Late afternoon update: The newest outlook for tomorrow from the Storm Prediction Center is much like that noted below, but with a bit stronger wording. The D.C. area is now under a high-end “slight risk,” and SPC will monitor potential as we get into tomorrow for a possible upgrade to ”moderate risk.” The main uncertainty continues to be how much shower and storm activity we see early, which might calm the atmosphere enough to temper later stuff. We’ll keep you updated as it unfolds.
From 2:00 p.m.: For the second time in the past 10 days, a powerful early fall cold front will blast through the region, setting the stage for a potentially dangerous line of thunderstorms Tuesday afternoon and evening.
Based on current information (subject to change a bit), the most likely timing for thunderstorms is between 2-8 p.m. (from west to east) Tuesday. A strong squall line may sweep through much of the region at this time, although a few individual more isolated storms could develop ahead of this main line. The worst of the storms may coincide with the heart of Tuesday’s PM commute.
The biggest potential hazard with these storms is damaging winds capable of felling trees and power lines. There is a power outage risk with this event if storms reach their potential.
Extremely heavy rain is likely when these storm pass through. A few locations could receive 1-2” of rain in a short period of time, sufficient to cause some flash flooding in low lying areas (e.g. places like Bloomingdale). The general rainfall potential, however, is closer to 0.5-0.75” as the storms - while likely heavy - should be fast movers. Thus, widespread flooding issues are not expected.
The other hazard we’ll be monitoring, but is likely to affect the fewest people, is the potential for tornadoes. As low pressure developing along the front strengthens and winds turn with height, some thunderstorms may rotate and produce a tornado or two.
Based on recent experience, this event seems to have similar potential to September 8 thunderstorm outbreak. (The set up is much more comparable to that than say, the June 29 derecho.) However, since it is coming through during the work week - perhaps around rush hour - it could be more disruptive than Sept. 8 event which occurred on a Saturday afternoon.
NOAA’s Storm Prediction Center has placed the entire Washington, D.C. and Baltimore regions under its “slight risk” designation for severe thunderstorms and indicates there’s about a 30 percent of severe weather within 25 miles of a point in our area. The risk of severe weather extends from central North Carolina through southern New York State.
A primary point of a concern is very strong low level winds - at an altitude of 3,000-5,000 feet or so - coming in from the south at about 55-70 mph ahead of the cold front. Should thunderstorms develop, vertical motions within them (i.e. the downdrafts) could easily force these strong winds down to the ground.
Writes the National Weather Service Office in Sterling:
SOUTH-SOUTHWEST WINDS AROUND 50 TO 60 KNOTS WILL BE JUST A FEW THOUSAND FEET FROM THE SURFACE DUE TO A STRENGTHENING LOW-LEVEL JET. IF A QUASI LINEAR CONVECTIVE SYSTEM [I.E. A SQUALL LINE] DEVELOPS...THESE WINDS WILL MIX DOWN TO THE SURFACE CAUSING AN ENHANCED THREAT FOR DAMAGING WIND GUSTS.
The tornado risk is not as high as the straight line damaging wind risk but low levels winds will be turning enough to very closely monitor thunderstorms for possible rotation:
STRONG SPEED/DIRECTIONAL SHEAR AT THE LOW-LEVELS OF THE ATMOSPHERE ALSO INDICATES AN ISOLATED THREAT FOR TORNADOES ALONG THE QUASI LINEAR CONVECTIVE SYSTEM SHOULD IT DEVELOP.
Both the straight line wind and tornado threat are somewhat conditional on the amount of instability that develops. If the sun comes out Tuesday morning and heats up the atmosphere, that will fuel the atmosphere and increase the possibility of damaging thunderstorms.