A severe weather outbreak is ramping up over parts of the eastern United States this afternoon, with an area that doesn't typically see much severe weather bearing the brunt of today's action. The risk for EF-2 or stronger tornadoes is 1000 times higher than normal in some spots.
The Storm Prediction Center has issued a 10% risk for tornadoes across parts of the southern Appalachians, including parts of southeastern Kentucky, northeastern Tennessee, southern West Virginia, and western Virginia. A 5% risk for tornadoes exists over a larger area, including larger cities such as Detroit, Cleveland, Pittsburgh, Washington, and Baltimore.
On the above map, I've overlain historical tornado tracks between 1950 and 2013 to illustrate how few tornadoes have occurred over the area under the highest risk today. The most recent tornado outbreak in the 10% area was back on March 2, 2012.
To further illustrate how rare tornadoes are in this part of the country, especially this late in July, we can look at the area's severe weather climatology to figure out how much higher than normal is today's tornado risk. On any given July 27 between 1982 and 2011, significant tornadoes occurred within 25 miles of the highest risk area only 0.01% of the time. Since today's risk is 10%, it means that today's risk for significant (EF-2 or stronger) tornadoes is 1000 times higher than normal.
[We arrive at the "1000 times higher than normal" by dividing today's 10% risk by the area's 0.01% climatological risk for July 27.]
Later on the evening and into tonight, the severe weather threat will shift east to the heavily populated I-95 corridor from Virginia to Massachusetts. Storms that form between the mountains in the ocean will pack the potential for isolated tornadoes, damaging winds in excess of 60 MPH, and hail the size of quarters or larger.
The Storm Prediction Center is responsible for issuing severe weather forecasts as well as severe thunderstorm and tornado watches, while local National Weather Service offices are responsible for issuing severe thunderstorm and tornado warnings.
[Images: author / author / SPC || Tornado risk map updated at 430PM EDT to reflect the 400PM forecast update.]