Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 1:27 PM GMT on September 06, 2011
East Texas' dangerous fires continued to rage out of control yesterday, thanks to gusty north winds associated with the passage of a cold front and the remnant circulation of Tropical Storm Lee. Since Saturday, wildfires have torched over 500 homes in East Texas and killed two people. At Austin Bergstrom Airport, sustained winds of 20 - 25 mph, gusting to 30 - 35 mph blew much of the day yesterday. Tropical Storm Lee's remnants didn't bring any clouds or moisture to Austin yesterday, and the temperature climbed to 91°, with a humidity of just 11%. With the region enduring it's driest 1-year drought on record, yesterday's heat, dryness, and winds resulted in critical fire conditions. The forecast today for Austin is much better--winds will be only 5 - 10 mph, which should give firefighters the upper hand in many of the blazes, despite low humidities that will be in the 15 - 25% range. NOAA's Storm Prediction Center is not predicting that critical fire weather conditions will return during the remainder of the week. You can monitor today's fire activity by using our wundermap for Austin with the fire layer turned on.
Figure 1. True-color image taken by NASA's Aqua satellite of the the fires burning near Austin, Texas on September 5, 2011. Image credit: NASA.
Video 1. Video shot by a motorist on Highway 21 near Austin, Texas of the smoke from the fires that raced through Bastrop County on September 4, 2011.The highway closes and the motorist is forced to turn around.
Texas' unprecedented heat
As I reported in yesterday's post, there has never been a Texas summer hotter than the summer of 2011. The summer of 2011 now holds every major heat record for the city of Austin, including most 100° days (67 so far), hottest month in recorded history (August, breaking the previous record by a remarkable 2.1°), hottest summer (by 1.1°), and hottest day in history (112°F, tied with Sep, 5, 2000.) As wunderground's weather historian Christopher C. Burt documents in his latest blog post, the situation is similar across the rest of the state. Seventeen major cities in Texas recorded their hottest summer on record in 2011. Most of these stations had records extending back more than 100 years, and several of the records were smashed by an amazing 3.4°F--at Lubbock and at Wichita Falls. Neighboring states also experienced unprecedented heat, with Oklahoma recording America's hottest month by any state in recorded history during July, and Shreveport, Louisiana breaking its record for hottest month by 3°F in August. Mr. Burt commented to me: " I do not believe I have ever seen a site with a long period of record, like Shreveport, where records go back to 1874, break its warmest single month on record by an astonishing 3°. This is unheard of. Usually when a site breaks its single month temperature record, we are talking about tenths of a degree, rarely a whole degree, let alone 3 degrees! Hard to believe, frankly." Texas has also had its worst fire season on record, with over 3.5 million acres burned this year, and it's driest 1-year period in recorded history.
Figure 2. Observed soil moisture for Sunday Sep 4, 2011. Soil moisture is expressed in percent, with 50% being a historically average soil moisture level. Very dry soils, with moistures in the driest 1% - 30% in history (red and orange colors), were present over much of the south, where Lee dropped its heaviest rains. These dry soils have limited flooding damage. Image credit: N OAA Climate Prediction Center.
Heavy rains from Lee create significant flooding
Tropical Storm Lee is no more, but its remnants are marching slowly northeastwards along a stalled cold front, bringing torrential rains. Jackson, Mississippi received 11.68" in a 24 hour period yesterday, which is that city's heaviest 24-hour rainfall on record, according to wunderground's weather historian, Christopher C. Burt. Their previous record was 8.54", set April 11 -12, 1979. Fortunately, Jackson was in severe drought, and the dry soils were able to absorb a significant amount of rain before the local rivers began flooding. The Pearl River at Jackson rose above flood stage this morning, and is expected to crest at moderate flood stage late this week. Chattanooga, Tennessee also set its record for the wettest 24-hour period in its history, with 9.85" falling yesterday. The previous record was 7.61", set on March 30, 1886. Again, the dry soils that were present before the event started will help keep river flooding in the minor to moderate range on area rivers. Soils are at near-average moisture levels in Central Pennsylvania, where Lee's remnants are expected to drop over seven inches of rain over the next two days. These rains should cause moderate and possibly major flooding in Pennsylvania. Also of concern is the potential for tornadoes today. NOAA's Storm Prediction Center (SPC) has logged 25 tornado reports over the past three days from Lee, including three near Atlanta, Georgia yesterday. More tornadoes are likely today over North Carolina, Southern Virginia, and Northern South Carolina, where SPC is predicting a "Slight Risk" of severe weather.
Lee's heaviest rain amounts, by state, as of 4 am CDT today:
Holden, LA: 15.43"
Florence, MS: 13.45"
Tillman's Corner, AL: 11.74"
Milton, FL: 10.03"
Chattanooga, TN: 9.85"
Rome, GA: 5.70"
Roanoake, VA: 4.30"
Bluefield, WV: 3.14"
Bridge City, TX: 3.12"
Flatwoods, KY: 3.67"
Figure 3. Predicted rainfall for the 2-day period 8am EDT Tuesday - 8 am EDT Thursday, Sep 8, 2011. Lee's remnants are expected to bring a large swath of 7+ inches of rain into Central Pennsylvania. Image credit: NOAA/HPC.
Hurricane Katia strengthened this morning into the Atlantic's first Category 4 hurricane of the year, but has slipped slightly in intensity due to an eyewall replacement cycle, and is now a strong Category 3 hurricane with 125 mph winds. Latest satellite loops show that the eye has now disappeared, and the hurricane is having trouble maintaining its eyewall in the face of moderate wind shear of 10 - 15 knots on its northwest side. Continued weakening to a Category 2 storm is a possibility, though Katia will probably re-strengthen later today or on Wednesday once it manages to build a new eyewall.
The computer models continue to agree that a low pressure system over the Eastern U.S. associated with the remnants of Tropical Storm Lee will turn Katia to the north well before the storm reaches the U.S. As the storm moves northwards past North Carolina, Katia will get caught up in west-to-east moving winds associated with the jet stream, and taken northeastwards out to sea. No land areas are in Katia's cone of uncertainty, and Katia's outer rainbands should remain just offshore from North Carolina, New England, and the Canadian Maritime provinces at the point of closest approach. The main impact of Katia will be high surf leading to beach erosion and dangerous rip currents. Long period swells from Katia have arrived at the coast, and the entire U.S. East Coast will receive an extended multi-day period of high surf. The East Coast is lucky that Tropical Storm Lee came along, since Lee helped to create the steering pattern that will keep Katia from hitting the U.S.
Figure 4. Morning satellite image of Hurricane Katia.
95L off the coast of Africa
A large tropical wave with plenty of intense thunderstorm activity and spin is located about 700 miles west-southwest of the Cape Verde Islands off the coast of Africa. This wave, Invest 95L, is under a moderate 10 - 20 knots of wind shear, and is headed west to west-northwest at 15 mph. Recent satellite loops show that 95L has an elongated circulation center; this will need to tighten up into a more circular shape before the storm can become a tropical depression. Water vapor satellite images show that 95L is embedded in a very moist environment. Ocean temperatures are near 28°C, which is 1.5°C above the 26.5°C threshold usually needed for a tropical storm to form. With wind shear predicted to remain low to moderate the next four days, the atmosphere expected to stay moist, and ocean temperatures predicted to gradually warm, I don't see anything that would keep 95L from becoming a tropical depression in 1 - 2 days. NHC is giving 95L a 70% chance of developing into a tropical depression by Thursday. There is a large amount of model support for development of 95L into a tropical depression, with most of the models predicting it could be a weak tropical storm by the time it reaches the Northern Lesser Antilles Islands on Friday or Saturday. Residents of the islands should anticipate the possibility of tropical storm conditions arriving as early as Friday. Most of the models predict 95L will follow a path near or slightly north of the Northern Lesser Antilles and Puerto Rico, then curve northwestwards, on a trajectory that would likely miss the Bahamas.
New Gulf of Mexico disturbance
A cold front swept into the Gulf of Mexico off the coast of Texas behind Tropical Storm Lee yesterday morning, and has stalled out along a line from Tampa, Florida to Mexico's Bay of Campeche in the southern Gulf of Mexico. Heavy thunderstorms have begun to build along the tail end of this front in the Bay of Campeche, but are still not very concentrated or organized. Most of the computer models develop a tropical depression in the Bay of Campeche late this week, and these same models did very well at anticipating the formation of Tropical Storm Lee in the Gulf of Mexico last week. Given the moderate wind shear, warm waters, and presence of an old cold front to serve as a nucleus for development, a new Gulf of Mexico tropical depression by late this week appears likely. The path such a storm might take would depend strongly on where the center forms. A more northerly formation location near the top of Mexico's Yucatan Peninsula would likely result in a northward motion towards the Florida Panhandle. This is the solution of the European Center Model (ECMWF), which takes a weak tropical storm with a central pressure of 1000 mb into the Florida Panhandle on Sunday. A more southerly formation location might lead to the storm getting trapped in a region of weak steering currents, resulting in a slow, erratic motion in the southern Gulf. This is the solution of the latest runs of the GFS, NOGAPS, and UKMET models. NHC is giving this disturbance a 20% chance of developing into a tropical depression by Thursday.
Figure 5. Volunteers with Portlight.org disaster relief charity take a break from their Hurricane Irene relief efforts in Pink Hill, NC. From their latest blog post:Please help as you can. And please remember in your thoughts and prayers those in the path of Irene.
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