Flooding, tornadoes for TX, OK; dangerous 92L forms; 4th hottest summer for U.S.

Dangerous flash flooding is occurring in Eastern Oklahoma this morning, where rainfall rates as heavy as three inches per hour from the remains of Tropical Storm Hermine are occurring. A large area of Eastern Oklahoma received 3 - 6 inches of rain last night and this morning, with radar-estimated rainfall amounts as high as fifteen inches (Figure 2.) Yesterday, Hermine killed two people in Texas, who were attempting to cross flood waters in their vehicles. Hermine dropped 6 - 8 inches of rain in the Dallas/Fort Worth area, creating moderate to major flooding along the Trinity River in Dallas. Two tornadoes touched down near Dallas, and NOAA's Storm Prediction Center logged a total of eight tornado reports in Oklahoma and Texas from Hermine. The latest rainfall totals from NOAA's Hydrometeorological Prediction Center show 15.62" fell in Georgetown, Texas, with fifteen locations in Texas receiving over ten inches of rain. Big city rainfall totals included 7.57" at Austin, 6.73" at San Antonio, 6.52" at Dallas, and 7.20" at Fort Worth.


Figure 1. Radar estimated rainfall for the Dallas/Fort Worth, Texas area from Hermine shows a large area of 6+ inches of rain, with maximum amounts of ten inches.

Figure 2. Radar estimated rainfall for eastern Oklahoma, where up to fifteen inches of rain fell last night and this morning.

Potentially dangerous Lesser Antilles tropical disturbance 92L forms
A tropical disturbance (92L) has developed over the extreme southeastern Caribbean just north of the coast of South America, over the southernmost Lesser Antilles Islands. Surface observations indicate that pressures have been slowly falling at a number of stations, and satellite loops show a modest region of heavy thunderstorm activity is building. A strong flow of upper level easterly winds is creating a moderate 10 - 20 knots of wind shear, and the waters are plenty warm for development. Water vapor satellite loops show a large area of dry air lies over the northern Caribbean, but this dry air should not interfere with development over the next two days.

The disturbance is slowly drifting westward, but steering currents favor a more northwest motion Friday and Saturday. Lower shear lies over the Central Caribbean, away from the coast of South America, so any northward component of motion will allow for more significant development. There is drier air to the north, but 92L is steadily moistening the atmosphere in the Caribbean, so dry air may not be a problem for it. There is substantial model support for development. The disturbance is in a dangerous location for development, and gives me the greatest concern of any Atlantic disturbance so far this year. The models predict that by Saturday, 92L will bring heavy rains to Puerto Rico. These rains will then spread to the Dominican Republic on Sunday, and Haiti, Jamaica, and eastern Cuba on Monday. The longer range track of 92L is uncertain, and will strongly depend on where the storm drifts during the next two days. The ECMWF and GFS models predict a more southerly path through the Western Caribbean towards Belize and Mexico's Yucatan Peninsula, and the NOGAPS and Canadian models predict a more northerly path along the length of Cuba towards Florida. NHC is giving 92L a 40% chance of developing into a tropical depression by Saturday. Stay tuned.


Figure 3. Morning satellite image of the potentially dangerous disturbance in the Southeast Caribbean.

Igor
Tropical Storm Igor is barely hanging on in the face of 20 - 25 knots of winds shear, courtesy of strong upper-level winds out of the east. The shear has exposed Igor's low level circulation to view, and the storm has just one small spot of heavy thunderstorms near its center. Once Igor gets another 200 miles away from Africa, the shear should decline to the moderate range, 10 - 20 knots, and allow steady strengthening to occur. Waters are warm, 28°C, and the Saharan Air Layer (SAL) is well north of Igor, so the storm should intensity once the shear drops. The models are pretty unanimous about developing Igor into a hurricane 3 - 5 days from now. Igor will track west to west-northwest over the next week, with long range forecasts from the GFS and ECMWF models putting the storm several hundred miles northeast of the Lesser Antilles Islands a week from now. Climatology shows that about 20% of all tropical cyclones that have existed at Igor's current position have gone on to hit the U.S. East Coast; these odds are 10% for the U.S. Gulf Coast, 5% for Puerto Rico, and 10% for Canada. The forecast steering pattern for the coming two weeks from the GFS model shows a continuation of the pattern we've seen all hurricane season, with regular strong troughs of low pressure moving off the U.S. East Coast. This pattern favors Igor eventually recurving out to sea without affecting any land areas, and the odds of Igor hitting land are lower than climatology.

Elsewhere in the tropics
The GFS, NOGAPS, and ECMWF models predict the development of a new tropical wave off the coast of Africa 4 - 6 days from now.

Hottest summer in history for 50 million Americans
The U.S. had its fourth warmest summer since record keeping began 116 years ago, according to statistics issued yesterday by the National Climatic Data Center. Only 1936, 2006 and 1934 were hotter. Ten states had their warmest summer on record--Rhode Island, New Jersey, Delaware, Maryland, Virginia, North Carolina, Tennessee, South Carolina, Georgia and Alabama. Seventeen other states had a top-ten warmest summer, including five states that had their second warmest summer in history (Figure 4.) No states had a top-ten coldest summer. Record daily highs outpaced record daily lows by about 4 to 1 during the summer, with 5,287 daily record highs set, and 1,426 record lows. The summer warmth was a pretty remarkable swing from this past winter, which was the 18th coldest in U.S. history.


Figure 4. State-by-state temperature rankings for the summer of 2010. Image credit: National Climatic Data Center.

According to Chris Burt, author of Extreme Weather, approximately 50 - 60 million Americans experienced their hottest summer ever. No summer in U.S. history has affected so many Americans as "hottest-summer-on-record". The following large cities all posted a record hottest summer:

Washington D.C. National Airport: 81.3° (old record 80.0° summer of 1943)
Dulles Airport, VA: 77.8° (old record 76.8° summer of 2007)
Richmond, VA: 81.3° (old record 80.0° summer of 1994)
Atlantic City, NJ: 77.5° (old record 75.8° summer of 2005)
Philadelphia, PA: 79.6° (old record 78.9° summer of 1995)
New York City (Central Park): 77.8° (old record 77.3° summer of 1966)
Trenton, NJ: 77.7° (old record 76.5° summer of 1898)
Wilmington, DE: 77.8° (old record 77.7° summer of 1900)
Baltimore, MD: 79.2° (old record 79.1° summer of 1943)
Norfolk, VA: 81.1° (old record 80.0° summer of 1994)
Tampa, FL: 84.5° (previous record 84.2° in 1998)
Lakeland, FL: 84.6° (previous record 84.4° in 1987)
St. Petersburg, FL: 85.6° (old record 84.6° in 1987)

Santa Barbara, CA was the only major U.S. city that had its coldest summer on record, though several other California cities were unusually cool. San Diego had its 3rd coolest summer, and the Los Angeles airport had its 2nd coolest summer.

Fortunately, it was a very wet summer, and the record heat did not lead to widespread drought. Summer 2010 ranked as the 16th wettest summer in the 116-year record for the contiguous U.S. Wisconsin had its wettest summer on record, and six other states had a top-ten wettest summer. No state had a top-ten driest summer.

Next post
I'll have an update Friday morning.

Jeff Masters