Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 221841 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 241 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 /THROUGH TONIGHT/...
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Complex weather situation continues to unfold across the area...as initial MCS is currently impacting the northern half of the forecast area between DC and Balt. Early arrival of QLCS has tempered instability somewhat...which has kept the strongest updrafts rather isolated. Furthermore, poor mid-/upper-level lapse rates coupled with relatively weak shear have tended to limit vertical parcel accelerations...and will likely keep strongest winds more isolated-to-widely scattered. Greatest threat for damaging wind over the next hour will reside across N Prince Georges and Anne Arundel counties. While cloud cover will likely limit additional heating out ahead of the line...with MLCAPE values peaking near 1500 J/Kg...modest cold pool will allow for continued localized instances of wind gusts AOA 50 kts as this activity continues to push east across that area. Storms that are developing and moving into the NW portion of the area and across N WV will continue to sag into the area through the evening. While the current line may work over much of the northern half of the area, weak (but sufficient) instability may lead to isolated instances of gusty winds with the most vigorous activity as it crosses the area. Precipitable water has surged to near 2 inches this...which would allow for locally heavy rainfall in the heaviest activity. However, storm speeds should keep hydro threats generally isolated. Though, a slightly greater threat could develop if cell training allows for multiple rounds of heavy rainfall over the same area. Heat Advisory also continues along and east of I-95 through today. Cloud cover has tempered warming a bit...keeping most places just shy of the 105F heat index threshold. Though, multi- day nature of threat justifies headline even if heat indices only marginally meet the criteria.
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&& .SHORT TERM /SUNDAY THROUGH MONDAY/...
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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.
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&& .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
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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.
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&& .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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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. 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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Winds will remain below SCA values through Monday. However, strong thunderstorms capable of producing gusty winds above Special Marine Warning criteria are possible late this afternoon and evening 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... DC...Heat Advisory until 7 PM EDT this evening for DCZ001. MD...Heat Advisory until 7 PM EDT this evening for MDZ011-013-014- 016>018-504-506. VA...Heat Advisory until 7 PM EDT this evening for VAZ052>057. WV...None. MARINE...None. && $$ SYNOPSIS...MSE NEAR TERM...MSE SHORT TERM...MSE LONG TERM...KLW AVIATION...MSE/KLW MARINE...MSE/KLW

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