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Posted at 06:00 PM ET, 07/11/2011

PM Update: Hot and sticky with storm threat

Heat advisory noon to 8 p.m. Tuesday | Code orange air quality alert Tuesday

9:30 p.m. update: The line of severe storms is now racing south and east out of the metro area across Southern Maryland and the Chesapeake Bay. The Severe T’storm Watch has been cancelled north of D.C. and west of Fairfax/Prince William counties. While it remains in effect until 11 p.m for D.C., Fairfax County and points south and east, the immediate metro area is pretty much done with storms for the night. (10:40 p.m.: A number of trees down in Frederick County and other outer suburbs, plus one in Mount Vernon. Closer in, 7 large trees down in Reston, but otherwise storms were mainly a lightning show with some power outages.)

8:15 p.m. update: Severe thunderstorms move northwest to southeast through the metro area during the next couple hours with dangerous lightning and possibly damaging winds over 60 mph. Take cover until these storms pass your location.

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6:00 p.m. update: A Severe Thunderstorm Watch has been issued for the mountains of Virginia, Maryland and eastern panhandle of West Virginia through 11 p.m. (7:15 p.m.: Severe T’storm Watch extended to include D.C. metro area.) This watch is for the same complex of storms that knocked out power to a record 817,000 customers in Chicago earlier today. Odds seem to be increasing that at least parts of the metro region will experience what’s left of these storms after about 8 or 9 p.m. tonight (10 p.m. to midnight most likely timing). While the storms are showing signs of weakening, there’s a slight possibility some of the storms remain severe. We’ll keep you posted.

From 3:45 p.m.: It’s hot out there! And that continues through tomorrow before a bit of a break. Highs are reaching or headed for the mid-90s many spots, with very few clouds to take shade under. If D.C. gets 95+ today (94 so far), and picks up another tomorrow, we’re already knocking on the yearly average (~11, 1981-2010) for such days with at least several more weeks where we can add more. At least some 80s are on the horizon...

Through Tonight: Not the most pleasant evening for outdoor activities, drink lots of water! We’re keeping an eye on a damaging convective system to our northwest. It should weaken a bit one way or another, and these things tend to die out in the mountains or continually redevelop south/southeast which could lead to it (or the worst) missing south. But if it survives/arrives (30-40% chance for something), it should get here during the late evening or toward midnight. With dew points well into the 70s and plenty of clouds, temperatures don’t drop much. If we don’t get any rain to cool us, lows in the mid-70s to near 80 seem a good bet. Rain could get us into the low-to-mid 70s.

Tomorrow (Tuesday): It’s going to be gross as we’re still ahead of the front and the heat ticks up. Under partly sunny skies, and a heat advisory, highs head mainly for the upper 90s, with urban centers potentially hitting 100+. Some possible thorns in the forecast: extra clouds, and the intensity of downsloping west winds. That downsloping may lead to a bit of drying in the afternoon, yet with temperatures as high as they are it might not be too noticeable. There could be an isolated to scattered storm late-day or as the front passes overnight.

As Jason noted earlier, “Tuesday’s record highs of 99 (from 1908) at Reagan National, and 97 at BWI and Dulles (from 1908 and 1986) will all be in play.”

See Jason Samenow’s forecast through the weekend. And if you haven’t already, join us on Facebook and follow us on Twitter .

100-degree days: Both D.C. and Baltimore have already had one day 100+, which is about the 30-year average for the year. It’s exactly 1.2 days and 1.1 days, respectively. However, the long-term average is less than one, so it’s not the most common event. The last time D.C. has seen back-to-back years with more than one 100+ day was 1987-88 with three and seven. In 2010, there were four at D.C. and seven (tied for the record) at Baltimore.

Pollen update: Walter Reed’s Susan Kosisky writes, “Tree count is LOW at 0.96 gr/cubic meter, grass pollen is MODERATE at 4.79 gr/cubic meter and weed pollen is LOW (NAB range) at 5.43 gr/cubic meter which is more Moderate for our local area weed count. Mold spores are MODERATE at 12641.15 spores/cubic meter.”

By and Jason Samenow & Dan Stillman  |  06:00 PM ET, 07/11/2011

Categories:  Forecasts

 
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