Area Forecast Discussion
Issued by NWS Sterling, VA

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905 FXUS61 KLWX 150229 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 929 PM EST Thu Dec 14 2017 .SYNOPSIS... High pressure will build overhead through tonight. Low pressure will develop along the North Carolina coast Friday and move out to sea Friday night. High pressure will return for Saturday before weak low pressure impacts the area early next week. && .NEAR TERM /THROUGH FRIDAY/...
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Plenty of high clouds tonight tonight in response to strengthening jet max and low pres approaching from the southwest. Recent model trends have shown a little bit more QPF along the western shore of the Chesapeake Bay and westward extent of precip tomorrow afternoon resulting in a little bit of snow to the VA portion of the Capital Beltway. A 3-hr period of potential light snow accumulation is possible mainly between 18Z-21Z. The NAM and Canadian hi-Res remain on the snowier side of the guidance while the Euro and latest GFS on the drier side. Looks like between a quarter to half inch of snow could accumulate along and east of I-95 based on the Euro and GFS QPF using 13 to 1 SLRs. On the worst case scenario, up to 1.5 inches could accumulate using the less reliable NAM and Canadian hi- res models. Previous discussion... Northern stream upper level trough will be quickly approaching the region during the morning Friday. At the same time, additional shortwave energy will be rapidly pushing eastward from the lower Mississippi River Valley and across the Appalachians by the afternoon. These two shortwaves are expected to remain un-phased until well offshore, which will keep any precipitation on the low side. That being said, surface low pressure is expected to develop along the baroclinic zone left by today`s frontal passage near the North Carolina coastline Friday morning and move northeastward and offshore Friday afternoon. This now looks like it will be just close enough, when combined with the forcing from the northern stream trough and intensifying jet max to bring some light snow to portions of the region on Friday. Model differences still exist with respect to westward extent, but the trends today have been for high chances for our region. Have increased snow probabilities to likely across portions of southern Maryland and to chance as far west as the I-95 corridor for late Friday morning, peaking in the afternoon, and ending during the early evening. Have shown accumulations of less than one inch for the Maryland counties that border the Chesapeake Bay, going to a trace or less west of I-95. However, uncertainty is still quite high at this time. Highs during the daytime remain on the cold side, in the 30s area-wide.
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&& .SHORT TERM /FRIDAY NIGHT/...
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Flow will turn northwest following the system`s departure Friday night, with a brief period of upslope snow showers likely along the Allegheny Front. Accumulations will be low though as moisture will be quite shallow, up to about an inch of snow or so. Lows Friday night in the 20s. High pressure will then build in south of the region on Saturday, with dry conditions expected. Highs Saturday in the upper 30s to mid 40s and lows Saturday night in the 20s.
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&& .LONG TERM /SATURDAY THROUGH THURSDAY/... High pressure, centered to our south on Sunday, will generate southerly flow over our region, allowing for high temperatures to reach the mid to upper 40s for most of our CWA. A shortwave trough will move across the area Sunday night into early Monday. This will increase PoPs over our area, but still uncertain how much QPF will be associated with it and any p-types. Guidance suggests that mid to upper level energy could bring a slight chance of precipitation between Monday and Tuesday with still above normal temperatures -reaching the 50s. A front could bring a chance of precipitation Tuesday night into early Wednesday. High pressure builds behind this front Wednesday into Thursday bringing dry and more near-normal high temperatures. && .AVIATION /02Z FRIDAY THROUGH TUESDAY/... Predominantly VFR conditions are expected through Saturday night. Gusty northwest winds up to around 25 knots are expected through today, diminishing by sunset. A period of light snow or snow showers are possible across the eastern terminals later Friday...but confidence is low at this time. This may bring a period of reductions, with highest chances at DCA/BWI/MTN. High pressure will then build in for Saturday. Mainly VFR conditions expected early Sunday into Tuesday. However periods of sub-VFR conditions are possible at times as showers could move through the terminals. && .MARINE... SCA is in effect through the rest of the afternoon for all waters with gusty northwest winds following the frontal passage this morning. Winds will gradually lessen this evening and tonight, but will likely remain gusty enough over much of the Chesapeake Bay and lower tidal Potomac to keep SCA going until midnight. Winds will then drop below SCA criteria later tonight and remain that way through Friday as high pressure moves overhead. Low pressure will then develop and move out to sea later Friday. Gusty winds will develop behind the front Friday night into Saturday. SCA goes into effect Friday night and may need to be extended into Saturday. Winds are expected to stay below small craft advisory threshold Sunday into Tuesday. Winds could increase Tuesday night. && .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until midnight EST tonight for ANZ531>534- 537-539>541-543. Small Craft Advisory from 8 PM Friday to 6 AM EST Saturday for ANZ530>534-537-539>541-543.
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&& $$ SYNOPSIS...MM NEAR TERM...LFR SHORT TERM...MM LONG TERM...IMR AVIATION...MM/IMR MARINE...MM/IMR

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