Area Forecast Discussion
Issued by NWS Sterling, VA

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771 FXUS61 KLWX 280045 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 845 PM EDT Mon Mar 27 2017 .SYNOPSIS... A cold front will cross the area late Tuesday. High pressure builds over the region Wednesday into Thursday. Another disturbance will approach the area late Thursday into early Saturday. && .NEAR TERM /UNTIL 7 AM TUESDAY MORNING/... Although instability meager (around 500 J/Kg on 00z LWX RAOB), it`s been sufficient for a line of scattered showers to develop in the Shenandoah Valley. This cluster is now (as of 8:30 pm) emerging off the Blue Ridge headed toward DC. Mesoscale guidance has been suggesting such a scenario since late this afternoon. Behind that, the nearest precipitation is in the Ohio Valley. However, dewpoints are on the higher side (in the upper 50s) and positive vorticity advection will be increasing overnight. Have targeted PoPs in the database for the evening hours, per CAMs, but will be transitioning to a more widespread but low-end probability for the pre-dawn hours as forcing grows. The other consideration will be the fog potential overnight, as there will be plenty of low-level moisture and little flow (but with an easterly component). Am still going with patchy wording but for a larger portion of the domain. (Visibility values are lower too, although that`s not reflected in public forecasts.) Will need to see how things transpire due to residual cloudcover, which may be an impediment. && .SHORT TERM /7 AM TUESDAY MORNING THROUGH WEDNESDAY/... Another warm day tomorrow as moisture/warmth pools ahead of cold front. SPC continues to delineate the southern half (i.e., from DC south) and east of the Blue Ridge in a marginal risk for severe weather. While CAPE is higher tomorrow (e.g., ~500-800 J/kg MLCAPE with the higher amounts to the south) shear is also considerably less (i.e., ~20-25 knots). Thus, expect the possibility of a few poorly-organized marginally-severe thunderstorms across the risk area. Any threats expected to be isolated and confined to damaging wind gusts and small hail. Chances for precipitation should be ending late Tuesday evening as the shortwave moves off the coast and cold front pulls away from the area. High pressure returns Tuesday night and Wednesday. Temperatures Wednesday cool nearly ten degrees from Tuesday (though still remain above normal). && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Thursday starts out dry as a high pressure builds into the NE CONUS. Meanwhile, a strong closed low will be moving northeast from the southern plains and into the Mid-Atlantic. This system will bring decent amounts of moisture into the region, with rain starting Thursday night and remain into sometime Saturday. Many GEFS members bring the highest QPF amounts on Friday night. Conditions become dry behind this system Saturday and into Monday. High temperatures will be in the 50s and 60s, and lows in the 40s. && .AVIATION /01Z TUESDAY THROUGH SATURDAY/... A couple of showers are meandering near IAD this evening, but are not causing any flight restrictions. These showers heading toward DCA. Expect the same result. Flow will be light tonight. In the DC/Baltimore corridor, there will be an onshore component, with relatively high dewpoints and potentially patchy wet ground. That means that fog development is a real possibility. Guidance has been emphatic that it once again will be dense. Am uncertain if that is an artifact of model bias. It still looks like the best chances for fog or low clouds will be at IAD/MTN/BWI/DCA. Went lower than previous TAFs, but still kept forecast at IFR due to reduced confidence. A chance for thunderstorms returns to the terminals Tuesday afternoon...with the possibility of gusty winds and small hail in the strongest storms. Conditions should improve late Tuesday evening through Wednesday with the return of VFR as high pressure builds into the area. Dry/VFR conditions expected for Thursday before rain approaches over the area Thursday night, possibly bringing sub-VFR conditions over the terminals Friday into early Saturday. && .MARINE... LIght winds on the waters tonight. Thunderstorms will again be possible Tuesday afternoon, some of which could produce SMW- level winds. Wind gusts will then increase Tuesday night into Wednesday behind frontal passage...with SCA possible as early as early Wednesday morning...but likely during the day Wednesday. Winds expected to diminish later Wednesday. Dry conditions expected for Thursday before rain approaches over the area Thursday night bringing showers over the waters into part of Saturday. Wind gusts will be near the small craft advisory threshold mainly Friday night into Saturday... therefore SCA possible at times. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...MSE NEAR TERM...HTS SHORT TERM...MSE LONG TERM...IMR AVIATION...HTS/MSE/IMR MARINE...HTS/MSE/IMR is the U.S. government's official web portal to all federal, state and local government web resources and services.