Today’s severe weather risk – pack an umbrella

A weather trifecta may be setting up today; heavy rain, strong winds, and an isolated tornado are all possible as a cold front and a whole lot of moisture meet in the skies of Virginia today.

September 18, 2012; 6:28 AM • by Weather Dan

Update #3 — September 18th, 4:55 PM

The tornado watch for the Richmond metro area has been cancelled. It remains active for areas east of Henrico, Hanover, Chesterfield, and Dinwiddie Counties.

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Update #2 — September 18th, 3:50 PM

The strongest of the storms are moving through the Richmond metro area now. While some showers and storms are still possible after this line passes, this represents the bulk of the severe thunderstorm threat. Portions of the tornado watch to the west have already been cancelled, and I expect that this watch will be pared back further after it clears I-95.

However, for those of you east of I-95, the severe threat still exists and should be taken seriously. A couple tornado warnings have been issued for other parts of this line of storms near Washington, DC and Raleigh. Straight-line winds are still the biggest concern with these storm as they cruise east, but the possibility does exist for a quick tornado embedded within the most intense parts of the line. Remain alert until the watch is canceled for your locality.

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Update #1 — September 18th, 11:45 AM

The National Weather Service has issued a tornado watch until 7:00 PM. Be safe!

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Original — September 18th, 6:25 AM

A strong upper-level trough and surface cold front are moving eastward into the Virginias today, and we’re looking at some very unsettled weather as a result. The rain Monday evening was just a preview of what just may turn into a three-ring weather circus today. Look for precipitation to begin between 10am and noon and last until after sunset. It will be off and on at times, with the last of the rain finally moving through the area overnight tonight.

The first component is the rain. While central Virginia will be relatively dry, rainfall amounts of up to an inch of rain are not out of the question for most of the region. Some locally heavier amounts – possibly up to two inches or more – are possible in some places by 8am Wednesday.

Along with this comes with a threat for some severe thunderstorms beginning around noon. The sun is not your friend today. The atmosphere is quite unstable, and it won’t take much to trigger thunderstorms, especially along the leading edge of the moisture as it moves into central Virginia this morning. Any prolonged periods of sunshine will help to destabilize the atmosphere that much more.

Thunderstorms that do develop today will present a strong risk for damaging wind gusts. The map above shows the risk for wind gusts in excess of 60 mph; strong, possibly damaging winds will accompany storms even if warnings are not issued.

Also of concern is the threat for a tornado or two today. While the risk will be relatively isolated, a 10% forecast is a rather significant one. The wind dynamics needed are in place and will continue to be through the day today. However, the most favorable conditions for tornadoes and rotating thunderstorms will be to the north of central Virginia. That said, if the sun does make an appearance, that will only serve to enhance the risk for severe thunderstorms.

There are some limiting factors here, however. The biggest of these, as I’ve mentioned, is the instability. Without that, what may otherwise have been a severe thunderstorm could be limited to just a heavy downpour. Of the two scenarios, this is probably more likely than a tornado, but you can’t completely ignore the tornado risk. Stay safe today!

September 18, 2012 6:28 AM 0