Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather
By: Dr. Jeff Masters , 2:14 PM GMT on April 25, 2007
A killer tornado swept through Piedras Negras, Mexico, crossed the Rio Grande River, and brought devastation to the small town of Eagle Pass, Texas, at about 7pm CDT last night. The tornado killed three and injured 87 in Mexico; seven died in Texas, and 74 were injured. Five of the U.S. deaths occurred in a single mobile home when it was picked up and tossed into an elementary school. The tornado destroyed 20 homes, two schools, and the local sewage treatment plant in Eagle Pass. Killer tornadoes in Mexico are rare, as most of the country is too far south to get tornado weather, and is sparsely populated in the Texas border regions that are prone to tornadoes. Mexico's worst tornado that I could find record of occurred in 2004, when a tornado killed 32 in Piedras Negras.
Tornadoes were also reported last night in Oklahoma, Kansas, and eastern Colorado. More tornadoes are possible today, as the Storm Prediction Center (SPC) has placed much of the Mississippi Valley under it's "Slight Risk" area. This is a major step down from yesterday's risk level, though, when SPC had portions of Texas under its "High Risk" area (although Eagle Pass was in the "Moderate Risk" area). This is the third time in 2007 that SPC issued a "High Risk" forecast. A "High Risk" forecast was also issued for the EF4 March 1 Enterprise, Alabama tornado that killed 20. It's been a bad year for tornadoes--last night's storm brought the 2007 U.S. death toll to 59. The average tornado death toll for the entire year has been just 46 the past three years. It's only April, and we still have the peak tornado months of May and June to get through.
Figure 1. Radar reflectivity image of the April 24, 2007 tornado as it approached Eagle Pass, Texas. Note the distinctive hook shape of the radar echo, which is characteristic of supercell thunderstorms that spawn strong tornadoes.
Figure 2. Radar velocity image of the April 24, 2007 tornado as it approached Eagle Pass, Texas. Note the area of blue and red echoes just south of the circle with a "+" inside it that marks the location of Eagle Pass. The blues and reds show that strong winds going both towards and away from the radar exist in a small area, denoting the presence of a parent mesocyclone (rotating thunderstorm) and a tornado.
Figure 3. Vertically integrated Liquid Water (VIL), in kilograms per square meter, for the April 24, 2007 tornado as it approached Eagle Pass, Texas. VIL is a measure of how much water is in the storm, when measured from the surface to the top of the storm. The Eagle Pass thunderstorm had cloud tops at 56,000 feet, so a column of air one meter square extending from the surface to 56,000 feet had up to 70 kilograms of liquid water in it. That's a lot of water available for hailstones to grow in, and large hail up to 1.75 inches in diameter was observed with this storm. A VIL of at least 50 is typically required to get large hail two inches in diameter in April in the Plains. A VIL of about 65 is needed later in summer, when the thunderstorms grow taller and more liquid water is needed to make large hail. See the Oklahoma Climatological Survey VIL help page for more information.
We've saved a 300 Mb radar animation of the Eagle Pass tornado for those interested.
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