Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 190213 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 913 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 SUNDAY/...
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Overall think the forecast for the cold front and upslope snow is on track, with only minor refinements to the grids. Have moved up the start of the Wind Advisory for the lower elevation zones west of the Blue Ridge to 1 AM, which is approximately when the front will enter the area. The nowcast portion of the forecast has been more challenging though, as per usual, models are trying to lift the warm front northward too quickly through the rain-induced cold pool in the ridge/valley region. The front has eked through Winchester in the past hour. Meanwhile south of the front, it has been quite gusty (SSW-NNE oriented valleys and VA Piedmont with some gusts 30-35 mph) and temperatures have surged into the upper 50s to lower 60s. Many areas will probably see temps rise as we pass midnight with low 60s across much of the CWA (except northwest) early Sunday morning as the cold front moves in. After a lull in precip expected this evening, showers will move in ahead of the cold front late 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.
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&& .SHORT TERM /SUNDAY NIGHT 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 /02Z SUNDAY THROUGH THURSDAY/...
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MRB is still stuck north of the warm front although the main issue now is light fog/mist. Winds now SE, so expect this concern to be alleviated shortly. 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 cigs, 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.
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&& .MARINE...
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In general winds will be sub-gale most of the night as the potent low level jet will have trouble mixing down, though winds are now approaching 30 knots in some locations. 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 warning remains in effect. 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.
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&& .TIDES/COASTAL FLOODING...
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Water levels have become steady on either side of a foot, with only Havre de Grace`s tide still occurring. 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. Given lower astronomical values, am thinking we can escape flooding, although it will be close at Straits Point and Annapolis.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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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 1 AM to 3 PM EST Sunday for MDZ003-502. Wind Advisory from 4 AM to 3 PM EST Sunday for MDZ004>006-011- 013-014-016>018-503>508. Coastal Flood Advisory until midnight EST tonight for MDZ508. 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 1 AM to 3 PM EST Sunday for VAZ027>031. Wind Advisory from 4 AM to 3 PM EST Sunday for VAZ040-052>054- 501-505-506. WV...Wind Advisory until 3 PM EST Sunday for WVZ501-503-505-506. Wind Advisory from 1 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...Gale Warning until 6 PM EST Sunday for ANZ530>543.
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&& $$ SYNOPSIS...RCM NEAR TERM...ADS/RCM SHORT TERM...RCM LONG TERM...KLW AVIATION...ADS/RCM/KLW MARINE...ADS/RCM/KLW TIDES/COASTAL FLOODING...DHOF/ADS

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