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Posted at 01:09 PM ET, 04/26/2011

Dangerous severe weather outbreak threatens for second straight day

Large tornadoes possible in mid-South, extreme flooding in central U.S.


(NOAA SPC)

UPDATE, 3 p.m.:A large “particularly dangerous situation” tornado watch has been issued for southern and central Arkansas, northern Louisiana, extreme southeast Oklahoma, and northeast and north central Texas for destructive tornadoes and large hail to four inches in diameter.

From 1 p.m.: Yesterday, more than 400 severe weather reports were logged by the National Weather Service, including 38 tornado reports. The worst tornado, estimated to be one half mile wide, occurred in Vilonia, Arkansas where there was major structural damage and at least four lives were lost. In many of the same areas where severe weather occurred yesterday, there is a moderate to high risk of another outbreak today. Along and to the northwest of the areas impacted by severe thunderstorms, significant flooding has occurred and is ongoing.

Today’s Severe Weather Threat

The Storm Prediction Center is highlighting a HIGH RISK of severe thunderstorms from northeast Texas across southern Arkansas into extreme western Tennessee, including Memphis. A MODERATE RISK of severe thunderstorms surrounds the high risk area (see map above). SPC writes conditions will be favorable for “long-track/damaging tornadoes” in the moderate to high risk areas.

AccuWeather says the following cities could be impacted by tornadic storms: Tyler, Texas, Broken Bow, Okla., Little Rock, Ark., Shreveport, La., Batesville, Miss., and Memphis, Tenn.with Dallas “a close call.”

Greg Forbes, severe weather expert at The Weather Channel, forecasts a high probability of tornadoes in south Arkansas, northeast Texas, and northwest Mississippi using his Tornado Condition (TORCON) index.

Today’s Flooding Threat

NOAA’s Hydrometeorological Prediction Center (HPC) writes a “significant heavy rainfall event across the Ohio and lower to middle Mississippi River valleys is expected to persist through the middle of the week.” Flood watches and warnings are in effect across these areas. Some of the three-day rain totals are staggering, including 17.09” near Springdale, AR...Fayetteville, AR: 12.51”...Poplar Bluff, MO: 12.86. In Poplar Bluff, a levee along the Black River is compromised in several locations “maximizing the potential of an eventual failure of the levee system” according to the National Weather Service. Officials have evacuated 500 structures in the flood threatened area.


Rainfall predicted by NOAA’s Hydrometeorological Prediction Center through 8 a.m. Thursday morning. (NOAA HPC)

Before the rain ends Wednesday into Thursday morning, 2 to 4 inches additional rainfall is possible with up to 5 inches from eastern Arkanasa to southern Indiana. In Kentucky, the Governor has declared a state of emergency due to the rising flood waters.

Yesterday’s Vilonia, Arkansas Tornado

Based on reports that the Vilonia, Arkansas tornado scoured pavement (Highway 64), there is some speculation it reached EF-5 intensity, the strongest possible tornado on this scale, with winds to over 200 mph. If that’s true, it would be a first for the state. Patrick Marsh, a research meteorologist at the University of Oklahoma, writes: “Officially, there has never been a F/EF-5 tornado in Arkansas.” For his part, Marsh cautions “any talk of ratings ... is premature” - as the NWS still needs to do a complete assessment of the damage to determine the EF-level.

The tornado was also extraordinary for its presentation on doppler radar. Meteorologist Paul Douglas at WeatherNation writes: “It’s VERY rare to see an actual tornado showing up on Doppler, unless a). the tornado is extremely large, or b). the portable Doppler radar is within a few miles of the tornado.” A three dimensional doppler image of the tornado - from the NWS Office in Little Rock is shown below.


Three dimensional image of the velocity couplet associated with the Vilonia, AR tornado, indicative of strong winds blowing toward and away from the location of the tornado. (NWS)

Storm Chaser Reed Timmer tweeted the radar presented “one of the strongest couplets” he’d ever seen indicative of winds blowing violently toward and away from the radar site at the location of the tornado. Additional radar imagery of the Vilonia tornado is available here.

Another tornado outbreak tomorrow?

NOAA’s SPC is predicting a moderate risk of severe thunderstorms from Mississippi and Alabama into the Tennessee Valley tomorrow, with the possibility of more dangerous tornadoes. It will be the third consecutive day the SPC has forecast at least a moderate risk of severe storms. A slight risk of severe storms will reach the East coast from Vermont through Washington D.C. all the way into the Florida panhandle.

By  |  01:09 PM ET, 04/26/2011

Categories:  Thunderstorms, U.S. Weather

 
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