Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 180736 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 336 AM EDT Tue Apr 18 2017 .SYNOPSIS...
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High pressure will build over the area through early Wednesday. The next cold front will cross the area Friday. High pressure builds over New England for the weekend while low pressure moving in from the west will impact the Mid Atlantic Sunday.
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&& .NEAR TERM /TODAY/...
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As of 3am, surface high pressure is across the NErn CONUS and Ern Canada under an upper trough (axis is Burlington, VT to Pittsburgh). High clouds persist over the LWX CWA as we are in the right entrance region of the SWly jet on the east side of the trough. The upper trough axis will shift east off the northern Mid-Atlantic coast around 12Z, which should make for a mostly sunny sunrise. The trailing surface ridge axis will shift east across the area late morning through midday. A light onshore flow will develop through this time - the onset of cold air damming. Abundant sunshine will overcome light mixing today, low 70s I-95 and west, upper 60s east of I-95 where the onshore flow will be most notable (Bay water temps are upper 50s).
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&& .SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/...
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Ongoing light rain over SWrn VA should enhance development of cold air damming across the area by this evening. A good general rule is 24 hours from onshore flow onset to drizzle onset. Therefore, patchy drizzle and chances for rain were added to the forecast Wednesday afternoon. Max temps decreased to around 60F inland. The GFS and ECMWF have the wind shifting S to SWly Wednesday night. However, another good rule is to not believe models that erode CAD overnight. Therefore, the rain and drizzle were maintained until 6am Thursday. Low pressure develops as it shifts east across the Great Lakes Thursday. An associated warm front will lift north across the central Mid-Atlantic with a threat for elevated and becoming surface based thunderstorms spreading in from the SW through the day. 12 to 14C 850mb temps overspread the area with MaxT in the 80s.
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&& .LONG TERM /FRIDAY THROUGH MONDAY/...
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A cold front will be slowing southward through the Mid Atlantic Friday. This will produce low chances for showers...but temperatures should still be able to climb to the lower 80s south/mid to upper 70s north. Saturday an induced high pressure wedge will move over the region allowing for a partly sunny and cool day. Highs will be below normal - in the lower to mid 60s. Sunday not looking too great: high pressure will move off to the east allowing upper level low pressure to move into the Appalachians. The atmosphere should be stable so am forecasting the p-type as straight rain. Likely PoPs. Highs only in the upper 50s/lower 60s. The upper low may be slow to dislodge itself from the east coast, keeping the area mostly cloudy Monday. Highs still in the 60s.
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&& .AVIATION /07Z TUESDAY THROUGH SATURDAY/...
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VFR under high pressure today as light Nly flow shifts east. Onshore flow then into Wednesday night as cold air damming sets up. IFR conds/rain/drizzle developing Wednesday afternoon and continuing as late as Thursday morning. A warm front lifts over the area Thursday with southerly flow and scattered thunderstorms possible over the DC metros in the afternoon. Showers expected Friday, then VFR Saturday.
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&& .MARINE...
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Light Nly flow becomes onshore today as high pressure moves east over the area. Onshore flow then through Wednesday night as cold air damming sets up. Warm front lifts north through the area Thursday with afternoon/ evening thunderstorms possible. Small craft advisory may be needed for Friday into Friday night with wind gusts reaching near 20 kt.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ .PRODUCTS...BAJ/WOODY!

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