* Flash flood watch 3 p.m. through late tonight *
1:25 p.m. update: We've started a live blog on the approaching squall line. Follow this link: Squall line targets D.C. area for P.M. commute (LIVE BLOG)
12:25 p.m. update on storm timing: Given storms are already into western Maryland and eastern West Virginia, we think models are too slow with the onset of strong storms. Our revised time window for the worst conditions is 3-7 p.m. General rains and some gusty winds (but not severe) will continue for several hours after that.
From 11:07 a.m.: A lengthy line of heavy showers and thunderstorms stretching from New Orleans to Toronto will plow eastward today, bearing down on the East Coast and Washington D.C. area tonight. This squall line has the potential to produce some damaging wind gusts capable of downing trees and power lines, and leading to power outages.
The most likely time of arrival for the squall line in the D.C. area is between 7 p.m. and midnight, although isolated to scattered showers and storms could develop out ahead of the line late this morning and this afternoon.
The 1,300 mile-long squall line has a history of producing damaging winds. On Tuesday, the National Weather Service Storm Prediction Center SPC) logged over 300 severe wind reports in the Lower to Mid- Mississippi Valley and Tennessee Valley. More than 40 (and counting) such reports have come in so far today.
As the squall line raced through Wheeling, West Virginia this morning, a 62 mph wind gust was reported. The Weather Channel says there are numerous reports of trees down in southwest Pennsylvania and north-central West Virginia.
A large area along and out ahead of the southern part of the line is under a tornado watch while a severe thunderstorm watch stretches from southwest Virginia through West Virginia into western Pennsylvania. SPC says areas to the east of these watch areas have a slight risk of severe thunderstorms. Additional watches will probably be issued later today.
In the D.C. area, the main risk will be gusty winds with the squall line, with a good chance of gusts from 35-50 mph, and possibly isolated gusts to 60 mph. SPC says there is a 15-30 percent chance of damaging gusts within 25 miles of a point in our region.
Peak gusts are expected to be below those of the June 29, 2012 derecho, which reached 70-80 mph. While this squall line is very dynamic, the derecho contained vastly greater amounts of energy.
In terms of timing, the squall line this evening offers the most likely opportunity for heavy rain and strong winds, but some showers and thundershowers may develop out ahead of the line late this morning and this afternoon.
Very strong winds are moving through the region at high altitudes, and any showers that develop could transport some strong gusts down to the surface. We cannot rule out that a brief, weak tornado could spin-up somewhere, but think the risk is relatively low (2 percent or less within 25 miles of a given location).
Rainfall amounts should generally range from one-half inch to 1.5 inches. As a lot of that rain will fall in a short time, a flash flood watch has been issued. Areas that typically flood during heavy rain events like the Bloomingdale area of the District should be prepared for high water.
This dynamic, powerful weather system has developed as a result of a tremendous clash of the air masses. Temperatures ahead of the cold front along which the squall line has formed are some 20 to 30 degrees above normal, and in record territory. Yesterday, 179 record high temperatures were tied or set according to the National Climatic Data Center.
Behind the front, temperatures crash. Areas in the central U.S. that were in the 60s yesterday are in the 30s and 40s today.
Stay tuned for additional updates throughout the day.