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Meteorology of Saturday's Colombian Flood Disaster That Killed 254

By: Jeff Masters and Lee Grenci , 4:07 PM GMT on April 03, 2017

At least 254 people were killed in the in the city of Mocoa (population 40,000) in southwest Colombia near the border of Ecuador early Saturday, when torrential rains triggered a debris flow on a nearby mountain that surged into the town as a huge wall of water carrying tons of mud and debris. The disaster is the fourth deadliest weather-related disaster in Colombia’s recorded history. Reports from Colombia indicate that 130 mm (5.1”) of rain fell during a short period on Friday night and early Saturday morning, with the heaviest of the rain falling in just two hours, between 23:00 Friday, 31 March and 01:00 Saturday, 01 April, 2017. The rains fell on soils that were already wet from unusually heavy rains during March; the Mocoa region received about 50 percent more precipitation than usual during the month of March. The heavy rains of Saturday morning triggered a debris flow down the Taruca ravine on the northwest side of Mocoa, and this landslide, accompanied by floodwaters, poured into the Sangoyaco River and rampaged through the city of Mocoa. According to a USA Today interview with Jonathan Godt, coordinator of the U.S. Geological Survey’s landslide hazards program, “That mixture can move at 35-40 miles an hour, and because it’s so dense it has a lot more momentum and destructive power than water alone.”

Figure 1. Mocao, Colombia after the Saturday, April 1, 2017 flood.  A video from early Saturday morning showed cars and trucks being swept down the streets. Image credit:  Ejército Nacional de Colombia via Facebook.

Contributing causes to the disaster
The fundamental cause of the disaster was that the city of Mocoa was situated in a vulnerable location—in a valley surrounded by steep slopes, close to the Mocoa, Mulato, and Sancoyaco rivers. Deforestation on the surrounding slopes may have contributed to the landslide and flood. President Juan Manuel Santos blamed climate change for triggering the flood, and he has a point—increased evaporation from warming oceans have caused a significant rise in atmospheric water vapor and very heavy rainfall events like the Mocoa event in recent decades. The Mocoa rains were triggered by a very moist flow of air from the tropical Atlantic, where ocean temperatures were near average (see the meteorological analysis below.) The rainy season in Colombia extends from March to mid June, so additional floods and landslides can be expected the next two months.

Figure 2. Aerial view of Mocao, Colombia after the Saturday, April 1, 2017 flood.  Image credit: Cesar Carrion/Colombian Presidential Office via AP.

Colombia’s history of weather disasters
According to EM-DAT, the international disaster database, Colombia’s most expensive and second deadliest weather-related disaster occurred in 2010 - 2011, when almost non-stop heavy rains caused three separate billion-dollar flooding events, killing 418 people. EM-DAT lists one other flood that killed more than 200 people in Colombia: a December 1971 flood in Magdalena and Cauca Valsfive that killed 307. EM-DAT also lists five landslides in Colombia’s history that have killed at least 200 people:

640 killed in Villatina on Sep 27, 1987
300 killed in Quebrada Blanca on June 28, 1974
200 killed in Bogota on June 28, 1973
200 killed on June 21, 1986
200 killed in December 1971

Meteorology of the disaster
Wunderblogger Lee Grenci looked in detail at the meteorology of the disaster, and what follows is his analysis.

Below is the animation of enhanced satellite images from GOES-13 during the period, 0045 UTC to 0715 UTC April 1 (nighttime in Colombia). You can see a mesoscale convective system (MCS) . This may more properly have been called a mesoscale convective complex (MCC), given the symmetry of the cirrus canopy toward the end of the loop--but the duration of the large, circular cirrus canopy did not last very long, however, and must last at least 6 hours to be labeled a MCC. Needless to say, the MCS was slow-moving, if not stationary, for several hours. The cloud-top temperatures at the peak of deep, moist convection were as low as -90 degrees Celsius, indicative of heavy thunderstorms quite capable of producing very heavy rain.

Figure 3. The animation of enhanced infrared images from GOES-13 during the period, 0045 UTC to 0715 UTC on April 1, 2017. Courtesy of Penn State.

Figure 4. A relief map of Colombia, Courtesy of Wikipedia. The disaster occurred in the town of Mocoa, which lies in southwest Colombia in the department of Putumayo at an elevation of nearly 2000 feet.

Given the quasi-stationary nature of the MCS, it’s pretty clear that the initiation of heavy thunderstorms was orographic in nature. Check out (below) the GFS model analysis of 700-mb streamlines (about 10,000 feet) at 00 UTC on April 2. Please note that I’m using 700 mb as a proxy for the lower troposphere, given the high elevations in this region (mountains to the west of Mocoa are as high as 15,000 feet). The analysis shows a strong easterly flow from the tropical Atlantic that moved upslope over the high terrain, paving the way for heavy thunderstorms. The rapid, extreme run-off from the lofty mountains set the stage for devastating flooding and mudslides.

Figure 5. The GFS model analysis of 700-mb streamlines at 00 UTC on April 2, 2017. Courtesy of Penn State.

Figure 6. The GFS model analysis of 850-mb streamlines at 00 UTC on April 2, 2017. Courtesy of Penn State. Lower-level winds come into South America from the tropical Atlantic and Caribbean.

Moisture from the Caribbean, where ocean temperatures were up to 1°C (1.8°F) above average might have played a role in the MCS, but I believe the mountains to the north probably blocked some of this low-level moisture. To support my claim, take a look at the GFS analysis of precipitable water (PWAT, Figure 7). In my opinion, the broad east-to-west band of high PWAT, with an embedded area of 2.5+ inches, was aimed (given the prevailing easterly flow) directly at the location of the MCS.

Figure 7. The GFS model analysis of precipitable water (PWAT expressed in inches) at 00 UTC on April 2, 2017. Larger image. Courtesy of Penn State.

Final blog post in old software system
This post is scheduled to be the final one in the old software system; the new “Category 6” will use the Drupal software package, and commenting will be via the Disqus software package. The current plan is to leave the current blog archive up for two more weeks, after which time a full archive of all the old blogs will be made available, along with the first 500 comments in each post (no graphics or videos posted to the comments will be saved, though.) WU member Patrap figured out that the Internet Archive's “Wayback Machine" has backups of the blogs; for example, one can access old posts by changing the “entrynum=“ portion of the URL below:

Our FAQ page has more detailed info on how this transition will occur. Blog commenting is not going away! Check back later on Monday for the first "new" Cat 6 post. 

Jeff Masters and Lee Grenci


Significant Tornado Threat in TX/LA on Sunday

By: Bob Henson , 2:00 PM GMT on April 02, 2017

A worrisome juxtaposition of very high instability and extreme wind shear will fall into place across a large swath of eastern Texas and northern Louisiana on Sunday, setting the stage for a potential round of multiple strong tornadoes. Update (12:30 pm CDT]: The NOAA/NWS Storm Prediction Center has placed the region under a high risk of severe weather (its highest risk category), with a chance of significant tornadoes (EF2 or stronger).

Figure 1. The Day 1 convective outlook from NOAA/SPC issued at 11:27 am CDT Sunday morning.

Figure 2. The probability of a tornado within 25 miles of a given point on Sunday, as estimated by NOAA/SPC in its 11:27 a.m. CDT outlook. The crosshatched area indicates where significant tornadoes (EF2 or stronger) are most likely to occur. Image credit: NOAA/NWS/SPC.

The air mass ahead of a cold front moving across central Texas is expected to be quite volatile by midday Sunday, with very rich moisture flowing north from the Gulf of Mexico beneath seasonably cold air aloft. Instability may vault into or above the range of 2000 - 3000 joules per kilogram, which is more than enough for significant severe weather.

Especially worrisome is the fact that winds in the warm sector over southeast Texas and western Louisiana will be from the southeast but will veer quickly to the south and southwest with altitude, while strengthening markedly. Models suggest that the resulting wind profiles will have a classic sickle shape favorable for storm rotation. Because of the especially deep moisture, cloud bases will be quite low. This will add to the tornado potential (because of the added buoyancy from water vapor condensing within the strongly sheared zone), while also raising the risk that tornadoes will be harder to see.

Figure 3. WU composite of NEXRAD radar as of 8:20 am CDT Sunday, April 2, 2017.

Today’s two main storm modes
A powerful mesoscale convective system (MCS) was already plowing toward the Austin/San Antonio region at 8 am CDT Sunday (see Figure 3 above). This MCS is expected to continue intensifying as it expands north and east through the day, with a risk of embedded tornadoes as well as very strong downbursts (straight-line winds). The HRRR mesoscale model suggests that this MCS could make it through northern Louisiana and southeast Arkansas and on into into northern Mississippi by Sunday night. By late tonight, a solid line of intense storms may extend from the MCS all the way to the Gulf Coast of Louisiana.

Ahead of this MCS, there will be plenty of other thunderstorms over eastern TX and much of Louisiana, likely including the Houston metro area. Any of the strongest storms that can remain more isolated have the potential to be more supercellular, with the highest tornado threat. Tornadic supercells may erupt on the early side for a spring outbreak—perhaps before noon CDT.

A particular zone of concern highlighted by the high-risk area will be near a warm front expected to lie from west to east across east-central Texas and into central Louisiana. In this zone, the low-level winds will have a strong easterly component, maximizing the vertical wind shear and the potential for strong tornadoes. Even though Houston and New Orleans will be south of the warm front, residents should keep on alert as they are not free and clear of Sunday’s tornado threat. The most volatile conditions should remain southeast of the Dallas-Fort Worth area, although DFW could see heavy rain on the north side of the evolving MCS. Very heavy rain will also plaster most of Louisiana; flash flood watches covered the entire state on Sunday morning.

Figure 4. The 12Z Sunday run of the HRRR mesoscale model projected that the significant tornado parameter (STP) would be at very high values, in the range of 4 to 8, over parts of southeast Texas and southwest Louisiana at 19Z (2:00 pm CDT) Sunday, April 2, 2017. The STP values are shown atop projected surface wind plots. The highest STP values can be seen near the warm front, where surface winds have the strongest east-to-west component. STP values indicate where large-scale conditions are supportive for significant tornadoes within a supercell, not where supercells or tornadoes will actually form. Image credit: College of DuPage.

Model-generated indices of severe weather point to the serious nature of Sunday’s set-up. The significant tornado parameter (STP)—a composite index that estimates the likelihood of EF2 - EF5 tornadoes by taking into account instability, wind shear, and how much storms are “capped” by a warm layer aloft—could reach unusually high values of 5 or more in places where the air mass is unaffected by ongoing storms. Most tornadoes rated EF2 or stronger occur where the STP is at least 1, and a study of more than 1000 such U.S. tornadoes found that the average STP value at the time of the tornadoes was 2.2.

Severe weather is expected to continue through the night and on into Monday, as the system quickly progresses into the southeast U.S. If the atmosphere is not heavily suppressed by leftover clouds and rains from overnight storms on Sunday night, a new batch of supercells—potentially including tornadoes—may develop from south Alabama into north Georgia, possibly affecting the Atlanta area. NOAA/SPC has parts of this region under an enhanced risk for severe weather in its Day 2 outlook, with southern Alabama upgraded to a moderate risk for Monday in an update issued at 12:30 pm CDT Sunday.

Figure 5. Top: Damage from the F4 tornado that tore through Paris, TX, on April 2, 1982. Bottom: Approximate track of the Paris tornado (based only on starting and ending points). Top image credit: SPC/Wikimedia Commons. Bottom image credit: NOAA/NCEI.

A somber anniversary: 35 years since the Paris tornado and the first-ever PDS watch
For longtime residents of northeast Texas, Sunday’s tornado threat is bound to bring up memories of a tragic day 35 years ago. On the afternoon of April 2, 1982, the region was hammered by a violent tornadic supercell that plowed through the small city of Paris, killing 10 people and damaging or destroying some 3000 homes. The F4 tornado moved through the heart of Paris (see Figure 5), and the parent supercell spun off other twisters as it charged eastward.

Hailstones up to a phenomenal 6” in diameter fell less than an hour before the Paris tornado struck. A news clip archived on YouTube vividly conveys the aftermath in Paris. Meteorologist Roger Edwards, then a teenager, watched the storm unfolding well to his northeast after he bicycled to a vantage point near his home in Dallas. The NWS office in Fort Worth has a detailed webpage on the event.

The Paris supercell was the “tail-end Charlie” of a devasting line of tornado-spawning storms associated with a very intense upper-level trough and surface low moving through the central U.S. On April 3, 1982, Wisconsin set a state low-pressure record of 28.45” at Green Bay (since eclipsed by another storm in 2010).

Early on April 2, 1982, the NOAA/NWS Storm Prediction Center—then known as the National Severe Storms Forecast Center—issued one of the first convective outlooks (perhaps the very first) to include a “high risk” designation. Later that day, forecaster Bob Johns issued the first-ever tornado watch to be identified as a “particularly dangerous situation” (PDS). This strong wording was borne out by events to come, as 81 preliminary tornado reports were logged from Texas to Ohio in a 25-hour span on April 2-3, 1982. A total of 77 tornadoes were confirmed, with 29 fatalities and 170 injuries.

Bob Henson

Severe Weather Tornado

Meet the New Director of the National Hurricane Center, Dr. Gail Spinner

By: Jeff Masters , 1:31 PM GMT on April 01, 2017

National Hurricane Center (NHC) director Dr. Rick Knabb announced last week that he was leaving NHC to take a position as The Weather Channel’s chief hurricane expert this summer. The excellent Dr. Knabb served ably for five years as NHC director, and will be missed. His successor has already been named—it will be Dr. Gail Spinner, the first-ever female head of the agency. Dr. Spinner comes from a distinguished career at the National Science Foundation-funded University Program for Hurricane Observations and Research In the Atlantic (UPHORIA), where she served a the lead scientist in their Boulder, Colorado laboratory. Her background is a colorful one; before earning her Ph.D. in Tropical Meteorology at the University of Alaska-Fairbanks, Dr. Spinner had a 4-year career as a professional ice skater for Stars on Ice. She also appeared last year on “Dancing With the Stars”, where former Texas Governor and current Secretary of Energy Rick Perry was her dancing partner (“good thing I wasn’t wearing ice skates for that”, she confided in an interview, “or I would have carved up his klutzy feet big time!”)

Figure 1. Outgoing National Hurricane Center director Rick Knabb speaks during a televised forecast regarding the threat of Hurricane Matthew in Miami. (AP Photo/Wilfredo Lee)

Figure 2. Incoming National Hurricane Center director Dr. Gail Spinner puts on one of her spin moves at an Stars on Ice show on February 21, 2006 at the Palavela in Turin, Italy. Image credit: Yuri Kadobnov/AFP/Getty Images.

Dr. Spinner already swirling things around at NHC
At her first press conference, held Saturday at NHC headquarters in Miami, Dr. Spinner announced the first of several major changes for the organization. “I’m concerned that as the strongest hurricanes in the Atlantic aren’t getting the attention they deserve,” she announced, “particularly since climate change is expected to make the strongest storms stronger. In the Northwest Pacific, when their equivalent of a hurricane—a typhoon—reaches maximum sustained wind speeds of 150 mph, the U.S. Navy’s Joint Typhoon Warning Center calls the storm a ‘Super Typhoon’. Nothing beats the drama and attention a Super Typhoon gets when it bears down on a populated area, and deservedly so. Super Typhoons cause more than half of all the damage and deaths attributed to Pacific typhoons. Similarly, it’s been shown that in the Atlantic more than half of all hurricane damage is done by Category 4 and Category 5 storms, even though they make up just six percent of all landfalls. These storms should be called ‘Super Hurricanes’ to give them the notoriety and attention they deserve. As it stands now, the only ‘Super’ storm there has ever been in the Atlantic was Super Storm Sandy in 2012. Surely Hurricane Katrina deserved to be called ‘Super Hurricane Katrina’, don’t you think?”

When asked if NHC would consider expanding the Saffir-Simpson Scale to include a new “Category 6” designation, given that the climate change is expected to increase the number of super-duper Super Hurricanes, Dr. Spinner had an emphatic “no”. “We’re not going to expand the Saffir-Simpson Scale to add a ‘Category 6.’ I’ll give you three reasons for that. Number one, a Category 5 storm is already catastrophic, so there is nothing to be gained from a warning perspective from having a higher rating. Number two, NHC is trying to de-link the Saffir-Simpson Scale from the damages a storm can cause, since the storm surge of a hurricane often does not scale with the Saffir-Simpson winds. That’s why NHC is debuting new ‘Storm Surge Warnings’ this year, separate from the usual Hurricane Warnings for wind. And finally, we couldn’t use a Category 6 anyway, since some wise guys at Weather Underground trademarked the term.”

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