Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 170100 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 800 PM EST Sat Dec 16 2017 .SYNOPSIS... Weak backdoor front will slip south into the area late tonight and Sunday. A weak upper level disturbance will cross the area Sunday night. A warm front will lift north across the region Monday. A cold front will drop south across the region Tuesday night. High pressure will dominate Wednesday and Thursday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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Forecast looks on track, so made no significant changes. Just nudged to current observations, with traditional radiational cooling spots cooling off a bit quicker this evening. Otherwise, mid and high clouds come in after midnight likely causing temperatures to level off. A weak backdoor front was dropping south of the NY/PA border as of early this evening, but with no precipitation associated with it. Previous discussion... High pressure dominates at present, centered to the south, but a cold front is slowly slipping toward us from the north. The front will drop into the region late tonight, but with limited if any sensible weather impact. High pressure will be the true dominating factor in the weather tonight, with radiational cooling allowing temps to drop back below freezing in most areas. Winds will lighten quickly after sunset.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY NIGHT/...
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Backdoor front stalls out in the area Sunday before essentially dissipating. Thus, temps on Sunday, while starting chilly, should not be significantly different from today. However, clouds will be increasing ahead of a weakening shortwave which is currently over western Texas. The clouds will move in, but the rain will be falling apart as the shortwave rides over the top of a ridge to our south and forcing falls apart. Temps will be marginal, so will have to look for potential mixed precip at elevation and at night, mainly along the fringes of the CWA (Allgheny Front and Mason-Dixon line), but right now it looks like impacts will be minimal. (730 PM update...added a slight chance of freezing drizzle for the Allegheny Front north of Pendleton County WV given temps of 29-32 F and shallow moisture). Warm front lifts north Monday with temps rising into the 50s most areas. Upslope showers may linger along Allegheny Front, but most areas dry. Staying mild at night with lows above freezing most spots.
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&& .LONG TERM /TUESDAY THROUGH SATURDAY/... Split upper level flow will be in place during the middle of the week. Tuesday will likely be mild (50s to perhaps lower 60s) if not a bit cloudy in between northern and southern stream systems with a light southwest surface flow. Trough axis from northern stream system will drop through the area by Tuesday night, although it will be dry outside of a little upslope snow. Wednesday could be breezy in the wake of the trough, and temperatures will return closer to normal. The 12Z model suite is in a bit better agreement with keeping a southern stream low pressure system suppressed to the south (i.e., Carolinas) Wednesday afternoon and night. With that said, forecast QPF is close enough to central VA that it will bear watching since precipitation could take wintry form. High pressure slides across the Great Lakes toward New England Thursday into Friday. The high eventually becomes in a position favorable for cold air damming, although at this time there aren`t any strong indications of clouds or precipitation, as a subtropical ridge encroaches on the southeastern states. This is subject to change pending any weak waves. Temperatures should remain fairly close to normal. The next upper trough will push toward the east coast Friday into Saturday. This is the time period where guidance begins to deviate the most since the northern and southern streams could eventually phase. Meanwhile the subtropical ridge could attempt to stay dominant to the southeast. Precipitation chances will be increasing since the associated cold front could be slow moving. Precipitation types could eventually become tricky over parts of the area depending where the baroclinic zone settles and if additional energy rides along it. Overall not looking at any major blasts of cold air within the forecast period though. && .AVIATION /01Z SUNDAY THROUGH THURSDAY/...
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VFR likely through Monday night. Main concern is isolated showers late Sunday/Sunday night which may briefly drop cigs and vis, mainly near/northwest of MRB to MTN. Winds will stay fairly light through Monday night. VFR is anticipated for Tuesday through Thursday at the moment, although a low pressure system won`t be too far away to the south on Wednesday that is worth watching. Northwest winds could gust to 25 kt on Wednesday behind a low pressure trough.
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&& .MARINE...
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Winds have subsided, so dropped the SCA. Light winds are anticipated through Monday night. Southwest winds will begin increasing Tuesday. A trough of low pressure will cross Tuesday night, with winds becoming northwesterly and increasing further. A Small Craft Advisory will be possible through Wednesday. Lighter winds can be expected Wednesday night into Thursday as high pressure builds into the area.
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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...RCM NEAR TERM...RCM/DHOF SHORT TERM...RCM/DHOF LONG TERM...ADS AVIATION...ADS/RCM/DHOF MARINE...ADS/RCM/DHOF

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