures under control in a world with ever-increasing CO2 emissions, we would have to inject an ever increasing amount of sulfur into the atmosphere. His computer model results showed that if a funding lapse occurred in, say, the year 2075, the atmosphere would rapidly warm by 5-9°F (3-5°C) over most of North America, Europe, and Asia, within a decade of cessation of the geoengineering efforts. The resulting shock to ecosystems would be extremely dangerous to civilization.

Richard Turco of UCLA estimated that injecting enough sulfur in the stratosphere to properly geoengineer the climate would require 3000 aircraft sorties per day, and cost $50-$100 billion per year. Model results he presented showed a large amount of uncertainty as to what might happen, and he cautioned that there was "no guarantee of success, and failure would be catastrophic".

A. Robrock of Rutgers disagreed with Dr. Turco, and estimated that the cost of injecting the required amount of sulfur into the stratosphere would by less that $5 billion per year, provided the U.S. military would let scientists use 167 of the existing fleet of 522 F15C Eagle jets to do the job. After all, he reasoned, why wouldn't the military want to use their aircraft to confront our enemy (global warming?) High-altitude fighter jets would be required to do the job, since ordinary jetliners cannot fly high enough to penetrate into the stratosphere. He cautioned that such a fleet of aircraft would have to fly three missions per day, and their exhaust gases would probably cause significant destruction of Earth's protective ozone layer. Furthermore, modeling studies show that we don't know what size particles to make, where to put the sulfur, and what uneven effects the efforts might have on Earth's climate. He concluded, "there are many reasons not to do geoengineering".

A more ecological approach to geoengineering was presented by Phil Rasch of Pacific Northwest National Laboratory, and by Jim Haywood of Britain's Met Office Hadley Center. They proposed building a fleet of wind-powered ships known as Fletter vessels (Figure 1) that would spray large amounts of sea salt into the air in regions where there are existing stratocumulus clouds. The sea salt would act as nuclei around which moisture could condense, making the clouds more reflective. A fleet of approximately 66 of these vessels would be required to seed the clouds over 30% of the globe, to balance a doubling of atmospheric carbon dioxide. However, they cautioned that while this solution would be relatively cheap, the technology to implement this scheme would be difficult. Furthermore, studies performed with climate models showed that the resulting climate shift would not be uniform, and many areas would experience drought. In particular, Dr. Haywood showed the possibility of severe drought in the Amazon rain forest and in the Southwest U.S.

Figure 1. A conceptual picture of Flettner spray vessel with Thom fences. These wind-driven vessels have vertical spinning cylinders that use the Magnus effect to produce forces perpendicular to the wind direction. Anton Flettner built a ship using this technology that crossed the Atlantic in 1926. The proposed geoengineering Flettner vessels would sail over ocean regions covered with stratocumulus clouds and make the existing clouds whiter by spraying small salt particles into the air. Image is copyright J. MacNeill 2006. For more information on these vessels, see Salter at al., 2008, "Sea-going hardware for the cloud albedo method of reversing global warming", Philosophical Transactions of The Royal Society A, 366, Number 1882, pp3989-4006, 13 November 2008.

Katharine Ricke of Carnagie Mellon University cautioned that the foreign policy community has virtually no awareness of geoengineering issues, and would be totally unprepared for the possibility of some country deciding to unilaterally attempt a geoengineering program on their own. She suggested that an effort needs to be made to promote international agreements on geoengineering, perhaps including binding treaties.

Jeff Masters

Climate Change

Is the Atlantic hurricane season getting longer?

By: JeffMasters, 6:34 PM GMT on December 16, 2008

It seems like there have been an unusual number of early and late season tropical storms and hurricanes in the Atlantic in recent years. In 2008, we had four named storms in July, and the second most powerful November hurricane on record. Both 2007 and 2005 had rare December storms, and 2003 featured Tropical Storm Anna, the first April tropical storm ever recorded. This year, Hurricane Tomas made 2010 the fourth straight year with a November hurricane, something that has never occurred in the Atlantic since accurate records began in 1851. Is hurricane season getting longer? Dr. Jim Kossin of the University of Wisconsin published a 2008 paper in Geophysical Research Letters, "Is the North Atlantic hurricane season getting longer?" He concluded that yes, there is an "apparent tendency toward more common early- and late-season storms that correlates with warming Sea Surface Temperature but the uncertainty in these relationships is high".

Figure 1. Observed sea surface temperature (SST) trends during the official North Atlantic hurricane season (June-November) for the period 1950-2007. Units are °C per century. The dashed rectangle denotes the tropical storm formation region south of 30° North latitude and east of 75° West longitude. Data are from the NOAA Extended Reconstructed Sea Surface Temperature V3 product [Smith et al., 2008]. Image credit: Kossin, J., 2008, "Is the North Atlantic hurricane season getting longer?", Geophysical Research Letters, Vol. 35, L23705, doi:10.1029/2008GL036012, 2008.

Dr. Kossin utilized the "best track" database of Atlantic tropical cyclone activity going back to 1851. However, since lack of satellite and aircraft reconnaissance data before 1950 makes the early part of this record suspect, he limited his analysis to the period from 1950 onward. The era of best data--the satellite era beginning in 1980--was also looked at separately, to ensure the highest possible data quality. The area studied was only a portion of the Atlantic--the tropical storm formation region south of 30° North latitude and east of 75° West longitude. This region has shown considerable warming of the Sea Surface Temperatures (SSTs) since 1950, in excess of 1°F (0.6°C) (Figure 1). A statistical method called "quantile regression" was employed. The method looked at how certain thresholds that mark the beginning and end of hurricane season have changed over the years. For example, the date where 5% of all tropical storms form earlier than that date, was called the 0.05 quantile, and the date where 5% of all tropical storms form later than that date, was called the 0.95 quantile. Kossin was able to show that the date of the 0.05 quantile got steadily earlier and the date of the 0.95 quantile steadily got later since 1950. Hurricane season for both the period 1950-present and 1980-present got longer by 5 to 10 days per decade.

Figure 2. Trends in tropical storm formation dates, in the region south of 30° North latitude and east of 75° West longitude, at the quantiles. Trends are based on the periods (left) 1950-2007, and (right) 1980-2007. The dates (month/year) associated with the 0.05, 0.25, 0.50, 0.75, and 0.95 quantiles for each period are shown on the top axis (these threshold dates are based on the full sample for each period). Shading denotes the 90% confidence interval. Image credit: Kossin, J., 2008, "Is the North Atlantic hurricane season getting longer?", Geophysical Research Letters, Vol. 35, L23705, doi:10.1029/2008GL036012, 2008.

Relationship with Sea Surface Temperature
The broadening of the Atlantic hurricane season found was strongly dependent upon Sea Surface Temperatures. Both the onset and end of hurricane season shifted by 20 days per degree C of warming of the SST. With global warming projected to increase tropical Atlantic SSTs 1-2°C by the end of the century, can we then expect a 40-80 day increase in the length of hurricane season? Dr. Kossin doesn't explore this possibility, and doesn't blame the observed increase in the length of the season on human-caused global warming of the oceans. There is reason to believe that future warming of the Atlantic SSTs won't necessarily broaden the area over which tropical storms will form, though. Papers by Henderson-Sellers et al. (1998) and Knutson et al. (2008) theorize that as SSTs warm, the lowest temperature at which tropical storms can form will also increase. The current minimum temperature of 26.5°C (80°F) may increase to 28.5°C for a 2°C warming of Atlantic SSTs. Johnson and Xie (2010) have found observational evidence that the lowest temperature at which tropical storms can form has indeed been increasing at about 0.1°C per decade in the Atlantic, in line with climate model predictions.

Henderson-Sellers, A., et al., 1998, "Tropical Cyclones and Global Climate Change: A Post-IPCC Assessment", Bull. Am. Meteorol. Soc. 79, 19–38.

Johnson, N.C., and S.P. Xie, 2010, "Changes in the sea surface temperature threshold for tropical convection", Nature Geoscience doi:10.1038/ngeo1008

Knutson, T.R., J.J. Sirutis, S.T. Garner, G.A. Vecchi, and I.M. Held, 2008, Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions", Nature Geoscience 1, 359 - 364 (2008), doi:10.1038/ngeo202

Kossin, J., 2008, "Is the North Atlantic hurricane season getting longer?", Geophysical Research Letters, Vol. 35, L23705, doi:10.1029/2008GL036012, 2008.

Jeff Masters

Wunderground launches high-definition radar product

By: JeffMasters, 2:46 PM GMT on December 15, 2008

The Terminal Doppler Weather Radar (TDWR) is an advanced technology weather radar deployed near 45 of the larger airports in the U.S. The radars were developed and deployed by the Federal Aviation Administration (FAA) beginning in 1994, as a response to several disastrous jetliner crashes in the 1970s and 1980s caused by strong thunderstorm winds. The crashes occurred because of wind shear--a sudden change in wind speed and direction. Wind shear is common in thunderstorms, due to a downward rush of air called a microburst or downburst. The TDWRs can detect such dangerous wind shear conditions, and have been instrumental in enhancing aviation safety in the U.S. over the past 15 years. The TDWRs also measure the same quantities as our familiar network of 148 NEXRAD WSR-88D Doppler radars--precipitation intensity, winds, rainfall rate, echo tops, etc. However, the newer Terminal Doppler Weather Radars are higher resolution, and can "see" details in much finer detail close to the radar. This high-resolution data has generally not been available to the public until now. Thanks to a collaboration between the National Weather Service (NWS) and the FAA, the data for all 45 TDWRs will be made available in real time over the next few months via a free satellite broadcast (NOAAPORT). Six radar sites are already available (Figure 1), and the remaining radars will be added by June 2009. I'm pleased to announce that the Weather Underground is now making the TDWR data available to the public, and will be adding new sites as they become available. We're calling them "High-Def" stations on our NEXRAD radar page. The six TDWR sites available so far are:

Detroit, MI
Cleveland, OH
Fort Lauderdale, FL
Miami, FL
West Palm Beach, FL
Newark, NJ

Since thunderstorms are uncommon along the West Coast and Northwest U.S., there are no TDWRs in California, Oregon, Washington, Montana, or Idaho.

Figure 1. The network of 45 Terminal Doppler Weather Radar (TDWR) stations in the U.S.

Summary of the TDWR products
The TDWR products are very similar to those available for the traditional WSR-88D NEXRAD sites. There is the standard radar reflectivity image, available at each of three different tilt angles of the radar, plus Doppler velocity of the winds in precipitation areas. There are 16 colors assigned to the short range reflectivity data (same as the WSR-88Ds), but 256 colors assigned to the long range reflectivity data and all of the velocity data. Thus, you will see up to 16 times as many colors in these displays versus the corresponding WSR-88D display, giving much higher detail of storm features. The TDWRs also have storm total precipitation available in the standard 16 colors like the WSR-88D has, or in 256 colors (the new "Digital Precipitation" product). Note, however, that the TDWR rainfall products generally underestimate precipitation, due to attenuation problems (see below). The TDWRs also have such derived products as echo height, vertically integrated liquid water, and VAD winds. These are computed using the same algorithms as the WSR-88Ds use, and thus have no improvement in resolution.

Improved horizontal resolution of TDWRs
The TDWR is designed to operate at short range, near the airport of interest, and has a limited area of high-resolution coverage--just 48 nm, compared to the 124 nm of the conventional WSR-88Ds. The WSR-88Ds use a 10 cm radar wavelength, but the TDWRs use a much shorter 5 cm wavelength. This shorter wavelength allow the TDWRs to see details as small as 150 meters along the beam, at the radar's regular range of 48 nm. This is nearly twice the resolution of the NEXRAD WSR-88D radars, which see details as small as 250 meters at their close range (out to 124 nm). At longer ranges (48 to 225 nm), the TDWRs have a resolution of 300 meters--more than three times better than the 1000 meter resolution WSR-88Ds have at their long range (124 to 248 nm). The angular (azimuth) resolution of the TDWR is nearly twice what is available in the WSR-88D. Each radial in the TDWR has a beam width of 0.55 degrees. The average beam width for the WSR-88D is 0.95 degrees. At distances within 48 nm of the TDWR, these radars can pick out the detailed structure of tornadoes and other important weather features (Figure 2). Extra detail can also been seen at long-ranges, and the TDWRs should give us more detailed depictions of a hurricane's spiral bands as it approaches the coast.

Figure 2. View of a tornado taken by conventional WSR-88D NEXRAD radar (left) and the higher-resolution TDWR system (right). Using the conventional radar, it is difficult to see the hook-shape of the radar echo, while the TDWR clearly depicts the hook echo, as well as the Rear-Flank Downdraft (RFD) curling into the hook. Image credit: National Weather Service.

No change to time resolution
Like the old NEXRAD data, the new TDWR data will update once every six minutes. The NWS advertises that the TDWR data will be sent out within one minute of when it is measured. The TDWR does scan the atmosphere once per minute at the lowest elevation angle of the radar, but unfortunately, there are no plans to make this rapid scan data available via the free public NOAAPORT feed.

Attenuation problems
The most serious drawback to using the TDWRs is the attenuation of the signal due to heavy precipitation falling near the radar. Since the TDWRs use the shorter 5 cm wavelength, which is closer to the size of a raindrop than the 10 cm wavelength used by the traditional WSR-88Ds, the TDWR beam is more easily absorbed and scattered away by precipitation. This attenuation means that the radar cannot "see" very far through heavy rain. It is often the case that a TDWR will completely miss seeing tornado signatures when there is heavy rain falling between the radar and the tornado. Hail causes even more trouble (Figure 3). Thus, it is best to use the TDWR in conjunction with the traditional WSR-88D radar to insure nothing is missed.

Figure 3. View of a squall line (left) taken using a TDWR (left column) and a WSR-88D system. A set of three images going from top to bottom show the squall line's reflectivity as it approaches the TDWR radar, moves over the TDWR, than moves away. Note that when the heavy rain of the squall line is over the TDWR, it can "see" very little of the squall line. On the right, we can see the effect a strong thunderstorm with hail has on a TDWR. The radar (located in the lower left corner of the image) cannot see much detail directly behind the heavy pink echoes that denote the core of the hail region, creating a "shadow". Image credit: National Weather Service.

Range unfolding and aliasing problems
Another serious drawback to using the TDWRs is the high uncertainty of the returned radar signal reaching the receiver. Since the radar is geared towards examining the weather in high detail at short range, echoes that come back from features that lie at longer ranges suffer from what is called range folding and aliasing. For example, for a thunderstorm lying 48 nm from the radar, the radar won't be able to tell if the thunderstorm is at 48 nm, or some multiple of 48 nm, such as 96 or 192 nm. In regions where the software can't tell the distance, the reflectivity display will have black missing data regions extending radially towards the radar (Figure 4). Missing velocity data will be colored pink and labeled "RF" (Range Folded). In some cases, the range folded velocity data will be in the form of curved arcs that extend radially towards the radar.

Figure 4. Typical errors seen in the velocity data (left) and reflectivity data (right) when range folding and aliasing are occurring. Image credit: National Weather Service.

Ground clutter problems
Since the TDWRs are designed to alert airports of low-level wind shear problems, the radar beam is pointed very close to the ground and is very narrow. The lowest elevation angle for the TDWRs ranges from 0.1° to 0.3°, depending upon how close the radar is to the airport of interest. In contrast, the lowest elevation angle of the WSR-88Ds is 0.5°. As a result, the TDWRs are very prone to ground clutter from buildings, water towers, hills, etc. Many radars have permanent "shadows" extending radially outward due to nearby obstructions. The TDWR software is much more aggressive about removing ground clutter than the WSR-88D software is. This means that real precipitation echoes of interest will sometimes get removed.

For more information
For those of you who are storm buffs that will be regularly using the new TDWR data, I highly recommend that you download the three Terminal Doppler Weather Radar (TDWR) Build 3 Training modules. These three Flash files, totaling about 40 Mb, give one a detailed explanation of how TDWRs work, and their strengths and weaknesses. There is also a full product documentation guide available. I'll be adding the info in this blog entry into the radar help link available on each of our radar pages.

No Atlantic named storm likely this week
The models continue to indicate an extratropical storm that has the potential to evolve into a named subtropical storm will form in the middle Atlantic by Thursday. However, it now appears that there will be too much wind shear for Subtropical Storm Rene to form out of this system.

I'm in San Francisco this week for the annual meeting of the American Geophysical Union, the world's largest climate change conference. I'll be posting daily "post cards" from the conference this week.

Jeff Masters


Rare snowstorm hits Louisiana, Mississippi, and Texas

By: JeffMasters, 3:02 PM GMT on December 12, 2008

A rare early December heavy snowstorm hit Louisiana, Texas, and Mississippi yesterday, setting several records. It was the earliest measurable snowfall in recorded history at Beaumont, Port Arthur, and Lake Charles. Also, this snow event set the all time record snowfall amounts for the month of December at Beaumont, Port Arthur, Lake Charles, Lafayette, and New Iberia, and was the first measurable snowfall in recorded history for the month of December at Lafayette. In Mississippi, up to 5 inches of snow fell on areas south of Jackson. The snow knocked out power to 83,000 and caused numerous traffic accidents and road closures across Southeast Louisiana. The snow was caused by an upper-level low pressure system that deepened over the Gulf of Mexico. The snow was unusual because it occurred when the surface temperatures were 32 to 35 degrees.

The one inch of snow reported in New Orleans was that city's earliest snow on record. The previous earliest date for measurable snowfall in New Orleans was Dec. 22, 1989. New Orleans' last snowfall, in 2004, was a dusting. The record snowfall for the city is about 5 inches, recorded Dec. 30, 1963.

Figure 1. Yesterday's snowstorm brought a festive blanket of white to Magee, Mississippi. Image credit: SouthernLady.

A few selected snow amounts from yesterday's storm:

Amite 8.0 inches
rossroads 6.0 inches
Hammond 6.0 inches
Mount Herman 6.0 inches
Opelousas 6.0 inches
Washington 6.0 inches
Covington 6.0 inches
Baton Rouge 3.0 inches
Plaquemine 2.5 inches
New Orleans 1.0 inches
Lafayette 1.0 inches
New Iberia 0.8 inches
NWS Lake Charles 0.4 inches

Columbia 5.0 inches
Jayess 5.0 inches
Brookhaven 5.0 inches
Prentiss 4.0 inches

Lumberton 4.0 inches
West Beaumont 4.0 inches
Woodville 3.0 inches
Beaumont City 2.5 inches
SE TX regional Arpt 1.8 inches
Orange 1.0 inches
Port Neches 1.0 inches
Jasper 0.5 inches

So what happened to global warming?
Record snow events inevitably bring comments like, "so what happened to global warming?" First of all, no single weather event can prove or disprove the existence of climate change or global warming. One needs to look at the entire globe over a period of decades to evaluate whether or not climate change is occurring. It might be instructive to look at what global snow cover is doing this season (it's about 10% below average, Figure 2), but this doesn't mean global warming is occurring. This year's reduced snow cover could be due to natural seasonal variations. Only global average temperatures, when viewed over a time scale of decades, can prove or disprove the existence of global warming. Global average temperatures, when averaged over a decades-long period that removes the "bumps" associated with natural seasonal weather fluctuations, show that global warming is occurring.

Secondly, as both myself and wunderground climate change expert Dr. Ricky Rood have pointed out, global warming won't necessarily lead to a decrease in snowfall in all regions. If it is cold enough to snow, we may actually see increased snow in many locations. Global warming puts more moisture in the atmosphere, due to fact that higher global temperatures evaporate more moisture off of the oceans. I expect that coming decades will bring many record snowfalls, due to the increased moisture available in the atmosphere.

Figure 2. Northern Hemisphere snow cover on December 9, 2008 (blue areas) compared to average (green line). Northern Hemisphere snow cover was about 10% below average the first week of December. Since October, Northern Hemisphere snow cover has been about 5% below average. Bob Hart at Florida State has put together a nice set of regularly updating images showing the current state of global snow cover.

Subtropical Storm Rene possible in the Atlantic next week
The trough of low pressure that brought snow to the deep south will track eastward over the Atlantic over the next few days, reaching the central Atlantic north of the Lesser Antilles Islands by Monday. On Monday or Tuesday, the computer models unanimously agree that the southern portion of the trough will pinch off and form a "cut-off low"--an extratropical storm that is cut off from the jet stream. This low is expected to track slowly westward to a point midway between Bermuda and Puerto Rico by late next week. The low will be over waters marginally warm enough--25°C--to support formation of a subtropical storm, and phase space diagrams from Florida State indicate that this storm will be warm-cored. Wind shear is forecast to be low enough to allow a subtropical or tropical storm to form, and I give a medium (20-50% chance) that we will see Subropical Storm Rene in the Atlantic next week.

Jeff Masters

Winter Weather

Active 2009 hurricane season predicted by Colorado State scientists

By: JeffMasters, 4:20 PM GMT on December 10, 2008

It's going to be a moderately more active than average Atlantic hurricane season in 2009, according to the latest seasonal forecast issued by Dr. Bill Gray and Phil Klotzbach of Colorado State University (CSU) today. The CSU team is calling for 14 named storms, 7 hurricanes, 3 intense hurricanes, and an ACE index 30% above average (Accumulated Cyclone Energy (ACE) is a measure of the total destructive power of a hurricane season, based on the number of days strong winds are observed). An average season has 10 named storms, 6 hurricanes, and 2 intense hurricanes. The CSU forecast calls for a 63% chance of a major hurricane hitting the U.S., which is 11% above average. The odds for a major East Coast hurricane are put at 39% (a 31% chance is average), and odds for the Gulf Coast are put at 38% (30% chance is average). The CSU team's prediction of an above average hurricane season hinges on two main factors:

1) An El Niño event is not expected in 2009. The current pressure pattern in the Northeast Pacific is one frequently associated with the development of a La Niña event. A number of the computer models used to forecast El Niño are now calling for development of a La Niña event in 2009. Lack of an El Niño event in 2009 will lead to average to below average values of wind shear over the Atlantic, enhancing hurricane activity.

2) Sea Surface Temperatures (SSTs) in the North Atlantic have been anomalously warm in October and November. This implies we are still in the active phase of the Atlantic Multidecadal Oscillation (AMO), the active period of hurricane activity that began in 1995.

Figure 1. Accuracy of long-range forecasts of Atlantic hurricane season activity performed by Bill Gray and Phil Klotzbach of Colorado State University (colored squares) and TSR (colored lines). The skill is measured by the Mean Square Skill Score (MSSS), which looks at the error and squares it, then compares the percent improvement the forecast has over a climatological forecast of 10 named storms, 6 hurricanes, and 2 intense hurricanes. TS=Tropical Storms, H=Hurricanes, IH=Intense Hurricanes, ACE=Accumulated Cyclone Energy, NTC=Net Tropical Cyclone Activity. Image credit: TSR.

How good are these December hurricane season forecasts?
Keep in mind that these December forecasts are a research project, and as yet have shown no skill in predicting the activity of upcoming hurricane seasons. They make this clear in the introduction to the December forecast, stating, "our real-time forecasts issued in early December from 1992-2007 did not show skill in real time". The CSU team talks extensively about their "hindcast" skill with these December forecasts, which means they can successfully predict the behavior of past hurricane seasons using their methodology. In their words, "It is only through hindcast skill that one can demonstrate that seasonal forecast skill is possible. This is a valid methodology provided that the atmosphere continues to behave in the future as it has in the past." The problem is that the atmosphere often does not continue to behave in the future as it has in the past, and a technique that is successful in a hindcast will often fail in a forecast. In their 2007 December forecast, they showed that the correlation coefficient (r squared), a standard mathematical measure of skill, was near zero for their real-time December forecasts between 1992-2007. They made a successful December 2007 forecast which was not included in that analysis, and their December skill is probably slightly positive now.

Another way to measure skill is using the Mean Square Skill Score (MSSS), which looks at the forecast error and squares it, then compares the percent improvement the forecast has over a climatological forecast of 10 named storms, 6 hurricanes, and 2 intense hurricanes (Figure 1). The skill of the December forecasts issued by both CSU and Tropical Storm Risk, Inc. (TSR) have averaged near zero since 1992. Not surprisingly, the forecasts get better the closer they get to hurricane season. The TSR forecasts show more skill than the CSU forecasts, but it is unclear how much of this superiority is due to the fact that TSR issues forecasts of fractional storms (for example, TSR may forecast 14.7 named storms, while CSU uses only whole numbers like 14 or 15). TSR does an excellent job communicating their seasonal forecast skill. Each forecast is accompanied by a "Forecast Skill at this Lead" number, and they clearly define this quantity as "Percentage Improvement in Mean Square Error over Running 10-year Prior Climate Norm from Replicated Real Time Forecasts 1987-2006."

The June and August forecasts from CSU, TSR, and NOAA show some modest skill, and are valuable tools for insurance companies and emergency planners to help estimate their risks. The key problem with forecasts done in April or earlier is that the El Niño/La Niña atmospheric cycle that can dominate the activity of an Atlantic hurricane season is generally not predictable more than 3-6 months in advance. For example, none of the El Niño forecast models foresaw the September 2006 El Niño event until May of 2006. Until we can forecast the evolution of El Niño more than six months in advance, December forecasts of Atlantic hurricane activity are merely interesting mental exercises that don't deserve the media attention they get. There is hope for the December forecasts, since Klotzbach and Gray (2004) showed that their statistical scheme could make a skillful forecast in December, when applied to 50 years of historical data. However, these "hindcasts" are much easier to make than a real-time forecast. For example, before 1995, it was observed that high rainfall in the Sahel region of Africa was correlated with increased Atlantic hurricane activity. This correlation was used as part of the CSU forecast scheme. However, when the current active hurricane period began in 1995, the correlation stopped working. Drought conditions occurred in the Sahel, but Atlantic hurricane activity showed a major increase. The CSU team was forced to drop African rainfall as a predictor of Atlantic hurricane activity.

Klotzbach, P.J., and W.M. Gray, "Updated 6-11 Month Prediction of Atlantic Basin Seasonal Hurricane Activity," Weather and Forecasting 19, Issue 5, October 2004, pp. 917-934.

Jeff Masters

Severe weather outbreak in the South; 2008 the 2nd busiest tornado season

By: JeffMasters, 1:45 PM GMT on December 09, 2008

A major severe weather outbreak is possible today across portions of Louisiana, Mississippi, Arkansas, and surrounding states, in association with a strong cold front expected to sweep through the region. The Storm Prediction Center has placed portions of Louisiana and Mississippi under the "moderate Risk" category for severe weather today, and supercell thunderstorms with tornadoes are likely today along the cold front. Several tornado warnings have already been issued for Louisiana this morning, though no confirmed touch-downs have been reported yet.

Links to follow
Severe weather map
Interactive tornado map

Figure 1. Cumulative tornadoes for 2008, compared to the five most recent years. The confirmed tornado count for 2008 through September is 1600, making it the second busiest tornado season on record. Only 2004, with 1817 twisters, has had more. Image credit: NOAA Storm Prediction Center.

Tornado season update
As the end of the year approaches, the tornado season of 2008 has already established itself as the second busiest tornado season on record. The 1600 confirmed tornadoes this year through September are second only to the record 1817 tornadoes recorded in 2004. Reliable tornado records began in 1950, but tornado reports have grown steadily over the years due to an increase in population and an increase in interest and reporting ability. With just a few weeks left in the year, it seems unlikely 2008 will break the record for most tornadoes. The preliminary count for October and November was 46 tornadoes, which means we would need at a minimum 171 tornadoes over the the next three weeks to set a new record. This is pretty unlikely, unless we get a major outbreak today and two additional outbreaks later this month. Tornado deaths so far this year are at 125--the most since 1998, when 130 people died in tornadoes.

Jeff Masters


Increasing hurricane damages

By: JeffMasters, 9:53 PM GMT on December 05, 2008

A conference called the Hurricane Science for Safety Leadership Forum convened this week in Orlando to look at how we can better prepare for the inevitable hurricanes in our future. The conference brought together an interesting mix of experts--scientists from environmental groups like the National Wildlife Federation, insurance industry representitives, and a representative from the pro-business Competitive Enterprise Institute (CEI).

There are a number of interesting Powerpoint and video presentations posted on their web site, for those interested. The most eye-opening fact I saw came during a presentation done by Amanda Staudt of the National Wildlife Federation. In her presentation on the policy implication of hurricanes and climate change, she showed that the population of South Florida is projected to grow from a 1990 population of 6.3 million to a 2050 population of 15-30 million people. That's a startling increase in population. Higher and higher hurricane damage tabs are inevitable in coming decades, just from this huge increase in population. She goes further, showing that if the theoretical predictions for global warming by the end of the century come true--a 2-13% increase in hurricane winds due to ocean warming, a 10-31% increase in hurricane rainfall, and an increase in sea level of several feet--there is likely to be a huge increase in hurricane damage, and probably in deaths, as well.

I have a few comments on this. While I believe that hurricane damages will continue to grow primarily because of population increases, higher wealth, and poor land management, the contribution of increased damage due to global warming will start to become significant by the end of the century. The 5% increase in hurricane winds per °C of ocean warming theorized by hurricane researcher Dr. Kerry Emanuel (Emanuel, 2005) may not seem like much, it will make a significant difference in the destructive power of the strongest storms. A Category 4 hurricane does about four times more damage than a Category 3 hurricane, and 250 times more damage than a Category 1 storm (Figure 1). Given the expected increase of tropical sea surface temperatures of 1-2 °C by 2100, hurricane wind speeds should increase by 5-10%. Since the difference in wind speed between a Category 3 and Category 4 hurricane is about 15%, we can anticipate that the strongest hurricanes in 2100 will do 1 1/2 to 3 times more damage than they do now.

This may be an underestimate of the increase in damage, though. Global sea level rose about 0.75 feet last century, and is expected to rise 0.6 - 1.9 feet this century, according to the "official" word on climate, the 2007 report of the U.N.'s Intergovernmental Panel on Climate Change (IPCC). A paper published by Pfeffer et al. in Science this year concluded that the IPCC underestimated sea level rise, and that the "most likely" range of sea level rise by 2100 is 2.6 - 6.6 feet. If true, we can expect greatly increased damage from hurricane storm surges. However, it is possible that there will be fewer hurricanes by the end of the century, thanks to an increase in wind shear over the tropical Atlantic (Vecchi and Soden, 2007).

Figure 1. Potential hurricane damage as a function of Saffir-Simpson category for U.S. hurricanes between 1925-1995. If the median damage from a Category 1 hurricane is normalized to be a "one", then Category 2, 3, and 4 hurricanes were 10, 50, and 250 times more damaging, respectively. Data taken from Pielke, Jr. R. A., and C. W. Landsea, 1998: "Normalized Atlantic hurricane damage 1925-1995" Wea. Forecasting, 13, pp.621-631.

Better building codes
Congressman Bennie Thompson, D-MS, Chairman of the House Homeland Security Committee, helped to kick off the conference with opening remarks that underscored his intention to hold Congressional hearings on developing new building codes in hurricane-prone areas. He was hopeful that President-elect Obama and new incoming head of Homeland Security, Arizona Gov. Janet Napolitano, would work to adopt new, tougher building standards. "Take a look at the homes on the Bolivar Peninsula in Texas that are still standing after the hurricane," Thompson said. "We know how to build stronger homes. Now we just need to do it." Thompson said that while such legislation had been introduced in the past but failed, chances were better under an Obama administration of passage.

I think it is essential that more stringent and comprehensive building codes get adopted in hurricane alley to reduce the inevitable huge price tags from future hurricanes.

Emanuel, K. 2005, "Increasing destructiveness of tropical cyclones over the past 30 years", Nature, 436, 4 August 2005, doi:10.1038/nature03906.

Vecchi, G.A., and B.J. Soden, 2007, "Increased tropical Atlantic wind shear in model projections of global warming", Geophys. Res. Lett., 34, L08702, doi:10.1029/2006GL028905, 2007.

Jeff Masters

Climate Change

Venice suffers its fourth-worst flood since 1872

By: JeffMasters, 7:59 PM GMT on December 03, 2008

Venice, Italy, suffered ocean flooding 1.56 meters (5.1 feet) above mean sea level on Monday, December 1, its fourth-worst flood since modern record keeping began in 1872. The flooding was triggered by a strong low pressure system that passed through Europe, dumping rains of 1-2 inches that helped fill the salt-water lagoon surrounding Venice with high levels of river run-off. The counter-clockwise flow of air around the low brought warm, southerly winds up the length of the Adriatic Sea, which piled up high levels of ocean water into the lagoon. Sustained winds reached 30 mph on the morning of December 1 in Venice. High tides submerged the city again yesterday, but only to a depth of 1.02 meters. As the low pulls away from Italy and river run-off gradually subsides, no further flooding of Venice should occur this week.

Figure 1.Flooding in Venice from the December 1, 2008 flood. Image credit: Venice Water Authority.

Sinking land, rising seas
When Venice was founded sixteen hundred years ago, sea level was about six feet lower than today. Sea level has risen over the centuries as a natural response to our continued emergence from the last Ice Age, thanks to melting of glaciers and ice caps. Global sea level rose eight inches during the 20th century (2 mm/year). The sea level rose by only about 4-5 inches in the Adriatic Sea near Venice during that time, though. However, the rate of global sea level rise has increased by 50% to 3 mm/year in recent decades and was 5 mm/year near Venice during the period 1993-2008. As the seas have risen, Venice has sunk. Since 1897, natural compression of the sediments Venice sit on has resulted in the city sinking about 1.5 inches. Additional sinking of three inches due to pumping of fresh water from an aquifer beneath the lagoon between the 1920s and early 1970s has left the city 4.5 inches lower than at the beginning of the century. In 1900, famed St. Marks Square flooded at high tide six times per year, on average. By 1999, that number had increased to 99 times per year, thanks to the combined effects of higher sea levels and the sinking of the city.

Figure 2. Satellite image of Venice. The city is on an island in a lagoon that opens to the ocean in three places. Image credit: NASA Earth Observatory.

Jetties built at the three entrances to Venice's lagoon help to hold back the sea, but have also acted as barriers to the natural inflow of fresh sand to replenish beaches within the lagoon. This sand now accumulates along the jetties. The jetties also act as barriers that disrupt ocean currents and contribute to coastal erosion.

Barrier island erosion
Venice's lagoon is protected by a number of barrier islands. These islands are steadily eroding, due to construction projects, summer beach crowds, and beach-going vehicles. Sea walls were built along many of these barrier islands in the 14th century, but high tides now regularly overtop these sea walls.

Salt marsh erosion
Additional flooding in Venice occurs because the protective salt marshes in the lagoon surrounding the city are steadily eroding. This is largely due to the fact that 600 years ago Venice diverted the courses of the four major rivers that flowed into the lagoon, forcing them to empty into the ocean directly. This was done so that sediments would not build up and allow attackers to approach the city via land. Without new sediments to replenish them, the marshes in the lagoon have been steadily eroding away. Water pollution has also contributed to erosion problems. The pollution kills eelgrass, the chief building block of the salt marshes.

Protecting Venice
The "official" word on climate, the 2007 report of the U.N.'s Intergovernmental Panel on Climate Change (IPCC), predicts a 0.6 - 1.9 foot sea level rise by 2100. However, they cautioned in their report that due to the lack of knowledge about how melting glaciers behave, the actual sea level rise might be higher. A paper published by Pfeffer et al. in Science this September concluded that when considering these unknown glacier melting processes, the "most likely" range of sea level rise by 2100 is 2.6 - 6.6 feet (0.8 - 2.0 meters). These sea level rise possibilities make it imperative that Venice construct barriers to keep the sea out. In 2003, construction began on the MOSE Project, a $5.5 billion system of inflatable gates that will rise up out of the ocean bed to block incoming storm surges from the Adriatic Sea during storm situations. The barriers will block all three entrances to Venice's lagoon, but allow ship traffic to pass through using a lock system. The gates are scheduled to be completed in 2012. In theory, the gates will protect Venice up to a maximum sea level rise of two meters (6.6 feet), which should be adequate for the remainder of the century. However, some scientists have argued that the gates will only protect Venice up to a sea level rise of one foot, and will allow high levels of toxic pollution to build up in the lagoon without the cleansing action of the tides to disperse the pollutants. A 2005 paper warned that if sea level rises 0.5 meters (1.6 feet), the gates would still allow considerable flooding of Venice, and block or delay 2/3 of all shipping traffic trying to pass through the gates.

Figure 3. Schematic drawing of the MOSE gate in the inflated (closed) position. Normally, the gate lies flat on the ocean bottom. When a storm surge threatens, the hollow top of the gate will be filled with air, and the hinged gate will rise to the surface, blocking any incoming storm surges. Image credit: Venice Water Authority.

The Venice Water Authority has complete info on the MOSE construction project.
NOVA aired a 1-hour show in 2002 on the sinking of Venice, and has some nice interactive web features to look at.

Jeff Masters


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Category 6™


Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather