A large part of the eastern United States is on track to see two distinct and very dangerous severe weather events both today and tomorrow, with the worst outbreak expected to occur tomorrow over the Ohio Valley and Mid-Atlantic regions.
Saturday's Severe Weather
A moderate (4 out of 5) risk for severe weather is in effect this afternoon across a large swath of the Midwest, stretching from southern Iowa through eastern Indiana.
The main risk for tornadoes is centered over the western half of the moderate risk zone, where discrete thunderstorms are most likely and have the greatest chance to begin rotating. These discrete cells could also produce hail larger than golf balls according to the Storm Prediction Center.
Later in the afternoon, storms will form into one or more bow echoes (squall lines) and move through the moderate risk area, carrying with them a high chance of producing damaging winds in excess of 60 MPH.
Sunday's Severe Weather
Sunday's severe weather is focused on an area that doesn't typically see dangerous severe weather outbreaks — the eastern Ohio Valley, including most of West Virginia. The SPC has issued a moderate risk for severe weather tomorrow, with a 45% risk (see here for how severe weather probabilities work) for severe weather across parts of Ohio, Kentucky, and West Virginia.
To drive home how uncommon severe weather is in this part of the country during this time of the year, let's look at some severe weather climatology. The below map shows how likely significant damaging winds (greater than 75 MPH) are tomorrow, based on actual observations between 1982 and 2011.
The climatological likelihood of seeing significant damaging winds over most of West Virginia is just 0.05%. With tomorrow's risk sitting at 45%, this means that the risk for significant damaging winds is 900 times higher than climatological normal. That is outrageous, and it highlights how dangerous this is in an area not usually susceptible to severe weather of this magnitude during this time of the year.
As the storms move east away from the Ohio Valley, they will threaten the Mid-Atlantic as a "severe MCS," according to the SPC. In other words, a dangerous bow echo (squall line) will begin to move through the West Virginia Panhandle into the D.C., Baltimore, and Philadelphia areas with damaging winds. The "derecho" hype is already beginning, but a line of storms doesn't need to reach the official criteria for a derecho to be dangerous. A squall by any other name would smell as sweet.
It's important to keep in mind that the areas shaded in yellow — the slight risk — may also see severe weather tomorrow. The slight risk includes almost all of the major cities on on the east coast between Greensboro and Portland, Maine, including all areas to the west. Even a large portion of southern Ontario is under the gun for dangerous storms.
[Images via AP / SPC / author / SPC]