Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 090233 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 933 PM EST Fri Dec 8 2017 .SYNOPSIS...
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Low pressure system will impact the area overnight through Saturday. High pressure will return Sunday into Monday. Another low pressure will impact the area on Tuesday. Weak high pressure builds over the area for the second half of next week.
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&& .NEAR TERM /THROUGH SATURDAY/...
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Previous forecast looks pretty much on track. Have adjusted snowfall totals up to the highest advisory threshold 3-5" for the Baltimore and Washington Metro areas. Have expanded the Winter Storm Warning to include Spotsylvania and Stafford Counties. Prince Georges, Anne Arundel were on the cusp of getting upgraded to a warning too, but will wait for 00Z models for further evaluation. 12Z ECMWF has 850-700 thickness of 1555 bisecting St. Mary`s County at 06Z-12Z. This should delineate the rain/sleet/snow line. Looking for a local max snow accumulation band to extend through Calvert/Charles/King George Counties with lesser amounts to the southeast. To the northwest to the city centers, lesser QPF forecast but colder boundary layer will lead to near warning level snowfalls. Expecting a tight gradient to the northwest of the city centers, and currently putting that gradient through central Loudoun/northern Montgomery/central Frederick counties. Snowfall should get going in earnest after dawn and last through early afternoon, as two upper jet structures merge over the mid-Atlantic. We will be in the right rear quad of upper level jet centered over New England and the left front quad of upper level jet approaching from southeast US. They should merge by 12Z Saturday overhead, causing enhanced upward motion. The boundary layer will become much colder after noon Saturday, with another inch or two of fluffier snowfall before tapering to flurries at sunset.
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&& .SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/... Low pres moves farther offshore with precip ending but upper level trof crossing the area will result in snow showers over the mtns with a couple of inches of accumulation likely. Turning colder and windy late Sat night and Sun. && .LONG TERM /MONDAY THROUGH FRIDAY/... A highly amplified upper level flow pattern will continue to dominate the long term period with west coast ridging and east coast troughing. We`ll be in between shortwave troughs on Monday, so dry but chilly weather is expected. A surface low will be moving through the Great Lakes Monday night. Model consensus has the warm advection arm of this system dry except along the Appalachian crest, but if this changes, there could be mixed precipitation potential. The associated cold front and upper trough axis will swing through the area Tuesday and Tuesday night. Winds could be quite gusty behind the front. Upslope snows look likely, while there could be some snow showers or even squalls east of the mountains. Surface temperatures will likely remain above freezing (40s for early highs) until very late in the day though. The core of the cold air will be over the area Wednesday into Wednesday night. It`s possible some areas could fail to reach freezing for highs on Wednesday, then fall into the teens Wednesday night. There could be additional opportunities for flurries during this time. High pressure will nudge into the area from the south on Thursday, but the next shortwave trough will approach on Friday, likely driving another surface low through the Great Lakes. There`s some additional spread in the guidance by this point, although there will be a chance of wintry precipitation across parts of the area. && .AVIATION /03Z SATURDAY THROUGH WEDNESDAY/... Developing IFR/LIFR conditions in light/moderate snow late tonight into Sat. Precip ends by 21Z Sat with improving conditions Sat evening. Winds strengthen from the NW Sat night with gusts up to 30 kt possible into Sun. Winds diminish Sun night. An interval of sub-VFR clouds is possible Monday night. Brief visibility impacts in scattered snow showers will be possible Tuesday into Tuesday night. Wind gusts over 30 kt possible Tuesday through Wednesday. && .MARINE...
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Light and variable winds increasing late tonight across the southern waters. SCA issued earlier. Winds strengthen across all waters Sun with solid SCA conditions everywhere. Chance of gales appears relatively low attm. Light southerly flow is expected Monday with weak high pressure. Small Craft Advisory conditions could begin Monday night, but become more likely with winds out of the west-northwest Tuesday into Wednesday after a cold front sweeps across the area. A period of gale force winds will also be possible.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Winter Weather Advisory until 4 PM EST Saturday for DCZ001. MD...Winter Weather Advisory until 4 PM EST Saturday for MDZ006-011- 013-014-503>508. Winter Weather Advisory from 4 AM to 4 PM EST Saturday for MDZ004-005. Winter Storm Warning until 4 PM EST Saturday for MDZ016>018. VA...Winter Weather Advisory until 4 PM EST Saturday for VAZ025-026- 029-030-036>040-050>054-501-502-506>508. Winter Weather Advisory from 4 AM to 4 PM EST Saturday for VAZ505. Winter Storm Warning until 4 PM EST Saturday for VAZ057. Winter Storm Warning until 4 PM EST Saturday for VAZ055-056. WV...None. MARINE...Small Craft Advisory from midnight Saturday night to 6 AM EST Sunday for ANZ530>533-538>542. Small Craft Advisory from 5 AM to 4 PM EST Saturday for ANZ533- 541-542. Small Craft Advisory until 6 AM EST Sunday for ANZ534-537-543.
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&& $$ SYNOPSIS...LFR NEAR TERM...Lee SHORT TERM...LFR LONG TERM...ADS AVIATION...ADS/Lee/LFR MARINE...ADS/Lee/LFR

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