Area Forecast Discussion
Issued by NWS Sterling, VA

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180 FXUS61 KLWX 151451 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 951 AM EST Fri Dec 15 2017 .SYNOPSIS...
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Low pressure will develop along the North Carolina coast today, moving northeastward and out to sea tonight. High pressure will return for Saturday before weak low pressure impacts the area early next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Latest analysis shows low pressure developing off the North Carolina coast and an upper-level trough swinging through the Ohio Valley toward our area. The upper-level trough will pass through the area today. A strong jetmax aloft that is associated with the upper-level trough will swing through late this morning through this afternoon. Our area will be within the left exit region of the jetmax and that will enhance lift across our area which will also lead to the development of a coastal low off the North Carolina coast. This system is progressive...and that is due to energy digging into the Pacific Northwest and northern Rockies. This means that the system will not phase in time over our area. However...the jetmax is potent and this means that precipitation will develop overhead as it moves through and it will continue to develop and blossom as it is moving off to the north and east. Temperatures are cold enough for the p-type to be snow across our area. The best lift will be across the Washington and Baltimore metropolitan areas this afternoon into southern and central Maryland as well as northern Virginia. These areas have the best chance for accumulating snow...and again with this system developing overhead and moving northeast...the farther northeast you go the better chance for accumulating snow. Most likely snow accumulations are less than an inch south and west of the Potomac River across northern Virginia to around and inch near Washington DC...to 1-2 inches near Baltimore and northeastern Maryland. Farther west across central Virginia into the Shenandoah Valley...a brief period of light snow is possilbe late this morning through early afternoon but any accumulation will be light. A couple things to point out. First...temperatures were cold overnight and plenty of cloud cover will prevent temps from rising too much today. Even for areas that get above freezing...if it snows temperatures will drop below freezing due to evaporative cooling. Therefore...any snow that does occur will stick on paved surfaces and this will cause any untreated surfaces to become slippery. Slippery untreated surfaces will affect the evening commute for the Washington and Baltimore Metropolitan areas even if snow ends before. Second...there is a tight gradient overhead between no or little snow vs accumulating snow and that is setting up near the Potomac River. Therefore...any slight change in the gradient will have a significant impact on the forecast. This lowers confidence a bit. The snow will exit the area between 1 and 3 pm across central Virginia and the Shenandoah Valley and between 3 and 6 pm for most other areas. Snow could hold on another hour or two near Baltimore and extreme northeastern Maryland. High pressure will build overhead tonight...bringing dry conditions with temps below freezing. Slick spots that develop on untreated surfaces will last throughout the night.
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&& .SHORT TERM /SATURDAY/... High pressure builds in across the area on Saturday and Saturday night with dry conditions expected. Temperatures will top out during the day on Saturday in the upper 30s to middle 40s, with overnight lows in the 20s to near freezing in the metro area. High pressure will reside to our south on Sunday, resulting in southerly flow across the region. This will generate moderating temperatures in the mid to upper 40s. Conditions will remain dry during the day with high clouds beginning to stream overhead ahead of an approaching shortwave trough that will cross the region Sunday night and early Monday morning. There is a slight increase in precipitation as a result, but moisture looks limited and with marginal temperatures, most favored precipitation will be light rain showers across our northern zones. Lows overnight Sunday will bottom out in the mid to upper 30s. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... East coast 500 mb ridging will be the dominant feature for the extended forecast. Consequently, as shortwave energy traverses the CONUS Tuesday, the primary precip type will be rain. And, since the northern and southern streams will remain unphased, it`s looking like a lighter QPF amount for the DC/Baltimore metro. Will be carrying slight chance PoPs for most, chance for the mountains. High pressure and mild conditions will prevail for the balance of the week. && .AVIATION /15Z FRIDAY THROUGH TUESDAY/...
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Predominate VFR conditions are expected through Sunday. A period of light snow is expected across our eastern terminals this afternoon, which will result in IFR periods for IAD/DCA/BWI/MTN. High pressure builds in to the area Saturday and Sunday with light to moderate westerly winds of 10 knots or less. VFR will likely prevail Mon-Tue. There will be an impulse moving through, so precip and flight restrictions will be possible, but confidence low.
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&& .MARINE...
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Low pressure will develop to our south today and move out to sea tonight. A Small Craft Advisory is in effect for the Maryland Chesapeake Bay south of Drum Point and the lower Tidal Potomac River...closer to the gradient. High pressure will build to the south for tonight through Saturday...but a tightening gradient will result in SCA winds for portions of the waters. The gradient will subside Saturday night and Sunday. No hazards anticipated Monday into Tuesday. Winds will increase thereafter, but any flags may hold off til beyond current forecast period.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Winter Weather Advisory from 1 PM this afternoon to 7 PM EST this evening for DCZ001. MD...Winter Weather Advisory from 1 PM this afternoon to 7 PM EST this evening for MDZ004>006-011-013-014-503>508. VA...Winter Weather Advisory from 1 PM this afternoon to 7 PM EST this evening for VAZ054. WV...None. MARINE...Small Craft Advisory from 9 PM this evening to 7 PM EST Saturday for ANZ530>533-538>541. Small Craft Advisory from 6 AM to 7 PM EST Saturday for ANZ542. Small Craft Advisory from 3 PM this afternoon to 7 PM EST Saturday for ANZ534-537-543.
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&& $$ SYNOPSIS...BKF NEAR TERM...BJL/BKF SHORT TERM...BKF LONG TERM...HTS AVIATION...BJL/BKF/HTS MARINE...BJL/BKF/HTS

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