Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 070753 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 253 AM EST Thu Dec 7 2017 .SYNOPSIS...
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Ridging will remain in place through early Friday. Low pressure will pass east of the area late Friday into early Saturday. A clipper-like system will cross the region Saturday evening. High pressure will return Sunday into Monday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Dense high cirrus continue to trek from southwest to northeast across the area early this morning. This is due to the right entrance region of an upper level jet streak. The jet will weaken and pivot gradually southeast through the morning, which should result in a gradual (though not a total) reduction in high cirrus. A weak perturbation in the 700-500 mb flow will cross the region this afternoon and evening. A thin deck of low-to-mid- level cloudiness (broken by mid to late afternoon) is likely with this wave. It may also kick off a few light snow showers along the Allegheny Front early this evening, but any accumulation should be minimal. More dense high cirrus is expected to build from south to north across the area in response to another upper jet streak well ahead of developing low pressure over the far southeastern U.S.
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&& .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
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Aforementioned surface low pressure (weak and elongated) will pass well to the southeast of the area over the western Atlantic late Friday into early Saturday. Most (if not all) of the low to mid level forcing will remain out at sea. However, PVA ahead of the 500 mb trough and the right entrance region of a very strong upper jet should expand the light northwestern edge of the precipitation shield into at least southern Maryland late Friday into Friday night. At the onset, boundary layer temperatures will be marginal, so anticipate a light rain/snow mix. After sunset, temperatures cool and precipitation becomes all (light) snow. There is still a small subset of guidance that expands precipitation much further north and west, but given the current low track, find it hard pressed that anything measurable would make it west of I-95. Therefore, have confined PoPs to these areas, most likely for Saint Mary`s County. Given the light nature of the precipitation and the marginal boundary layer temperatures for a good portion of the event, snowfall accumulations should be an inch or less and confined to far southern Maryland. As the surface low ejects into the open Atlantic, a strong upper low/associated clipper-like surface feature will pivot from west to east late Saturday afternoon through Saturday evening. Believe scattered snow showers will result in typically favored upslope areas as well as northern and central Maryland.
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&& .LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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By Sunday morning, the coastal system will be well into the Atlantic, and the axis of the initial shortwave will be passing through. A second shortwave will cross the area Tuesday afternoon-night. Much like Saturday night, the primary consequence will be scattered snow showers... primarily across favored upslope areas, but would not preclude flakes just about anywhere in the area. The other result of this weather pattern will be a steady stream of cold air, which will be reinforced by the passing shortwave. The first day/night after these shortwaves (Sunday-Sunday night and Tuesday night-Wednesday) will be the coldest, with highs in the 30s and lows in the 20s. However the other periods won`t be that much better.
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&& .AVIATION /07Z THURSDAY THROUGH MONDAY/...
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VFR expected through Saturday night. Any restrictions in CIGS/VSBYS should be confined to S MD. Elsewhere, very brief restrictions are possible in snow showers, most likely Sat eve over highlands and near/N of Potomac River. Surface ridging likely results in light westerly flow AOB 10 kts through Sat, becoming NW 10-20 kts and gusty Sat night in the wake of a cold frontal passage. VFR should prevail Sunday-Monday.
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&& .MARINE...
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SCA in effect for middle MD portion of Chesapeake Bay and lower tidal Potomac through noon. Gradient just strong enough for some 20-knot gusts during this time. Winds go light this afternoon through Saturday before increasing out of the northwest Saturday night. SCA will likely be needed by Saturday night, with gale gusts possible as well by Sunday morning. A strong Small Craft seems likely on Sunday, although wouldn`t rule out close to Gale early. Winds will gradually subside through Monday.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until noon EST today for ANZ532>534-537- 540-541-543.
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&& $$ SYNOPSIS...DHOF NEAR TERM...DHOF SHORT TERM...DHOF LONG TERM...HTS AVIATION...HTS/DHOF MARINE...HTS/DHOF

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