Massive tornado outbreak kills 202; 100-year flood coming on Mississippi River

Posted by: JeffMasters

A stunning tornado outbreak of incredible violence has left at least 202 dead across the Eastern U.S.; injuries probably number over a thousand, with 600 injured in the town of Tuscaloosa alone. The tornadoes carved huge swaths of damage, completely flattening large sections of many towns, and damage from the storms is likely to be the greatest in history for any tornado outbreak. Hardest hit was Alabama, with at least 149 dead; at least 36 were killed in neighboring Mississippi. NOAA's Storm Prediction Center logged 160 preliminary reports of tornadoes between 8am EDT yesterday and 8am EDT today. At least 11 of these tornadoes were killer tornadoes; deaths occurred in six states. Damage from some of these storms appeared to be at least EF-4, and it is likely that there were multiple violent EF-4 or EF-5 tornadoes. The death toll makes the April 27 - 28 outbreak the third deadliest tornado outbreak of the past 50 years, behind the April 3 - 4, 1974 Super Outbreak (315 killed) and the 1965 Palm Sunday tornado outbreak (256 killed.)

Figure 1. Damage in Birmingham, Alabama from last night's tornado. Image posted to twitter.

Figure 2. Damage in Tuscaloosa, Alabama from last night's tornado. Image posted to twitter, photographer unknown.

Figure 3. Radar reflectivity image of the Tuscaloosa, Alabama tornado.

The 3-day total of preliminary tornado reports from this outbreak is 278, close to the 323 preliminary tornado reports logged during the massive April 14 - 16 tornado outbreak. That outbreak has 155 confirmed tornadoes so far, making it the largest April tornado outbreak on record. It is unprecedented to have two such massive tornado outbreaks occur so close together. According to a list of tornado outbreaks maintained by Wikipedia, only two other tornado outbreaks have had as many as 150 twisters--the May 2004 outbreak (385), and the May 2003 outbreak (401).

Figure 4. Satellite image of last night's storm at 8:15pm EDT April 27, 2011. Image credit: NASA/GSFC.

Tornado outbreak winding down today
Tornado warnings continue to be issued this morning along the cold front now pushing towards the Atlantic coast, and a tornado was reported at 7:35am EDT in McBee, South Carolina. NOAA's Storm Prediction Center has placed a large swath of the coast, from Florida to Vermont, in their "Slight Risk" region for severe weather. The high instability and high wind shear that triggered so many killer tornadoes yesterday is gone, and we should see only a few weak tornadoes today. No severe storms are predicted for Friday. Saturday has a slight risk of severe weather over Oklahoma and Texas.

Figure 5. Severe weather threat for Thursday, April 28, 2011.

Figure 6. Remarkable video of the tornado that hit Tuscaloosa, Alabama yesterday. Fast forward to minute four to see the worst of the storm.

Figure 8. Tornado near Empire, Alabama, moving rapidly down a hill.

Unprecedented flooding predicted on Ohio and Mississippi Rivers
This week's storm system, in combination with heavy rains earlier this month, have pushed the Ohio River and Mississippi River to near-record levels near their confluence. The Ohio River at Cairo, Illinois is expected to crest at 60.5 feet on May 1. This would exceed 100-year flood stage, and be the highest flood in history, besting the 59.5' mark of 1937. Heavy rains of 10 - 15 inches have inundated the region over the past few days, and one levee breach at Black River levee near Poplar Bluff, Missouri, has resulted in the evacuation of over 500 homes. Poplar Bluff has received 15.45" of rain since Friday morning. The greatest rain gauge-measured precipitation from the storm occurred in Springdale, Arkansas, where 19.70" inches has fallen since Friday morning.

Figure 9. The latest River Flood Outlook from NOAA shows major flooding is occurring over many of the nation's major rivers.

Record 100+ year flood expected on Mississippi River
Snow melt from this winter's record snow pack across the Upper Mississippi River has formed a pulse of flood waters that is moving downstream on the Mississippi, and is currently located in Iowa. When this floodwater pulse moves south of Cairo, Illinois over the next two weeks, it will join with the record water flow coming out of the Ohio River, and create the highest flood heights ever recorded on the Mississippi, according to the latest forecasts from the National Weather Service. Along a 400-mile stretch of the Mississippi, from Cairo to Natchez, Mississippi the Mississippi is expected to experience the highest flood heights since records began 100 or more years ago, at 5 of the 10 gauges on the river along this stretch. The records are predicted to begin to fall on May 3 at New Madrid, and progress downstream to Natchez by May 20. Areas that are not protected by levees can expect extensive damage from the flooding, and it is possible that the Army Corps of Engineers will have to intentionally dynamite a levee at Birds Point and New Madrid, Missouri to protect the town of Cairo from flooding.

The Mississippi River at New Madrid, MO, about 40 miles downstream of the confluence of the Mississippi and Ohio Rivers, is currently at 44', the 3rd highest flood in history. The river is predicted to crest on Tuesday very near the all-time record height of 48 feet. The NWS warns that at this height, "Large amounts of property damage can be expected. Evacuation of many homes and businesses becomes necessary." Previous record heights at this location:

(1) 48.00 ft on 02/03/1937
(2) 44.60 ft on 04/09/1913
(3) 43.60 ft on 04/04/1975
(4) 43.50 ft on 02/16/1950
(5) 42.94 ft on 03/17/1997

The timing of the floods crests will depend upon a complex mix a factors, including how much rain falls over the next month, the possible influence of southerly winds holding up the floodwater pulses, the potential opening of flood control structures and reduction of flows from flood control reservoirs, and potential levee failures (no levee has failed on the Lower Mississippi south of the Ohio River junction since 1950, however.) The Mississippi River is expected to crest at 17 feet at New Orleans on May 22, three feet below the top of the levees. This would likely require opening of the Bonnet Carre Spillway 28 miles upstream from New Orleans, to relieve pressure on the city's levees. Opening the spillway drains 250,000 cubic feet per second of flow into Lake Pontchartrain.

Helping out tornado victims
For those who want to lend a helping hand to those impacted by the widespread destruction this month's severe weather has brought, stop by the blog.

Related post: Are tornadoes getting stronger and more frequent? The answer is--we don't know.