Area Forecast Discussion
Issued by NWS Sterling, VA

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250 FXUS61 KLWX 230045 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 845 PM EDT Sat Jul 22 2017 .SYNOPSIS... High pressure remains off the southeast coast. Multiple upper level disturbances will be possible through Sunday. An upper level trough of low pressure will move over the northeastern US early in the week. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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What the first round didnt stabilize this afternoon, the second round did. A cluster of thunderstorms currently moving southeast of Point Lookout at this time. There is a gap from there back to round 3, which is entering the Potomac Highlands. However, a few cells have been popping up in the void between convective clusters. Mesoscale analysis suggests that a few hundred joules of MLCAPE present across much of the area. On the other hand, around or in excess of 1000 J/kg present across the Highlands southeast towards Charlottesville...which has not been worked over yet. Effective shear around 30 kt. So, we`re not out of the woods with respect to vigorous convection yet, but with the loss of daytime heating, its becoming less about winds and more about repeated rainfall. A composite of guidance sources suggest that the third round should be in the metros (I-95, Baltimore-Washington corridor) around 10 pm to midnight. This rapid propagation will be key, as precipitable water still in excess of 2 inches. If there has been one adjustment to make wrt timing, precip has been arriving quicker than progged. Due to that, accumulated rainfall thus far, and current radar presentation...have no plans for a Flood Watch at this time. Will be adjusting temperatures, too. Ultimately, the low temp will be whatever we have after round 3. Conditions should be quieter overnight. But in this air mass, cannot rule out a pop-up shower of thunderstorm at any time.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY/... Warm and humid weather Sunday as we continue to reside in the warm sector and high temperatures remain in the 90s with dewpoints in the 70s. Heat dome recedes just a bit to the SE as heights fall...which will keep the hottest temperatures across central VA...where a heat advisory appears more likely. Along/East of I-95...temperatures may be a degree or two cooler than today...though, prolonged nature of event, coupled with heat indices near 100 to perhaps 105F, suggest that a Heat Advisory may again be needed...especially considering the accumulated effect of the heat and humidity. Convective evolution tomorrow rather muddled and will likely depend on the spatiotemporal evolution of convection later tonight (which will allow finer scale details to become more clear). While CAPE/Shear parameters may become more favorable than they are today (which would enhance storm organization), timing and evolution will likely determine degree of coverage of the strongest storm. Gusty winds and a few instances of large hail are the primary hazards. A cold front will approach the region Monday before pushing across the region Monday night. Showers and thunderstorms will develop along and ahead of this front...some of which could be strong. Highs will reach the 90s again. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... A cold front should stall across the Virginia and North Carolina border Tuesday, allowing for a few showers to linger across our southern areas. High pressure will build in from the west and northwest Tuesday night and Wednesday. Temperatures will be nearly 10 degrees cooler than previous days with a comfortable relative humidity. The next cold front will arrive and push across the region Thursday into Friday. Another active pattern of showers and thunderstorms during the period and could linger into the weekend if the front stalls across portions of the region. A second area of high pressure will build in for the weekend, bringing drier and cooler conditions. && .AVIATION /01Z SUNDAY THROUGH THURSDAY/...
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Thoughts have not changed from this afternoon. VFR conditions will prevail through Monday...outside of thunderstorm activity. Multiple rounds of thunderstorms are possible during this time (with the best chance during the afternoons and evenings) with gusty winds and reduced visibility in the strongest storms. A third round of thunderstorms approaching this evening, and should be affecting the terminals for 2-3 hrs between 9pm and 1am. More storms likely tomorrow. Vfr conditions expected Tuesday through Wednesday night. Winds north 10 knots Tuesday, becoming northeast 5 to 10 knots Tuesday night. Winds light and variable Wednesday and Wednesday night.
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&& .MARINE...
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No changes to thoughts from this afternoon. Winds will remain below SCA values through Monday. However, strong thunderstorms capable of producing gusty winds above Special Marine Warning criteria are possible tonight and again Sunday/Monday. No marine hazards expected Tuesday through Wednesday night. Winds becoming northwest 10 knots Tuesday, then northeast 5 to 10 knots Tuesday night. Winds light and variable Wednesday and Wednesday night.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...None.
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&& $$ SYNOPSIS...MSE NEAR TERM...HTS SHORT TERM...MSE LONG TERM...KLW AVIATION...MSE/HTS/KLW MARINE...MSE/HTS/KLW

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