Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 080241 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 941 PM EST Thu Dec 7 2017 .SYNOPSIS... Surface ridge 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. && .NEAR TERM /THROUGH FRIDAY/...
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A weak cold front will pass through the area overnight. The boundary will have little moisture with it...so no precipitation is expected...but a wind shift to the west and northwest is expected behind the boundary. The longwave upper-level trough will remain to our west and our area will remain in the right entrance of a strengthening upper-level jet. Therefore...a bkn to ovc deck of high and mid-level clouds are expected over our area during this time. This will prevent radiational cooling to some extent...so min temps are forecast to range from the teens in the Allegheny Highlands...to the mid and upper 20s near Washington and Baltimore...to the lower and middler 30s in downtown Washington/Baltimore as well as southern Maryland. The longwave upper-level trough will west for Friday and a southwest flow aloft will usher in more high and mid-level clouds. Surface high pressure will build into the region from the west. Dry and chilly conditions are expected for most areas Friday with high temps in the upper 30s to lower 40s for most areas. Southern stream moisture will advance northward a bit for Friday...and this may be enough for light rain/snow across southern Maryland...especially during the afternoon. The lack of forcing should keep this moisture confined toward our extreme southeastern areas though. Any snowfall that does occur Friday afternoon should not accumulate with surface temps well above freezing.
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&& .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/...
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12 and 18Z model trends now show a more amplified sfc low pres system off the mid-Atl coast Fri night through Sat. This results in precip shield reaching farther NW into the cold air Fri night and Sat with latest Euro and GFS showing measurable precip as far west as the Blue Ridge mtns. Given these trends and better agreement between GFS and Euro increased PoPs and QPF on the nw side bringing chance of snow to the Rt 15 corridor with 1-3" most likely across the Washington and Baltimore Metro areas into southern Maryland. With the gradient between no snowfall and accumulating snowfall directly overhead...this lowers the certainty in the forecast because any slight change in the track will have a significant impact on the forecast. Also...temperatures are marginal. In fact much of the time it snows it may be near or slightly above freezing. This means that most of the accumulation will be on grassy and elevated surfaces. However...a slushy coating of snow cannot be completely ruled out...especially with snow early Saturday morning and again early Saturday evening before it moves out. Drier air will move in overnight Saturday...but a few snow showers cannot be ruled out with the northern stream shortwave passing through. Most of the accumulating snow should remain near/west of the Allegheny Front. More importantly...any slushy or standing water will freeze Saturday night with temps dropping below freezing. High pressure will build overhead for Sunday...bringing dry and chilly conditions. Sunshine should cause any icy roads to melt later Sunday morning. The high will shift to our south and east Sunday night and it will remain chilly.
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&& .LONG TERM /MONDAY THROUGH THURSDAY/... A cold front will sag across the Lower Great Lakes region Monday before pushing southward across the mid-Atlantic region Monday night and Tuesday. At the same time, a low pressure system will connect with the front then shove off to the east and offshore by late Tuesday. Ahead of the front and low pressure system will be the chance for rain and snow showers each period. Too early to tell if there will be any snow accumulations east of I-81, while the better chances for accumulations will be in the mountains with the passing energy and upslope flow. Temperatures will be around 10 to maybe 15 degrees below normal. While a ridge of high pressure is expected to keep most of the region dry but chilly Wednesday through Thursday, there is a potential for additional snow accumulations west of I-81, particularly in the Appalachian Mountains as a few more upper level balls of energy move throughout the Ohio Valley and mid-Atlantic region. Temperatures will remain below normal. && .AVIATION /02Z FRIDAY THROUGH TUESDAY/...
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VFR conditions are expected through Friday with a bkn/ovc deck of high and mid-level clouds. A brief period of west to northwest wind gusts around 20 knots are possible overnight behind a weak cold front. A period of snow is likely late Friday night through Saturday for the terminals. IFR conditions are possible during this time. A light snow accumulation is most likely...but most of that should be on grassy and elevated surfaces. Still...a coating of snow cannot be ruled out on paved surfaces as well. Brisk late Sat night and Sun with NW winds gusting up to 30kt. VFR conditions Monday and Monday night. Any brief rain or snow showers could reduce conditions to MVFR briefly. Winds generally southwest 5 to 10 knots each period.
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&& .MARINE...
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A weak cold front will pass through the waters late this evening and overnight. A period of SCA wind gusts are possible. Have noticed some 18-22 knot gusts upstream this evening. A Small Craft Advisory is in effect for the Bay and lower Tidal Potomac River. A brief period of SCA gusts are possible across the other portions of the waters as well...but confidence is too low for a headline due to the nocturnal inversion setting in. Winds will weaken Saturday as high pressure builds toward the waters. Winds strengthen Sat night and Sun with solid SCA conditions expected. Gales are also possible Sun. No marine hazards Monday and Monday night. Winds mainly southwest 5 to 10 knots each period.
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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 6 AM EST Friday for ANZ530>534-537- 539>541-543.
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&& $$ SYNOPSIS...BJL/LFR NEAR TERM...BJL/LFR SHORT TERM...LFR LONG TERM...BJL/KLW AVIATION...BJL/LFR/KLW MARINE...BJL/LFR/KLW

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