NWS Jackson, MS May 10-11, 2008 Severe Weather

May 10th & 11th Severe Weather

From the afternoon hours of May 10th into the morning hours of May 11th, a major outbreak of severe weather affected a large swath of the south central and southeastern United States, from Oklahoma and Kansas to the coastal areas of South Carolina and Georgia. Dozens of tornadoes, including a number of strong and violent tornadoes, extreme wind gusts, and large hail affected numerous states in this region. A total of 23 people were killed.

The National Weather Service office in Jackson is responsible for forecasts and warnings for the area of central Mississippi, southeast Arkansas, and northeast Louisiana generally along and south of the U.S. Highway 82 corridor. Our office forecast that significant severe weather associated with this outbreak would occur Saturday afternoon and night, generally north of Interstate 20. Fortunately, this did not happen, as most of the severe weather remained north and west of the area. Given our forecast of significant severe weather, and a number of areas under tornado watches for many hours without any occurrence of severe weather, we feel that we owe our customers and partners some explanation of what transpired with this event, and why severe weather did not occur in our service area.

The initial tornado watch for our area was issued during the late morning hours Saturday, and included areas over northeast Louisiana, southeast Arkansas, and western Mississippi generally along and north of I-20 and east of I-55. The rationale for this watch was that thunderstorms were already beginning to develop, and the atmosphere at that time was quite favorable for severe storms including tornadoes. There was some concern that "capping," or warmer air aloft in the atmosphere, might move in and keep the storms from being as strong, but given the volatility of the atmosphere the watch was issued. In the end, the "capping" won out, and no severe weather occurred during this watch.

As expected, the main event with this outbreak for our region occurred during the evening and overnight hours Saturday. Tornado watches were issued during the late afternoon for the Delta region of southeast Arkansas and northwest Mississippi, and then during the late evening were expanded south and east to cover the entire area north of I-20. Very intense thunderstorms were ongoing just to the northwest and west of our area in Arkansas at the time the watches were issued. Conditions over our region appeared "ripe" for the storms to move in to the area, or for new storms to develop. The atmosphere was very unstable, and wind shear (the change of wind with height in the atmosphere, a necessary ingredient for tornadoes) was high. In the end, the storms almost "split" in their motion. One large area of severe storms moved just south of due east and stayed just north of the area from Bolivar County east to Lowndes County. Significant damage was reported in some areas of north Mississippi from these storms. The other area of storms moved south-southeast from western and central Arkansas into north Louisiana. While this activity produced the only severe weather observed in the Arklamiss region (some downed trees and hail over parts of northeast Louisiana and southeast Arkansas), the most severe weather with it was to our northwest and west. When this activity moved south of Interstate 20, it decreased in intensity as expected, and no severe weather was reported south of I-20.

The below graphic shows the track of the intense, rotating storms (red lines), and the hourly position of a squall line (black dash/dot line), which developed and moved across the southeastern U.S. There is also a black line that shows the line that briefly formed and moved across northeast Louisiana and southeast Arkansas, producing the wind damage in that area. As you can see, a number of intense storms moved just north and northwest of our area - a difference of 25 to 50 miles would have brought significant severe weather impacts to our region.

Image Courtesy of Greg Carbin, Warning Coordination Meteorologist, Storm Prediction Center, Norman, OK

The staff of the NWS office in Jackson takes their duty to protect life and property very seriously. With the volatile atmosphere having the potential to produce intense severe weather that could have caused significant damage and/or casualties, we felt that this was clearly a situation where strongly worded forecasts were necessary to ensure that the public and officials were aware of the potential magnitude of the threat. This is particularly true given that it was occurring over a weekend when many people would be engaged in outdoor activities. Luckily, our area was spared the brunt of the weather, but only very small deviations in the atmosphere that are simply not readily forecast could have resulted in a much greater impact in our area.