Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 181956 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 256 PM EST Sat Nov 18 2017 .SYNOPSIS... A powerful cold front will cross the region late tonight. High pressure will slowly build south of the region late Sunday through Monday before passing off the East Coast on Tuesday. Another cold front will cross the region Tuesday night, with high pressure building back across the region for the latter half of the week. && .NEAR TERM /THROUGH TONIGHT/... Warm front has been very slowly pushing northward across the CWA this afternoon. As of this writing it stretches from near Bel Air MD southwestward to around Front Royal VA and west towards Petersburg WV. It should start to move again over the next few hours as southerly flow continues to increase just above the surface and ongoing rain showers diminish, so we still think warmer air will envelope the entire region this evening. Many areas will probably see temps rise as we pass midnight and we could be into the low 60s across much of the CWA early Sunday morning as the cold front moves in. Southerly winds have not been very effectively mixing down even in the warm sector, with gusts mainly in the 20 kt range, with a few gusts closer to 40 knots at the higher elevations of western VA and eastern WV where the wind advisory starts early. After a lull in precip expected this evening, showers will move in ahead of the cold front later this evening into the overnight, with potentially a line of gusty showers right on the front. Given strong pressure surge and potential for showers right on the front to effectively mix this wind down, decided to start wind advisory earlier than the synoptic wind would argue. Its possible that some areas see their strongest winds with the front itself. After frontal passage, temps will crash quickly back from high 50s and low 60s down to the 40s, with sub- freezing expected along the Allegheny Front. These areas will also get upslope flow, so snow is expected to start accumulating late tonight, and a winter weather advisory has been issued. Most of the accumulation will likely be after sunrise Sunday. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... Gusty winds are the main story as pressure rises rapidly behind the strong cold front on Sunday. Best wind aloft and mixing appear to coincide mid-late morning Sunday, with most likely region to get 45+ mph gusts being north-central MD, but given uncertainty decided to issue for all of DC metro and areas further west from there. As is typical, winds will diminish as we head south through the VA Piedmont, so no advisory for CHO metro. Temps will have difficulty rising with strong cold advection, but expect most places near I-95 to edge back into the 50s during the day. Upslope flow will continue Sunday morning along the Allegany Front, with continued snow showers. There may be a lull midday, with another surge in snow showers late in the day and early Sunday evening as another shortwave moves through. Thus, went a bit long on the advisory to catch both bursts. Accumulations will be somewhat elevation dependent, but local amounts of 6-7 inches aren`t out of the question. High pressure will then slowly build eastward to our south Sunday night and Monday. Upslope flow will diminish, allowing snow showers along the Allegany Front to end late Sunday night, and the gradient and winds aloft will both diminish as well, allowing winds to die down. That said, we still expect it to be breezy Monday. Otherwise, Monday will be cool but mostly sunny, with highs in the 40s to lower 50s after a night of below freezing temps region-wide. Not quite as chilly Monday night as the high slips further east, but still dry and mostly clear. && .LONG TERM /TUESDAY THROUGH SATURDAY/... A cold front will move across the lower Great Lakes region Tuesday and approach the mid-Atlantic region Tuesday night. There is some uncertainty with this front as to whether it produces precipitation or not late Tuesday and Tuesday night. The GFS model hints of a dry frontal passage as a broad area of high pressure covers the Southeast U.S., not allowing for a substantial moisture return flow. The European model hints of measurable rainfall across the eastern two-thirds of our region with high pressure farther offshore to allow for a modest moisture return flow. We will keep slight/chance of rain showers and slight/chance rain and/or snow showers in the west, for now. Temperatures will be a little chilly. High pressure will make an attempt to build in behind the weakening and departing cold front. Here again, there are discrepancies as to how far east the high will build in the mid- Atlantic and whether or not the preceding cold front will stall along the East Coast, allowing for a possible low to develop along it. The GFS model is weaker than the European with the building area of high pressure, while the GFS tries to develop a low along a stalled front. For now, we will lean toward dry and chilly conditions with high pressure building into the region Wednesday through Saturday. && .AVIATION /20Z SATURDAY THROUGH THURSDAY/... MRB has gotten stuck north of the warm front and has gone IFR with low cigs from the rain. Expect this to lift later this afternoon or evening, but timing it out is tricky given persistetnce of light rain and low level cold air. Elsewhere, gusty south wind and VFR cigs/vis with a stray shower is the rule through this evening. Later tonight, powerful cold front moves through with possible MVFR restrictions in showers, and gusts possibly up to 40 knots at frontal passage. VFR returns behind the front for Sunday and Monday, but gusty winds expected to continue Sunday with gusts still possibly reaching 40 knots, particularly BWI/MTN (much less chance at CHO). Winds diminish Monday as high pressure builds in. VFR conditions overall Tuesday through Wednesday night. Any rain showers could produce a brief period of MVFR conditions Tuesday night. Winds southwest 5 to 10 knots Tuesday and Tuesday night. Winds becoming northwest 5 to 10 knots Wednesday and Wednesday night. && .MARINE... SCA all waters at present as storm system approaches. SCA should be the rule tonight as the potenti low level jet will still have trouble mixing down. However, with the cold front passage late tonight, we could see gusts to 40 knots, particularly if a line of showers develops right on the front, so started the gale tonight. Best gale period is probably going to be mid-late morning Sunday, however, with diminishing winds thereafter. SCA will likely need to continue after the gale ends Sunday night and Monday, then sub-SCA may finally return Monday night for a time. Small craft advisories possible Tuesday ahead of a cold front. No marine hazards expected Tuesday night through Wednesday night. Winds southwest 10 to 15 knots gusts up to 20 knots Tuesday. Winds southwest around 10 knots Tuesday night. Winds becoming northwest around 10 knots Wednesday and Wednesday night. && .TIDES/COASTAL FLOODING... Strait`s Point peaked below flood stage this afternoon. The lower of the two astronomical tides is tonight so despite gradually increasing anomalies it shouldn`t hit again this evening. Highest anomalies are in the northern part of the bay so expect solid minor tidal flooding at Annapolis/Baltimore/Havre de Grace. DC SW Waterfront may come close as well but not enough confidence for a headline just yet but will monitor. It`s a bit odd to see Havre de Grace hitting minor but not Strait`s Point, probably because winds are more west of south near the mouth of the bay pushing water out, while winds north of southern MD are more southerly trapping water further north. Anomalies will begin to decrease as winds become westerly late tonight across the entire area, but water levels may not fall quickly enough to prevent flooding during tomorrow morning`s high tide cycle. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Wind Advisory from 4 AM to 3 PM EST Sunday for DCZ001. MD...Wind Advisory until 3 PM EST Sunday for MDZ501. Wind Advisory from 4 AM to 3 PM EST Sunday for MDZ003>006-011- 013-014-016>018-502>508. Coastal Flood Advisory from 7 PM this evening to midnight EST tonight for MDZ508. Coastal Flood Advisory until 9 PM EST this evening for MDZ014. Coastal Flood Advisory until 10 PM EST this evening for MDZ011. VA...Wind Advisory until 3 PM EST Sunday for VAZ025-026-503-504-507- 508. Wind Advisory from 4 AM to 3 PM EST Sunday for VAZ027>031-040- 052>054-501-505-506. WV...Wind Advisory until 3 PM EST Sunday for WVZ501-503-505-506. Wind Advisory from 4 AM to 3 PM EST Sunday for WVZ050>053-055- 502-504. Winter Weather Advisory from 4 AM to 11 PM EST Sunday for WVZ501-505. MARINE...Small Craft Advisory until 6 PM EST this evening for ANZ530>543. Gale Warning from 6 PM this evening to 6 PM EST Sunday for ANZ530>543. && $$ SYNOPSIS...RCM NEAR TERM...RCM SHORT TERM...RCM LONG TERM...KLW AVIATION...RCM/KLW MARINE...RCM/KLW TIDES/COASTAL FLOODING...DHOF is the U.S. government's official web portal to all federal, state and local government web resources and services.