Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 061533 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 1033 AM EST Wed Dec 6 2017 .SYNOPSIS... Weak high pressure will build into the area today and Thursday in the wake of last night`s cold front. Low pressure will pass to the east Friday and Friday night, then another low pressure will cross the region Saturday and Saturday night. High pressure will then return Sunday into Monday. && .NEAR TERM /THROUGH THURSDAY/...
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There will be some clearing in nw parts of the area today while dense cirrus persists over the southern and eastern part of the fcst area. Winds will gradually diminish through the day as pres gradient weakens. Temps will likely hold steady in the low to mid 40s. Tonight and Thursday, high pressure will continue to press in from the west, but with the front stalling along the southeast coast, we will continue to see high clouds streaming overhead. Cold advection will push temps below freezing in most spots tonight, but radiational cooling will not be exceptional with the high clouds and some lingering breezes. Warming on Thursday will also be modest with the high clouds, with temps likely to return to the 40s.
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&& .SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
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On Thursday night, the front which crossed our region overnight will remain stalled just off the southeast coast. High clouds will continue to stream overhead while northerly winds will continue to advect cold air across the region. Lows will generally be below freezing. An elongated wave of low pressure will ride northeast along the stalled front Friday and Friday night as a shortwave moves in from the west. Models continue to differ somewhat on the details but most skirt southern Maryland with some light precipitation, and given the antecedant air mass, odds are high that some snow will at least mix in. Some guidance is considerably further northwest, with the 0Z GGEM bringing precip well northwest of I-95, but at this point this seems to be an outlier. EC is closer to the middle of the road, bringing light precip right up to I-95 but no further. This scenario could bring light accumulations to southern Maryland, perhaps an inch, but warm boundary layer would fight against much impact, and accumulations further north and west are highly uncertain. Will need to closely watch this system for any further adjustments in track, but it must be noted that guidance has been wavering back and forth between further NW 12Z runs and further SE 0Z runs, so it has been hard to find much of a trend. Highs on Friday will depend on the system so have gone with near 40 in much of the CWA... these will be colder if it is cloudier with more precip, but could be milder if it stays further east and gets a bit sunnier. Lows Friday night will again be below freezing.
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&& .LONG TERM /SATURDAY THROUGH TUESDAY/... There are two periods of interest in the long term, aside from the overarching theme of persistent colder-than-normal temperatures. 1) A strong clipper-like system Saturday night, and 2) another upper trough/clipper Tuesday into Wednesday of next week. The first system will likely bring at least scattered snow showers to parts of the area. There are several moving parts. Some of the latest guidance suggests a weak offshore low/baroclinic zone may linger into Saturday while the clipper moves in from the west. This subtle increase in moisture lends further support to the notion of snow showers Saturday night. The most likely areas are along the western ridges of the Allegheny Front (upslope), and northern Maryland/along the I-95 corridor in concert with juxtaposition of better mid/upper forcing and moisture. Reinforcing cold follows for Sunday, then the next system pivots around the upper trough Tuesday into Wednesday of next week. && .AVIATION /16Z WEDNESDAY THROUGH SUNDAY/...
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Generally VFR at the terminals through the TAF period and probably through Friday night. VFR should be the rule. Winds will diminish a bit this afternoon and tonight but may be a bit reinvigorated on Thursday due to insolation. Winds will relax again Thursday night and should remain relatively light Friday and Friday night. Main concern Friday into Friday night is a wave of low pressure passing to the southeast along a stalled front. There is a chance it brings some snow to the region, which could, at minimum, bring IFR cigs and vis, but as of this writing, odds of precip reaching the terminals is below 50 percent. Sub-VFR possible in any snow shower activity over the weekend. Winds generally light and variable Saturday, become NW 10- 20 kts Sunday.
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&& .MARINE...
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SCA for all waters through 6PM as cold high pressure slowly builds in behind the cold front. Winds probably subside on most waters tonight, though cold advection over relatively warm bay waters might keep SCA gusts going over the middle bay. Winds get a bit reinvigorated during the day Thursday so have issued new SCA for the daylight hours. After that, winds likely relax Thursday night and remain below SCA through Friday night, though a passing coastal low pressure could bring some rain and snow to the waters late Friday and Friday night, which at minimum could reduce visibility significantly. Odds of this happening are highest in the middle Chesapeake Bay. SCA gusts likely out of the NW Saturday night into Sunday behind a strong clipper. Gales possible.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 6 PM EST this evening for ANZ530>543. Small Craft Advisory from 6 AM to 6 PM EST Thursday for ANZ530>534-538>541-543. && $$ SYNOPSIS...RCM NEAR TERM...LFR SHORT TERM...RCM LONG TERM...DHOF AVIATION...RCM/DHOF MARINE...RCM/DHOF

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