SPC issues rare High Risk for severe weather

Date Posted: March 2, 2012

In anticipation of a major severe weather outbreak — the largest so far this year — the NWS Storm Prediction Center has issued a High Risk for severe weather into tonight for parts of the central and eastern U.S., which as of Friday afternoon included parts of southern Indiana, southwest Ohio, a large portion of Kentucky and northern middle Tennessee.

With several tornadoes reported already, SPC is forecasting the development of a few strong, long-track tornadoes for these areas, with the possibility of severe storms and tornadoes in the area sounding the High Risk area, across parts of southern Indiana, southern Ohio, western West Virginia, Western Virginia, far western North Carolina, Kentucky, most of Tennessee, northern Georgia, the northern half of Alabama, parts of central and northern Mississippi and a small portion of northeast Louisiana. This is the second outbreak of severe storms in just three days across much of the central U.S., including areas still recovering in the wake of Wednesday’s destructive tornadoes.

“Today’s severe weather threat is being enhanced by very strong winds aloft, which will continue to overspread a warm, moist air mass ahead of an advancing cold front,” SPC Forecaster Liz Leitman said in a multimedia weather briefing posted on SPC’s webpage. “As the storm system advances northeastward through the afternoon, an increase in thunderstorm coverage and intensity is expected.”

NWS is emphasizing the need for people to be prepared for severe weather as part of its initiative to build a Weather-Ready Nation.

“Now is the time to identify safe shelter, such as a basement or an interior room on the lowest floor in a sturdy building, for when threatening weather approaches and when a warning is issued. Also, be sure to have a NOAA Weather Radio along with fresh batteries to ensure immediate awareness of this serious weather situation,” said NWS Director Jack Hayes. “It may be late winter, but severe weather is possible any time of year under the right conditions and we’re seeing those come together now. With spring severe weather season ahead, we all need to be aware and prepare.”

The High Risk is the highest of SPC’s three categories for severe weather used in its Day 1, 2 and 3 Convective Outlook products to symbolize the coverage and intensity of organized severe weather such as supercells, squall lines, and multicell thunderstorm complexes. SPC typically includes High Risk areas in its outlooks only a few times a year.

A high risk area suggests a major severe weather outbreak is expected, with a high concentration of severe weather reports and an enhanced likelihood of extreme severe weather, such as violent tornadoes or very damaging, convective wind events occurring across a large area. In a High Risk, the potential exists for 20 or more tornadoes, some possibly EF-2 or stronger, or an extreme derecho potentially causing widespread wind damage and higher end wind gusts (80+ mph) that may result in structural damage. There have been fewer than 90 High Risk days since 1990.