Category 6™

July Atlantic hurricane outlook

By: JeffMasters, 5:31 PM GMT on June 30, 2008

The first half of July is usually a quiet period in the Atlantic for tropical cyclone formation. Since 1995, six of 13 years (46%) have had a named storm form during the first half of July. The busiest first half of July occurred in 2005, when three hurricanes formed. These included Hurricane Dennis and Hurricane Emily--the strongest hurricanes ever observed so early in the season. As seen in Figure 1, most of the early July activity occurs in the Gulf of Mexico, Western Caribbean, and Carolina waters. However, a few long-track "Cape Verdes" hurricanes begin to occur. These are spawned by tropical waves that come off the coast of Africa. Tropical waves serve as the instigators of about 85% of all major hurricanes.

Figure 1. Tracks of all tropical storms and hurricanes since 1851 that formed July 1-15. North Carolina and the Gulf of Mexico coast from the Florida Panhandle to Texas are the preferred strike locations. Oddly, the Florida Peninsula has been struck by only two storms that formed in the first half of July.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) have cooled about 0.5°C in the past month over the region we care about the most--the hurricane Main Development Region that extends from the coast of Africa to the coast of Central America, between 10° and 20° latitude (Figure 2). SSTs are now about about 0.5°C below average in this region (compare the May 29 SST anomaly image. One notable exception is the region closest to the African coast, which is about 2°C above average. The reason for the cooling over most of the tropical Atlantic is that the strength of the Bermuda-Azores High has increased since May, driving stronger trade winds. These stronger winds cause more evaporative cooling of the sea surface (just like blowing on your wet skin cools it off). In addition, levels of Saharan dust coming off the coast of Africa in June increased dramatically compared to May, which had dust levels about 30% below average. In particular, a major dust storm that began about June 20 off the coast of Africa crossed all the way to Florida by Friday last week, bringing hazy skies across the entire tropical Atlantic (Figure 3). All that dust blocked sunlight, preventing the water from heating up as much as usual. Another large dust storm was observed leaving the African coast around June 14.

Figure 2. Sea Surface Temperature (SST) departure from average for June 30, 2008. Image credit: NOAA.

Figure 3. Satellite image of the West African coast from June 21, 2008, at 14:50 GMT. A huge dust storm moved off the coast of Africa June 20, and arrived over South Florida on June 27. Image credit: NASA.

Wind shear
Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart.

Wind shear during almost all of June was above average over the tropical Atlantic, making it a very quiet month. This shear was predominately caused by the two branches of the jet stream--the polar jet, which runs along the U.S.-Canadian border, and the subtropical jet, which runs through the Caribbean to North Africa. This is very typical for June, when the jet stream is still very active and quite far south. The jet stream will gradually weaken as summer progresses, bringing lower wind shear and greater chances for tropical storm formation. Indeed, the GFS model is predicting that very low levels of wind shear will develop in the region between Africa and the Lesser Antilles Islands late this week (Figure 4). This is echoed by all the other major global forecast models, which unanimously predict that a tropical depression may form between Africa and the Lesser Antilles Islands by late this week or early next week. The latest 2-week GFS forecast keeps wind shear below average levels through mid-July.

Figure 4. GFS model wind shear forecast for Friday, July 4, 2008. Wind shear is the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude) in meters per second (multiply by two to get the approximate wind shear in knots). In most circumstances, wind shear below 12 knots (6 m/s, the lighter red colors) is conducive for tropical storm formation.

Dry air and African dust
June and July are the peak months for dust coming off the coast of Africa. Despite the fact that the Sahel region of Africa has seen three straight years of average to above-average rains, which should result in soil stabilization and fewer dust outbreaks, June 2008 had high levels of dust coming from Africa. Expect dust from Africa to be a major deterrent to any storms that try to form between Africa and the Lesser Antilles Islands in July.

Steering currents
During June 2008, the Bermuda-Azores High (Figure 5) extended farther west than usual. Its strength was about 1 mb below average, which normally would drive slower trade winds than average. This did not occur, as the trade winds were about 2 m/s (4 knots) above average over most of the tropical Atlantic in June. For the first half of July, both the GFS and ECMWF long-range models are predicting that the Bermuda-Azores High will not extend so far west, as the jet stream will bring a persistent trough of low pressure over the East Coast. This pattern will tend to recurve most tropical storms that penetrate north of the Caribbean Sea. Steering current patterns are not predictable more than about two weeks in advance, and there is no telling if this favorable steering current pattern will persist into the peak of hurricane season.

Figure 5. Sea level pressure for June 2008 (left), and average sea level pressure from climatology (the years 1979-1996). The Bermuda-Azores High extended farther west than usual in June 2008, keeping low pressure entrenched in the U.S. Midwest, leading to major flooding. Image credit: NOAA/ESRL.

Recent history suggests a 46% chance of at least one named storm occurring in the first half of July. All of the major computer models predict the possible formation of a tropical depression in the region between Africa and the Lesser Antilles late this week or early next week. Given the high degree of model unanimity, marginally favorable sea surface temperatures, favorable wind shear, and unfavorable dust levels expected in this region, I put the chances of a tropical storm forming in the region at 30% over the next two weeks. I put the odds of a tropical storm forming elsewhere--in the Caribbean, Gulf of Mexico, or off the U.S. East Coast--at 30% as well, for a combined roughly 50% chance of a first half of July named storm.

I'll have an update Wednesday morning.
Jeff Masters

Midwest flood price tag $8 billion; Extreme Weather magazine review

By: JeffMasters, 4:54 PM GMT on June 27, 2008

The American Farm Bureau, a lobbying group that represents American farmers, estimated yesterday that crop damage from the Midwest's Flood of 2008 has amounted to $7 billion. More than half of this total--$4 billion--was in Iowa. Other states taking a hit from excessive wetness and flooding were: Illinois, $1.3 billion; Missouri, $900 million; Indiana, $500 million; Nebraska $500 million; and an additional $1 billion in remaining wet states. When added to the at least $1 billion in property damage the floods wrought (including $762 million in Cedar Rapids, Iowa), the $8 billion price tag of the Midwest Flood of 2008 ranks as the second most expensive U.S. non-hurricane flooding disaster on record. America's worst flood, the Midwest Flood of 1993, caused $26.7 billion in damage (adjusted to 2007 dollars).

The damage will continue to rise in coming days, as major flooding continues along the Mississippi and Missouri Rivers. A levee broke along the Mississippi just north of St. Louis this morning, sending flood waters towards the small town of Winfield. Heavy rains in excess of five inches have hit much of northern Missouri this week (Figure 1), and NOAA's Hydrometeorological Prediction Center is forecasting a high probability of heavy rain in the region today through Saturday morning. The culprit is a slow-moving low pressure system over Minnesota, which will drag a cold front through Missouri tonight. An additional 2-4 inches of rain will fall in some areas along the front. The additional rain should keep the Mississippi and Missouri Rivers in Missouri above flood stage for an extra day or two. Currently, these rivers are expected to reach their highest crests sometime between Monday June 30 and Wednesday July 2. The forecast looks somewhat drier for the Midwest next week, thankfully. The jet stream has regularly been taking a major dip southward into the Central U.S. the past two months, putting the favored track for rainy low pressure systems over the Midwest. The jet often gets "stuck" in a high-amplitude trough-ridge pattern which causes drought in one part of the country (California in this case) and floods in another. This "stuckness" often lasts for 3 months. The current 2-week forecast from the GFS and ECMWF models predicts a continuation of the "stuck" jet stream pattern, but decreasing in amplitude and sliding more to the east. This should result in the favored storm track moving more towards the East Coast, relieving flooding in the Midwest.

Figure 1. Precipitation for the 7 days ending on Friday, June 27, at 8am EDT. Image credit:NOAA.

Review of the new magazine, Extreme Weather
A beautiful new weather magazine called Extreme Weather has hit the bookstores this month. Published by Astronomy magazine, the new magazine features some truly spectacular weather photos, including a 12-page "Weathergallery" with awesome shots of tornadoes, lightning, floods, supercells, hail, hurricane winds, and waterspouts. The first article of the magazine features the equally fantastic photos of storm chaser Warren Faidley, who also happens to be the best writer among professional storm chasers, in my opinion. Additional articles in Extreme Weather include a balanced and interesting look at the hurricanes/global warming connection, plus some quality articles on dust storms, super cell thunderstorms, lightning, and the Great Galveston Hurricane of 1900.

Extreme Weather is not yet a regular publication; the editors are gauging interest to see if they wish to make it so. I whole-heartedly encourage them to do so--this magazine rocks! You can order a copy at their website, it's $7.95.

It's quiet in the tropical Atlantic. There are no threat areas to discuss, and none of the models are forecasting tropical storm formation in the next seven days.

Jeff Masters


California fires could reach record levels in 2008

By: JeffMasters, 9:42 PM GMT on June 25, 2008

An unusually early and dangerous fire season has hit California, where at least 33 fires burning over a total of 190,000 acres are active, according to the Interagency Fire Center. The fires were sparked over the weekend when an unusually far southward-moving storm system brought numerous thunderstorms to central and northern California. Over 8,000 lightning strikes hit the region. Most of these strikes were not accompanied by rain, since a very dry atmosphere at low levels caused much of the thunderstorm rain to evaporate before reaching the surface. The lightning strikes ignited an unusual number of fires, due to exceptionally dry vegetation in California. This year, the state experienced its driest spring season (March-April-May) since record keeping began in 1895, and much of the state is in moderate to severe drought.

Figure 1. Visible satellite image from NASA's Aqua spacecraft on Monday, June 23, 2008, showing smoke from hundreds of wildfires sparked by lightning in California. The red regions show where the satellite's sensor detected fires burning. The smoke has created air pollution levels in excess of the federal standards for fine particulate matter (PM2.5) over much of California's Central Valley. Image credit: NASA.

The forecast
With the dry season only beginning, it could be a record fire year in California. Even before last weekend's lightning storms, California had already seen an unusually large number of destructive wildfires, according to CalFire--90,000 acres had burned, compared to 42,000 acres during the same period last year. It is not unusual for large portions of the state to receive no rain at all in July and August, such as occurred last year (Figure 2). The jet stream typically moves far enough north in summer that the migrating low pressure systems that bring California most of its rain only hit the northernmost portions of the state. With high fuel levels due to a century of misguided fire suppression efforts, moderate to severe drought gripping the state, no rain in sight for months to come, and an above-normal chance of warmer than average temperatures forecast this summer for the state by NOAA's Climate Prediction Center, expect a record fire season in California.

Figure 2. Observed precipitation over California during 2007. Much of the state received no rain at all during July and August, which is a common occurrence. A little bit of thunderstorm activity did make it into the easternmost portion of the state, thanks to moisture flowing north-westward from the Arizona Monsoon. However, the Sierra Mountains block this moisture from reaching the central and western portions of the state. Image credit: NOAA.

It's quiet in the tropical Atlantic. There are no threat areas to discuss, and none of the models are forecasting tropical storm formation in the next seven days. Beyond a week from now, the GFS is hinting that the region off the coast of Africa could see some development, but it is still probably too early for this too occur, despite warmer than average sea surface temperatures in the region.

Jeff Masters


Fengshen: deadliest Western Pacific storm in 17 years

By: JeffMasters, 3:19 PM GMT on June 24, 2008

Typhoon Fengshen may be the deadliest Pacific tropical cyclone since 1991. The death toll in the Philippines now stands at 598 dead or missing on land, with another 800 missing and presumed dead in the wake of the sinking of the ferry MV Princess of Stars. Fengshen (the Mandarin Chinese name for the God of Wind) made landfall over the northern Philippines Saturday, triggering rains and landslides that destroyed 34,000 buildings and damaged 53,000 more, causing an estimated $100 million in damage. According to, the Philippines' deadliest tropical cyclones were Tropical Storm Thelma of 1991 (5101 dead) and Typhoon Ike of 1984 (1363 dead). Left off the list was Tropical Depression Winnie, which killed 1404 people in the Philippines November 29-20, 2004. It appears likely that the death toll from Fengshen will exceed Winnie's, making Fengshen the deadliest Western Pacific tropical cyclone since 1991's Tropical Storm Thelma.

Figure 1.The ferry MV Princess of Stars. Image credit: Sulpicio Lines.

The ferry and the forecast
The ferry MV Princess of Stars (Figure 1), operated by Sulpicio Lines, left the capital of Manila on Friday night before the storm, headed south for the 20-hour run to Cebu. At the time, Fengshen was a Category 1 typhoon, headed due west, and was located a few hundred miles south of the ship. As the ferry began passing through the outer spiral bands of Fengshen, the storm did a sharp (and poorly forecast) turn to the north-northwest and began a burst of rapid intensification to strong Category 2 status (110 mph winds), bringing very high waves and much higher than anticipated winds to the region the ferry was traversing. The waves battered the ship to the point where the engines stalled, and the ferry lay helpless until the strongest portion of the storm, the northern eyewall, passed over the ship and sank it. Why the ferry allowed itself to get so close to the storm in the first place is a mystery.

Figure 2.Visible satellite image of Fengshen at 4:55 GMT June 21, 2008, 25 minutes after radio contact was lost with the ferry MV Princess of Stars. The ship had left Manila in the Philippines about 8 hours prior to the accident for the 20 hour trip to Cebu. It appears that the ferry ran into the north eyewall of Fengshen when it was at peak intensity, with sustained winds of 110 mph. Fengshen was headed due west when the ferry set sail (track image, lower left), then made a sudden, poorly forecast turn to the north-northwest as the ferry approached the typhoon. Image credit: NASA.

Tropical Atlantic
There are currently no threat areas in the Atlantic to discuss. One computer model, the GFS, is forecasting a disturbed area of low pressure may form the the southern Gulf of Mexico's Bay of Campeche on Sunday. The other models don't see this happening, and instead put the focus of any development on the Pacific side of Mexico early next week. At present, this seems a more reasonable forecast.

Jeff Masters

Typhoon Fengshen one of the deadliest ever for the Philippines

By: JeffMasters, 1:48 PM GMT on June 23, 2008

The year 2008 continues to be a year of the natural disaster, as the death toll from Typhoon Fengshen in the Philippines appears likely to exceed 900. At least 163 died when the Category 2 typhoon made landfall over the northern Philippines Saturday, and 807 people are missing from the ferry MV Princess of Stars, which sank during the typhoon. Only 38 survivors of the 845 people who were on the ship have been found. According to, Typhoon Fengshen will rank as at least the seventh deadliest Philippines typhoon since 1947, and may be the third deadliest by the time all the casualties are counted. The Philippines' deadliest typhoons were Typhoon Thelma of 1991 (5101 dead) and Typhoon Ike of 1984 (1363 dead).

Figure 1.Category 2 Typhoon Fengshen as it made landfall in the Philippine Islands on June 21, 2008. Image credit: Navy Research Lab, Monterey.

Fengshen (the Mandarin Chinese name for the God of Wind), is the seventh named storm of the 2008 Western Pacific season, the the second to cause major loss of life. On May 17, Tropical Storm Halong made landfall on Luzon island in the Philippines. The storm caused 58 deaths and $94 million (USD) in damage, destroying 43,365 houses and damaging 188,830. All of last year's typhoons in the Western Pacific killed about 160 people, so the 2008 typhoon season is off to a very bad start. The City University of Hong Kong is predicting a slightly above average typhoon season in 2008, with 30 named storms and 19 typhoons. An average season has 27 named storms and 17 typhoons.

Tropical Atlantic
It's quiet in the Atlantic. There are no threat areas to discuss, and none of the models are forecasting tropical storm formation in the next seven days.

Jeff Masters

Two 500-year floods in 15 years

By: JeffMasters, 2:48 PM GMT on June 19, 2008

The U.S. Geological Survey has preliminary data showing that this month's floods on four of Iowa's rivers--the Cedar, Iowa, Shell Rock, and Wapsipinicon--were 500-year floods. Back in 1993, many rivers in the Midwest also experienced 500-year floods, so the region has endured two 500-year floods in the past 15 years. How can this be? First of all a definition--a 500-year flood is an event that has only a 0.2% chance of occurring in a given year, based on available river flow data. Of course, reliable data only goes back a century at most, so designation of a 500-year flood event is somewhat subjective. Still, it seems rather improbable that two such huge floods should occur within such a short time span, raising the question of whether the floods were, in part, human-caused.

In a provocative story in the Washington Post today, it was pointed out that part of the flooding is due to the draining of wetlands for farming purposes. As nature's natural buffers against flooding are drained and filled to provide room for more farmland, run-off and flooding are bound to increase. Furthermore, as more levees are built to protect more valuable farmland and new developments, flood waters are pushed out of the former areas they were allowed to spread out in and forced into river channels behind the new levees. Even higher levees must then be constructed to hold back the increased volume of water they are asked to contain.

Climate change contributing to flooding?
The heaviest types of rains--those likely to cause flooding--have increased in recent years (see my February blog, "The future of flooding", for more detail). According to the United Nations' Intergovernmental Panel on Climate Change (IPCC) 2007 report, "The frequency of heavy precipitation events has increased over most land areas". Indeed, global warming theory has long predicted an increase in heavy precipitation events. As the climate warms, evaporation of moisture from the oceans increases, resulting in more water vapor in the air. According to the 2007 IPCC report, water vapor in the global atmosphere has increased by about 5% over the 20th century, and 4% since 1970.

Over the U.S., where we have very good precipitation records, annual average precipitation has increased 7% over the past century (Groisman et al., 2004). The same study also found a 14% increase in heavy (top 5%) and 20% increase in very heavy (top 1%) precipitation events over the U.S. in the past century. Kunkel et al. (2003) also found an increase in heavy precipitation events over the U.S. in recent decades, but noted that heavy precipitation events were nearly as frequent at the end of the 19th century and beginning of the 20th century, though the data is not as reliable back then. Thus, climate change is likely partly to blame for increased flooding in the U.S., although we cannot rule out long-term natural variations in precipitation.

Figure 1. Forecast change in precipitation and runoff for the period 2080 to 2099 compared to 1980 to 1999. The forecasts come from the A1B scenario from multiple climate models used for the Intergovernmental Panel on Climate Change (IPCC) 2007 report.

The forecast
According to a multi-model consensus of the climate models run for the Intergovernmental Panel on Climate Change (IPCC) 2007 report, precipitation and river runoff for the Mississippi River drainage basin are expected to increase only slightly by the end of this century (Figure 1). However, more of this rain is expected to fall in heavy precipitation events, the ones most likely to cause flooding. As a result, the U.S. needs to prepare for an increase in the number and severity of 100-year and 500-year flooding events in the coming century.

Kunkel, K. E., D. R. Easterling, K. Redmond, and K. Hubbard, 2003, "Temporal variations of extreme precipitation events in the United States: 1895.2000", Geophys. Res. Lett., 30(17), 1900, doi:10.1029/2003GL018052.

Groisman, P.Y., R.W. Knight, T.R. Karl, D.R. Easterling, B. Sun, and J.H. Lawrimore, 2004, "Contemporary Changes of the Hydrological Cycle over the Contiguous United States: Trends Derived from In Situ Observations," J. Hydrometeor., 5, 64.85.

It's quiet in the tropics. There are no threat areas to discuss, and none of the models are forecasting tropical storm formation in the next seven days.

Jeff Masters


Iowa flood price tag at $2 billion and growing

By: JeffMasters, 5:30 PM GMT on June 17, 2008

Preliminary damage estimates from the June 2008 Midwest flood puts agricultural damage in Iowa alone at $1.0 billion. At least another $1.0 billion in property damage has likely occurred--$762 million of that in Cedar Rapids, Iowa. The price tag is sure to grow, as many locations downstream are facing record flood heights this week. Levee overtopping is possible in at least 28 locations along the Mississippi River and its tributaries in the coming days, according to the Army Corp of Engineers. This year's flooding is one of the ten most damaging non-hurricane flood events in the U.S. since 1980, according to the list of billion dollar weather disasters maintained by the National Climatic Data Center. The damage from this year's flood will not come close to the record $26.7 billion in damage from the catastrophic 1993 flood, though.

Figure 1. Number of billion-dollar weather disasters in the U.S. since 1980. While it is possible that climate change has contributed to the increase in billion-dollar disasters since 1980, increases in population and wealth are primarily responsible. Image credit: NOAA/NCDC.

Billion dollar flooding disasters, 1980-2007
Below is a list of all billion-dollar flooding disasters not due to a hurricane or tropical storm in the U.S. between 1980 and 2008. Two damage figures are given for events prior to 2007. The first figure represents actual dollar costs at the time of the event and is not adjusted for inflation. The value in parenthesis is the costs normalized to 2007 dollars using a GNP inflation index.

Midwest flooding of 2008. At least $2 billion in damage.

Northeast Flooding June 2006. Severe flooding over portions of the northeast due to several weeks of heavy rainfall, affecting the states of NY, PA, DE, MD, NJ, and VA. Over $1.0 billion in damage/costs; at least 20 deaths reported.

Texas Flooding October-November 1998. Severe flooding in southeast Texas from 2 heavy rain events, with 10-20 inch rainfall totals; approximately $1.0 (1.1) billion damage/costs; 31 deaths.

Northern Plains Flooding April-May 1997. Severe flooding in Dakotas and Minnesota due to heavy spring snow melt; approximately $3.7 (4.1) billion damage/costs; 11 deaths.

MS and OH Valleys Flooding and Tornadoes March 1997. Tornadoes and severe flooding hit the states of AR, MO, MS, TN, IL, IN, KY, OH, and WV, with over 10 inches in 24 hours in Louisville; estimated $1.0 (1.1) billion damage/costs; 67 deaths.

West Coast Flooding December 1996-January 1997. Torrential rains (10-40 inches in 2 weeks) and snow melt produce severe flooding over portions of California, Washington, Oregon, Idaho, Nevada, and Montana; approximately $3.0 (3.4) billion damage/costs; 36 deaths.

Pacific Northwest Severe Flooding February 1996. Very heavy, persistent rains (10-30 inches) and melting snow over Oregon, Washington, Idaho, and western Montana; approximately $1.0 (1.2) billion damage/costs; 9 deaths. Special Report

Blizzard of '96 Followed by Flooding January 1996. Very heavy snowstorm (1-4 feet) over Appalachians, Mid-Atlantic, and Northeast; followed by severe flooding in parts of same area due to rain and snow melt; approximately $3.0 (3.5) billion damage/costs; 187 deaths.

Texas/Oklahoma/Louisiana/Mississippi Severe Weather and Flooding May 1995. Torrential rains, hail, and tornadoes across Texas - Oklahoma and southeast Louisiana - southern Mississippi, with Dallas and New Orleans areas (10-25 inches in 5 days) hardest hit; $5.0-$6.0 (6.5-7.1) billion damage/costs; 32 deaths.

California Flooding January-March 1995. Frequent winter storms cause 20-70 inches rainfall and periodic flooding across much of California; over $3.0 (3.6) billion damage/costs; 27 deaths.

Texas Flooding October 1994. Torrential rain (10-25 inches in 5 days) and thunderstorms cause flooding across much of southeast Texas; approximately $1.0 (1.2) billion damage/costs; 19 deaths.

Midwest Flooding Summer 1993. Severe, widespread flooding in central U.S. due to persistent heavy rains and thunderstorms; approximately $21.0 (26.7) billion damage/costs; 48 deaths.

Texas/Oklahoma/Louisiana/Arkansas Flooding May 1990. Torrential rains cause flooding along the Trinity, Red, and Arkansas Rivers in TX, OK, LA, and AR; over $1.0 (1.4) billion damage/costs; 13 deaths.

Western Storms and Flooding 1982 - Early 1983. Storms and flooding related to El Niño, especially in the states of WA, OR, CA, AZ, NV, ID, UT, and MT; approximately $1.1 (2.2) billion in damage/costs; at least 45 deaths.

Gulf States Storms and Flooding 1982 - Early 1983. Storms and flooding related to El Niño, especially in the states of TX, AR, LA, MS, AL, GA, and FL; approximately $1.1 (2.2) billion in damage/costs; at least 50 deaths.

New Orleans still vulnerable to a strong Category 2 hurricane
NOAA announced Monday that the rebuilt New Orleans levee system cannot withstand a strong Category 2 or higher hurricane without overtopping occurring. The Army Corp of Engineers has been given $7.1 billion so far to repair New Orleans' levees in the wake of the catastrophic failures experienced during Hurricane Katrina of 2005. Congress is considering giving another $5.7 billion for the effort. While the levees may get overtopped in a strong Category 2 hurricane, they should be more resistant to catastrophic breaches such as occurred in Katrina, when several flood walls completely failed. The Army Corps says that they have fixed the levees with better support so that water won't be able to scour out the base, and put better clay in them to keep them from dissolving.

It's quiet in the tropics. There are no threat areas to discuss, and none of the models are forecasting tropical storm formation in the next seven days.

Jeff Masters


Flood waters recede in Cedar Rapids

By: JeffMasters, 3:58 PM GMT on June 16, 2008

The rampaging Cedar River is falling today, after cresting at an amazing 31.1 feet Friday in the town of Cedar Rapids, Iowa. The National Weather Service said the flow on the Cedar River through Cedar Rapids peaked at 149,500 cubic feet per second Friday, more than double the previous record of 73,000 in 1961. During the historic 1993 flood, the river hit only 19.27 feet, and the record flood of 1929 hit only 20.5 feet. The 2008 flood has hit levels expected only once every 500 years. The river was at 23 feet this morning, which is down 8 feet, but still 11 feet above flood stage, and 2.5 feet above the record high observed in 1929.

Figure 1. Total rainfall for the period May 16 - June 16, 2008. About 2/3 of the state has seen rainfall amounts in excess of 10 inches in the past month. Image credit: NOAA.

Eighth warmest May on record
May 2008 was the 8th warmest May for the the globe on record, according to statistics released by the National Climatic Data Center. The spring season--March, April, and May--ranked as the seventh warmest spring for the globe. La Niña continued to weaken in May, and near neutral conditions now prevail in the tropical Eastern Pacific.

For the contiguous U.S., May was the 34th coolest May since 1895, and spring season was the 36th coolest spring on record. For the spring, Missouri had its fourth wettest, Arkansas its sixth wettest, Indiana and Iowa their eighth wettest and Illinois its 10th wettest. California had its driest spring on record, while Nevada and Utah had their 10th and 11th driest on record.

Sea ice extent
May 2008 Northern Hemisphere sea ice extent was the tenth lowest on record for the month of May, 6% below its extent in 1979 when satellite measurements began, according to the National Snow and Ice Data Center. May was the sixth straight month that a new monthly minimum arctic sea ice record was not set, following a string of five months in a row where monthly records were set.

It's quiet in the tropics. There are no threat areas to discuss, and none of the models are forecasting tropical storm formation in the next seven days.

Jeff Masters


Historic flooding hits Iowa

By: JeffMasters, 5:07 PM GMT on June 13, 2008

Truly extraordinary flooding has hit the Cedar River in the town of Cedar Rapids, Iowa. Rainfall amounts in excess of 12 inches have fallen in the past ten days over the Cedar River watershed, which extends to the northwest of Cedar Rapids into southern Minnesota. The rains that fell during the weekend of June 7-8 were fueled in part by moisture from Tropical Storm Alma/Arthur, which affected Central America May 29 - June 2. The Cedar River is expected to crest today at 31.8 feet, which is an amazing 19.8 feet above the flood stage of 12 feet. During the historic 1993 flood, the worst in recent history, the river hit only 19.27 feet, 7 feet above flood stage. Nine rivers in Iowa are at all-time record flood levels, and Iowa Governor Chet Culver has declared 83 of the state's 99 counties state disaster areas. Additional heavy rainfall is not expected over the Cedar River watershed over the next two days, so today should mark the peak of this year's historic flooding.

Figure 1. Total rainfall for the period June 2 - June 12, 2008, as estimated by NASA's TRMM satellite. An additional three inches fell over portions of eastern Iowa in the 24 hours since this image was created.

New way to track river flooding on wunderground
Wunderground has added a way to track local river forecast levels and assess flood risk at Using data from the USGS (the U.S. Geological Survey), the product plots river data and forecasts on top of our interactive "WunderMap". Users can scroll across the country and zoom in and out to view in-depth observations from all major rivers in the U.S. Alternatively, one can click an option to view only the rivers with current flood alerts. Each river observation is color coordinated to reflect its dry/wet percentile and users can click on each observation point to view data and graphs that display Flow Rate, Percentile, Current Stage, Forecast Stage and Flood Stage. Flood Alert symbols will appear on every river icon whenever a river is in danger of flooding.

The WunderMap™ also allows one to choose from a variety of layers including current conditions, animated radar, severe weather and tornado warnings, live webcam images and animated infrared or visible satellite imagery showing cloud coverage. WunderMap™ is available as a link on every U.S. forecast page, just under the small radar image.

Tornado outbreak update
The tornado that stuck the Little Sioux Scout Ranch Boy Scout camp in western Iowa Wednesday night, killing 4 and injuring 48, was rated an EF-3 on the Enhanced Fujita scale. The tornado that hit Manhattan, Kansas the same day, causing major damage at Kansas State University, has been rated an EF-4. This is the tenth violent (EF-4 or EF-5) tornado this year, the most number of violent tornadoes since 13 were observed in 1999.

The Storm Prediction Center is calling for a "Slight" risk of severe weather across the Midwest today, from Michigan to Oklahoma. We can expect a few more tornadoes today in the affected region, although the primary severe weather threat will be damaging thunderstorm winds and large hail. The "Slight" risk of severe weather continues Saturday and Sunday across a large portion of the Midwest.

It's quiet in the tropics. There is some disorganized thunderstorm activity in the southern Gulf of Mexico associated with a surface trough of low pressure, but this activity is not likely to develop. None of the models are forecasting tropical storm formation in the next seven days.

Jeff Masters


Tornado kills four Boy Scouts in Iowa

By: JeffMasters, 1:10 PM GMT on June 12, 2008

Tragedy struck a Boy Scout camp in western Iowa last night, when a tornado swept through, killing four Boy Scouts and injuring at least 48 people. A tornado warning was issued by the National Weather Service 12 minutes before the tornado hit the camp at 6:35 pm CDT, but it appears the campers never heard the sirens, either because the sirens were too far away from the remote camp, or because the storm caused a power outage in the nearest town of Blencoe, which silenced that town's tornado siren.

Tornadoes also hit southern Minnesota, eastern Kansas, and eastern Nebraska yesterday, and the Storm Prediction Center recorded 52 tornado reports. A tornado caused major damage in Manhattan and Kansas State University, tossing cars and destroying several businesses. A half-mile wide tornado hit the town of Chapman, destroying 60 homes and killing one person. Another person died in a mobile home in the Jackson County town of Soldier. Yesterday's deaths bring this season's tornado death toll up to 118--the most since 1998, when 130 people were killed.

Figure 1. Radar reflectivity (top) and Doppler velocity (bottom) for the June 1, 2008 tornado that hit the Little Sioux Scout Ranch in western Iowa. The tornado was embedded in a line of severe thunderstorms that swept through the state, and did not exhibit the classic hook-shaped echo one commonly sees in tornadoes. The most dangerous tornadoes commonly show a hook echo and tend to be spawned by "discrete supercells"--isolated thunderstorms that are not embedded in a solid line of thunderstorms.

Tornado activity forecast
The Storm Prediction Center is calling for a "Slight" risk of severe weather across the Midwest today, from Michigan to Oklahoma. We can expect a few more tornadoes today in the affected region, although probably not as many twisters as were reported in yesterday's outbreak. The "Slight" risk of severe weather continues Friday across the Midwest, then shifts to the East Coast by Saturday.

An area of disturbed weather has developed in the southern Gulf of Mexico between the Florida Keys and Mexico's Yucatan Peninsula this morning. The thunderstorm activity is under about 20 knots of wind shear, and is moving northward into the central Gulf of Mexico. I'm not expecting this to develop, but we should keep an eye on it. None of the computer models are forecasting development of a tropical depression in the Atlantic in the coming seven days.

Jeff Masters


Invest 91 not expected to develop; wild June weather continues

By: JeffMasters, 4:21 PM GMT on June 11, 2008

A tropical wave approaching the coast of South America near the southernmost Lesser Antilles Islands has developed some spin and a bit a heavy thunderstorm activity. This disturbance has been labeled Invest 91 by NHC this morning, but does not appear to be a threat to develop into a tropical depression. Water temperatures are warm enough and shear is low enough to allow some development, but dry air on the west side of the disturbance is limiting its thunderstorm activity. The disturbance should move ashore over South America by Thursday before a tropical depression can form.

Wind shear is very high over the Caribbean, and is expected to remain high for the coming week. None of the computer models are forecasting tropical development in the coming week. However, wind shear is forecast to be low along the northern coast of South America and in the waters offshore of Panama and Coast Rica. We will need to watch this area for development early next week when the moisture from Invest 91 arrives.

Wild June weather across the U.S.
It's good to be back blogging again! I had a great week of relaxing and recharging in the Bitteroot Mountains of Montana the past week. It was pretty nuts to go for a hike yesterday--35° with heavy snow and driving wind--then come back to the sweltering east with the vegetation going bonkers from all the heavy rain of the past week. An unusually pronounced kink in the jet stream is responsible, which allows cold air to spill southward over the Western U.S., while at the same time pumping plenty of hot, moist air northwards into the Eastern U.S. The latest 2-week forecast from the GFS model indicates a continuation of a more active than usual jet stream the remainder of June, so expect plenty more wild weather this month. The main action today will be in Iowa, Nebraska, and eastern Kansas, which the Storm Prediction Center has placed at "Moderate" risk of severe weather.

Jeff Masters

Tropics remain quiet, typical for June

By: JeffMasters, 3:35 PM GMT on June 09, 2008

Although we got an early start to the 2008 hurricane season, things have since slowed down and returned to normal. I spent quite some time this morning scouring through the models and satellite images looking for some signs of tropical development, but none was to be found. The only action in the Atlantic is a weak upper level low moving from the Bahamas across South Florida today. Conditions are not favorable for development as the low enters the Gulf of Mexico.

Atlantic visible satellite

It is still very early in the season. Usually we wouldn't have seen a single storm by this point. Considering that the official long term 'forecasts' all called for climatology, this season remains unremarkable. We're still a couple months away from things getting very exciting.

Hurricane frequency by date (Credit: NOAA)

If you're looking for tropical development in the Atlantic at this time of year, your best bet is looking in the southern Gulf and western Caribbean. It is still very early to see development in the Atlantic proper. Wind shear is still strong at higher latitudes and the central Atlantic has yet to warm to temperatures conducive for tropical development.

June hurricane genesis index and locations (Emanuel and Nolan 2004)

Elsewhere in the country, the severe weather situation seems to have quieted down from earlier in the season. While there are still active areas associated with frontal boundaries draped across the eastern half of the continent, they are significantly less active than earlier this season. Usually by June the severe weather season shifts to the High Plains. Those storms are associated with potential vorticity anomalies advecting off of the Rockies. Below you will find a map of yesterday's severe reports. A similar, though slightly advected pattern is expected today.

Yesterday's severe weather reports from SPC

Bryan Woods, filling in for Jeff Masters

Jet stream moved northwards 270 miles in 22 years; climate change to blame?

By: JeffMasters, 4:11 PM GMT on June 05, 2008

Climate change is forcing the jet stream higher and closer to the pole in both the Northern and Southern Hemisphere, according research published this April in the journal Geophysical Research Letters. In their paper, "Historical trends in the jet streams", researchers Cristina Archer and Ken Caldeira of Stanford's Carnegie Institution of Washington analyzed data from 1979-2001, and found that the Northern Hemisphere jet stream moved northward at approximately 125 miles per decade (270 miles during the 22-year period of the study). The jet moved higher by 5-23 meters during this period, and the wind speeds decreased by about 1 mph. Archer and Caldeira's study confirms other research showing a poleward movement of the jet stream in recent decades (Fu et al., 2006; Hu and Fu, 2007). All of these changes are consistent with the behavior of the jet stream predicted by global warming theory. For example, Lorenz and DeWeaver (2007) found poleward shifts of the jet stream by 2100 in the forecasts of 15 climate models used to formulate the "official" word on climate, the 2007 Intergovernmental Panel for Climate Change (IPCC) report. However, the authors were careful not to say how much of this shift in the jet stream was due to natural causes versus human-caused climate change. It is unknown if the jet stream has natural decades-long changes in its path that could account for the observed poleward shift.

Figure 1. The jet stream is located where the strongest winds at the top of the troposphere are found (35,000-45,000 feet high, 200-300 mb in pressure).

Archer and Caldeira note that "These changes in jet stream latitude, altitude, and strength have likely affected, and perhaps will continue to affect, the formation and evolution of storms in the mid-latitudes and of hurricanes in the sub-tropical regions." They don't specify what these changes might be. There is very little research that has been done suggesting how changes in the jet stream might affect hurricane formation and strength. One effect we may begin to see in coming decades is a reduction and/or delay in the number of hurricanes that recurve northward out to sea. Recurvature occurs when a hurricane begins to "feel" the westerly winds of the jet stream. As the jet stream continues to move northward and weaken as the globe warms, we can expect that hurricanes moving though the Caribbean will be less likely to recurve, resulting in more hurricane strikes in Mexico and Central America. Unfortunately, the quality of the Atlantic hurricane database for non-U.S. landfalls is not very good, and it will be several decades before we will be able to tell if the number of hurricane landfalls in Mexico and Central America is increasing due to a poleward shift in the jet stream.

Fu, Q., C. M. Johanson, J. M. Wallace, and T. Reichler (2006), Enhanced mid-latitude tropospheric warming in satellite measurements, Science, 312, 1179, doi:10.1126/science.1125566.

Hu, Y., and Q. Fu (2007), Observed poleward expansion of the Hadley circulation since 1979, Atmos. Chem. Phys. Disc., 7, 9367.9384.

Lorenz, D. J., and E. T. DeWeaver (2007), Tropopause height and zonal wind response to global warming in the IPCC scenario integrations, J. Geophys. Res., 112, D10119, doi:10.1029/2006JD008087.

Jeff Masters

Climate Change

Above average hurricane season expected by Dr. Bill Gray's team

By: JeffMasters, 6:23 PM GMT on June 03, 2008

An above average Atlantic hurricane season is on tap for 2008, according to today's seasonal forecast issued by Dr. Bill Gray and Phil Klotzbach of Colorado State University (CSU). The Gray/Klotzbach team is calling for 15 named storms, 8 hurricanes, 4 intense hurricanes, and an Accumulated Cyclone Energy (ACE) index 150% of normal--unchanged from their April forecast. These numbers are also the what the Atlantic has averaged since we entered a period of above-average hurricane activity in 1995. An average season has 10-11 named storms, 6 hurricanes, and 2 intense hurricanes. The Klotzbach/Gray forecast calls for an above normal chance of a major hurricane hitting the U.S., both along the East Coast (45% chance, 31% chance is normal) and the Gulf Coast (44% chance, 30% chance is average). The Caribbean is also forecast to have an above average risk of a major hurricane.

The forecasters cite the continuation of the above-normal hurricane activity period that began in 1995, the expected lack of an El Niño event, the continuation of above average sea surface temperatures and below average pressure in the eastern Atlantic, and slower trade winds (which result in reduced evaporative cooling of the ocean), as the justification for their forecast of an above average hurricane season. It's of interest to look at the activity maps for the four analogue years that had similar atmospheric and oceanic conditions in May: 1951, 1961, 2000, and 2001. The 1961 season was the nastiest of these four seasons, with two Category 5 hurricanes. One of those storms, the notorious Hurricane Carla, hit Texas as a mighty Category 4 hurricane with a 22-foot storm surge. The other three seasons had no hurricanes that hit the U.S.

Figure 1. Sea Surface Temperature departure from average from May 29, 2008. Caribbean SSTs are near average, but are much above average in the Eastern Atlantic. Image credit: NOAA.

How good are the CSU forecasts?
The CSU forecast team has been making seasonal hurricane forecasts since 1984. Based on predictions of a below average, average, or above average season, they have done pretty well over the past nine seasons. Eight of the past nine forecasts have been correct. Their only failure occurred in 2006, when they called for a very active season, and it was a normal year with 10 named storms, 5 hurricanes, and 2 intense hurricanes. A more rigorous way of determining forecast skill is to compute the mathematical correlation coefficient. A correlation coefficient of 1.0 is a perfect forecast, and 0.0 is a no-skill forecast. The late May CSU forecasts have a respectable correlation coefficient of 0.57 for predicting the number of named storms (1984-2006). This decreases a bit to 0.46 and 0.42 for number of hurricanes and intense hurricanes, respectively. These are respectable correlation coefficients, and the late May/early June CSU forecasts are worth paying attention to. This is in contrast to the December and April CSU forecasts, which have had a correlation coefficient near zero (and thus no skill).

Other forecasts
The British private forecasting firm Tropical Storm Risk, Inc. (TSR), issues monthly 2008 Atlantic hurricane season forecasts. Their June 2 forecast calls for a 53% chance of an above average season. This compares to the 65% chance of an above average season given by NOAA in their May 22 forecast.

Last blog entry for a week
This will be my final blog entry until June 11. With the tropics looking quiet for the next week, it's a good time to do some hiking in SW Montana. In my absence, I've arranged for a few "canned" blogs of mine to be posted. If there's anything stirring in the tropics, I've arranged for a guest blogger, Bryan Woods, to jump in. Bryan has done a great job over the past three years blogging on the tropics over at Here's Bryan's bio:

Bryan received his BS in Meteorology from the University of Massachusetts in Lowell, MA in 2005, and his M.Phil. in Geology & Geophysics from Yale University in New Haven, CT in 2007. Bryan is currently a Ph.D. candidate at Yale where he is also the graduate and professional student body president.

Bryan has spent two summers working on a National Science Foundation (NSF) sponsored field micrometeorological research project in Atlanta, GA, studying evapotranspiration rates in urban forest canopies. Currently, Bryan's research is focused on combining wavelet techniques and aircraft data from the NSF/NCAR Gulfstream V to diagnose energy and momentum fluxes from atmospheric gravity waves. Bryan has spent the past three hurricane seasons writing blogs on the tropics for

Jeff Masters

Arthur claims the season's first victims

By: JeffMasters, 12:28 AM GMT on June 03, 2008

Reports coming in from Belize indicate that Tropical Storm Arthur hit that country hard. Rainfall totals of about six inches were measured at the airport, and up to 11 inches in the Corozal Town area. The resulting flooding has claimed the first lives of the 2008 Atlantic hurricane season--at least four people are confirmed dead, and another two are missing. Arthur's rains forced some rivers higher than was observed during devastating Hurricane Mitch of 1998. According to one government official, "We have been accustomed to flooding but no one expected a flood of this magnitude. Certainly we had no warning because these floods are beyond anything we have experienced in the last thirty to forty years and so that is one of the quirks of nature." If the minister quoted is correct, Arthur's flooding is the worst since Category 4 Hurricane Hattie hit the country in 1961, killing at least 275 people.

Figure 1. Upper left: An aerial view of the flooding in Dangriga Town. Upper right: Section of the Hummingbird Highway near Middlesex Village in Southern Belize washed away, making the roadway impassable.
Lower left: Kendall Bridge, Southern Belize, Washed Away by Flooding Waters. Lower right: Flooding in Sarawee Village, Southern Belize has left many homes under water. Image credit: Belize National Emergency Management Organization (NEMO).

The heavy rains from Arthur ended at about 10am EDT Monday June 2. One resident of Ambergris Caye wrote me to say she thought a supercell thunderstorm hit her island at 4:30am June 2, during the height of the rain. She heard the roar one hears from a tornado, she thought. Given the prodigious amount of rain that fell from the very intense thunderstorms over Belize at that time, I wouldn't be surprised if a supercell thunderstorm with a tornado did hit the island.

There has been no heavy rain over Belize for the 24 hours since Monday morning, but additional rains of 1-2 inches could fall over the next day or so, in association with the remains of Arthur.

Satellite estimated rainfall for the 24 hours ending at 8am EDT 6/02/08. The red bullseye marks where up to 11 inches of rain fell in just 24 hours, triggering serious flooding. Image credit: Navy Research Lab, Monterey.

Jeff Masters

June hurricane season outlook and Arthur/Alma post-mortem

By: JeffMasters, 2:35 PM GMT on June 02, 2008

The season's first tropical storm, Arthur, has come and gone. Arthur formed Saturday afternoon--one day before the official start of hurricane season--and immediately made landfall in northern Belize on the Yucatan Peninsula. Arthur brought heavy rain to Belize, southeast Mexico, Guatemala, and Honduras. Resulting flooding in Belize has killed at least two people, left five missing, and flooded many homes and businesses. Rainfall totaled 171mm (about 7 inches) at Tikal International Airport in Northern Guatemala, and satellite rainfall estimates suggest as much as 10 inches of rain may have fallen in some isolated areas of southeast Mexico. Arthur's remains will continue to soak the region with up to six more inches of rain in the coming two days. No computer models are suggesting that Arthur's center will drift over any ocean areas and rise from the dead again. However, a new low pressure area (91E) has developed in the Eastern Pacific off the southeast coast of Mexico, just south of Arthur's remains. Moisture from Arthur/Alma may fuel the development of a third tropical storm--Boris--which could form Tuesday or Wednesday, and move northwestward into Mexico.

Before Arthur was a he, he was a she--Tropical Storm Alma in the Eastern Pacific. Alma soaked Costa Rica and Nicaragua with up to ten inches of rain, damaging or blocking 117 roads and destroying a number homes in Costa Rica, where an estimated 1,500 people are homeless. In Nicaragua, three people died, ten are missing, and 25,000 people are homeless in the wake of the storm.

June Atlantic hurricane season outlook
June is typically the quietest month of the Atlantic hurricane season. On average, we see only one named storm every two years in June. Only one major hurricane has made landfall in June--Category 4 Hurricane Audrey of 1957, which struck the Texas/Louisiana border area on June 27 of that year, killing 550. The highest number of named storms for the month is three, which occurred in 1936 and 1968. In the 13 years since the current active hurricane period began in 1995, there have been ten June named storms (not including this year's Arthur). Five tropical storms have formed in the first half of June in that 13-year period, giving a historical 38% chance of a first-half-of-June named storm.

Figure 1. Tracks of all June tropical storms and hurricanes since 1851.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) are still quite cool in June, which limits the regions where tropical storm formation can occur. SSTs are typically too cold to allow storms to develop between the coast of Africa and the Lesser Antilles Islands, and there has only been once such development in the historical record (Figure 1). This year (Figure 2), SSTs are about 2°C above average off the coast of Africa, which has led to some unusually vigorous tropical waves for this time of year. SSTs near the Cape Verde Islands are about 25°C, and this will need to increase to at least 26°C before we need to be concerned about African tropical waves developing.

Typically, June storms only form over the Gulf of Mexico, Western Caribbean, and Gulf Stream waters just offshore Florida, where water temperatures are warmest. SSTs are 26°C-28°C, which is 0.5°C above average over most of this region. June storms typically form when a cold front moves off the U.S. coast and stalls out, with the old frontal boundary serving as a focal point for development of a tropical disturbance. African tropical waves, which serve as the instigators of about 85% of all major hurricanes, are usually too far south in June to trigger tropical storm formation. Every so often, a tropical wave coming off the coast of Africa moves far enough north to act as a seed for a June tropical storm. This was the case for Arthur this year (which also had major help from the spinning remnants of the Eastern Pacific's Tropical Storm Alma). Another way to get Atlantic June storms is for a disturbed weather area in the Eastern Pacific Intertropical Convergence Zone (ITCZ) to push north into the Western Caribbean and spawn a storm there. This was the case for Tropical Storm Alberto of 2006 (which may have also had help from an African wave).

Figure 2. Sea Surface Temperature (SST) departure from average for May 29, 2008.

Tropical Cyclone Heat Potential
It's not just the SSTs that are important for hurricanes, it's also the total amount of heat in the ocean to a depth of about 150 meters. Hurricanes stir up water from down deep due to their high winds, so a shallow layer of warm water isn't as beneficial to a hurricane as a deep one. The Tropical Cyclone Heat Potential (TCHP, Figure 3) is a measure of this total heat content. A high TCHP over 80 is very beneficial to rapid intensification. As we can see, the heat energy available in the tropical Atlantic has declined steadily since 2005, when the highest SSTs ever measured in the tropical Atlantic occurred. I expect that the TCHP will continue to remain well below 2005 levels this year, so we should not see any intense hurricanes in July, like we saw that year.

Figure 3. Tropical Cyclone Heat Potential (TCHP) for May 31 2005 (top), May 31 of last year (middle) and May 31 2008 (bottom). TCHP is a measure of the total heat energy available in the ocean. Record high values of TCHP were observed in 2005. TCHP this year is much lower. Image credit: NOAA/AOML.

Wind shear
Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart.

Wind shear over the past 11 days (Figure 4) has been unusually low over the Gulf of Mexico, Caribbean, and Central America. Th shear was 4-8 m/s (8-16 knots) below average last week, aiding the formation of Tropical Storm Alma/Arthur. The jet stream is usually very active and quite far south in June, bringing plenty of shear. The jet stream looped unusually far northwards during late May, but is forecast to return to a more normal position over the coming two weeks, increasing the shear over the June breeding grounds for tropical storms. The jet stream will gradually weaken and retreat northwards as summer progresses, bringing lower wind shear and greater chances for tropical storm formation.

Figure 4. Top: Average wind shear over the 11 days ending on May 30. Wind shear is the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude) in meters per second (multiply by two to get the approximate wind shear in knots). In most circumstances, wind shear above 20 knots (10 m/s, the blue colors in the top image) will act to inhibit tropical storm formation. Wind shear below 12 knots (6 m/s, the orange colors) is very conducive for tropical storm formation. Bottom: Departure of wind shear from average for the past 11 days in meters per second. Note the unusually low wind shear area near Central America where Alma/Arthur developed. Image credit: NOAA/CPC.

Dry air and African dust
It's too early to concern ourselves with dry air and dust coming off the coast of Africa, since these dust outbreaks don't make it all the way to the June tropical cyclone breeding grounds in the Western Caribbean and the Gulf of Mexico. Developing storms do have to contend with dry air from Canada moving off the U.S. coast; this was a key reason why last year's Subtropical Storm Andrea never became a tropical storm.

Steering currents
The steering current pattern over the past few weeks has typical for June, with an active jet stream bringing many troughs of low pressure off the East Coast of the U.S. These troughs are frequent enough and strong enough to recurve any tropical storms or hurricanes that might penetrate north of the Caribbean Sea. Steering current patterns are well-predictable only about 3-5 days in the future, although we can make very general forecasts about the pattern as much as two weeks in advance. At present, it appears that the coming two weeks will maintain the typical June pattern, bringing many troughs of low pressure off the East Coast capable of recurving any June storms that might form. There is no telling what might happen during the peak months of August, September, and October--we might be in for a repeat of the favorable 2006 steering current pattern that recurved every storm out to sea--or the unfavorable 2005 pattern, that steered so many hurricanes into the U.S.

Recent history suggests a 38% chance of a named storm occurring in the first half of June. Given the current two-week wind shear forecast, the odds are that Arthur will be the only tropical storm we'll see during the first half of June. Still, there will be "holes" opening up from time to time in the shear pattern, so we need to keep our eye on the Gulf of Mexico and Western Caribbean. None of the computer models are forecasting tropical storm development in the coming seven days.

I'll have an update Tuesday afternoon, when the latest Colorado State University Atlantic Hurricane season forecast by Phil Klotzbach and Dr. Bill Gray will be available.

Jeff Masters

The hurricane season of 2008 rings in with gender-confused Arthur

By: JeffMasters, 1:35 PM GMT on June 01, 2008

Buckle your seat belts, hurricane season is here! If the formation of Arthur on the day before hurricane season officially starts is any indication, we are in for a strange and unusual season. Alma, the Eastern Pacific tropical storm that hit Nicaragua Thursday, fell apart over the high mountains of Honduras. The remnants reorganized over the Western Caribbean on Saturday morning and became Arthur. Had Alma maintained her identity as a tropical depression during the crossing, she would have kept her name. As it was, Alma died, had a posthumous sex change, and became reborn as a man named Arthur. Only two tropical storms since 1949 have made the crossing from Pacific to Atlantic and maintained at least tropical depression status during the crossing:

Northeast Pacific Hurricane Cosme became Atlantic Tropical Storm Allison (June 1989).

A Northeast Pacific tropical storm (September-October 1949) became Atlantic Hurricane Storm #10 and made landfall in Texas.

Seven tropical cyclones have survived the crossing from Atlantic to Pacific. I'm not sure how many cases have occurred like Alma/Arthur, where the remnants of a tropical storm reform into a new cyclone in different ocean basin.

Did Arthur form over land?
Arthur was also unusual in that the first advisory position for the storm was inland over northern Belize, about 30 miles from the ocean. Technically, the storm probably formed while the center was just offshore or right at the coast, but NHC did not name it until the center was already inland. There is one other case of NHC issuing its first advisory on a system while it was over land--Hurricane Agnes, which became a tropical depression on June 14, 1972, while centered over the Yucatan Peninsula. Since the Yucatan is a relatively narrow strip of land with very warm ocean waters on three sides, one can form a tropical depression centered over land here in rare cases, when the large-scale atmospheric patterns are very favorable for tropical storm formation.

Figure 1. Track of Hurricane Agnes of 1972, which formed over the Yucatan Peninsula.

The future of Arthur
The primary threat from Arthur is rain. Heavy rains of up to five inches have fallen over portions of Belize and Southeast Mexico over the past 24 hours, and rainfall amounts of up to ten inches may accumulate in some regions. None of the models are bringing the center of Arthur over the Gulf of Mexico, so the storm should decay into a tropical depression later today, and then dissipate by Monday. What would really make for an odd season would be if Arthur died over Mexico, its remains drifted over the Eastern Pacific, then re-formed into Tropical Storm Boris. Some of the computer models were suggesting this yesterday, but are no longer doing so today.

Would Arthur have been named 30 years ago?
Arthur is one of those weak, short-lived tropical storms that may not have been recognized as a named storm thirty or more years ago. Arthur was named primarily based on measurements from a buoy that didn't exist 30 years ago, and from measurements from the QuikSCAT satellite, which didn't exist until 1999. There was one ship report that was used, though, and ship reports were heavily relied upon in the old days to name tropical storms.

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