Area Forecast Discussion
Issued by NWS Sterling, VA

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479 FXUS61 KLWX 091534 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1034 AM EST Sat Dec 9 2017 .SYNOPSIS...
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Low pressure passing to our east will bring accumulating snow across much of the area today. A cold front will cross the region tonight. A couple clipper-like systems will graze through the area next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Reflectivity`s on KLWX have been filling in this morning with light snow now encompassing much of the CWA. As a result of the slower onset of precipitation this morning and less moisture to work with than previously expected, have trimmed back snow total amounts a bit and removed Winter Storm Warnings from portions of southern MD and the I-95 corridor around Fredericksburg, VA. St. Mary`s county in MD remains in a Winter Storm Warning through this afternoon with total snow accumulations of 4 to 6 inches expected. For the latest snowfall forecasts and amounts, check us out on our winter weather page at weather.gov/lwx/winter. Northern and southern stream upper jets phasing over the area will continue the widespread light to at times moderate snow. A few bands of heavier snow are likely given the strong dynamics aloft. We have seen some bands set up west of the metro areas, extending from northern VA and into north central MD. Snow should begin to taper off late afternoon or early evening as is the consensus with model guidance, moving east of the I-95 corridor around nightfall. Western slopes of the Allegheny Front likely see a couple inches of snow as the northern stream wave/clipper moves across tonight. Held off on advisories for now since timing/amounts are threshold (2-3 inches in 12-18 hours). Some dry air could cut down on totals over the ridges.
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&& .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... Sunday looks dry, cold and breezy as an Arctic airmass moves into the area. Highs struggling to get out of the 30s and wind gusts up to 30 mph likely result in wind chills in the teens and 20s through the day. Another shortwave pivots across overnight Sunday night. This shortwave is much more pronounced in the NAM/GFS than the ECMWF, but still believe there will be enough forcing with the shortwave to get at least some scattered upslope snow showers. Temperatures will be a couple degrees warmer on Monday with less wind making it a bit more comfortable. All guidance then shows another shortwave/clipper late Monday night into early Tuesday. Warm air advection aloft over top cold air near the surface may result in a period of mixed wintry precipitation late Monday night into early Tuesday morning. && .LONG TERM /TUESDAY THROUGH FRIDAY/... By 12 UTC Tuesday, the trough axis would have swung through, bringing cold air advection back to the area. Since temperature profiles would be subfreezing throughout the column, any potential precipitation would revert back to snow...or melt if boundary later temperatures were warm enough. However, with downslope flow, its unclear how much precip there would be (mtns aside). Snow showers/squalls could jump the divide given sufficient shortwave support, but that aspect hasn`t been resolved yet. Will be maintaining a chance/slight chance of precip, with greater probabilities closer to the mountains...and trough axis. This would be a favorable pattern for several inches of upslope snow showers. Will bump PoPs up to likely Tuesday. Potential will gradually subside through Wednesday as low- level ridging builds. Heights will be moderating somewhat through the midweek, while remaining troffy. Then another shortwave will drop into the trough axis on Thursday. While agreeing on the overall concept, GFS and ECMWF depicting completely different tracks for the vortmax and surface reflection (western New York vs eastern Kentucky). That would have a substantial impact on the forecast late Thu into Fri. At this stage, believe its wise to have a chance of precip to the forecast Thursday, lingering into Thursday night. Suspect it primarily will be snow. && .AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
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IFR/possible LIFR likely through 18-20z in -SN. Moderate to heavy bursts of snow possible at times. VFR returns this evening. Light NE flow becomes NW 10-20 kts after midnight, with gusts to 25+ knots likely Sunday. VFR likely Monday, with sub-VFR possible late Monday night into early Tuesday morning. Mixed wintry precipitation is possible during this time. Winds will be light out of the south Monday into Monday night. Gusty winds on northwest flow will impact terminals Tuesday into Wednesday; 30 kt winds possible. Ceiling will be low, but its unclear if that will turn our as low VFR or high MVFR. There could also be brief vsby restrictions in a potential passing snow shower.
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&& .MARINE... SCA gusts expected out of the NE on the northwestern fringes of low pressure passing over the western Atlantic. Gusts will peak over the lower tidal Potomac River and lower Maryland portion of the Chesapeake Bay today. A lull is expected this evening before northwest flow kicks in after midnight. Solid SCA conditions are expected after midnight through Sunday, and could linger into Sunday night. Light winds are expected Monday before winds increase again ahead of a clipper late Monday night. Gusty northwest winds will impact the waters behind an upper level disturbance. It will be high end Small Craft criteria if not low end Gales. Have that possibility in the synopsis, and will add to the Hazardous Outlook as well. && .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Winter Weather Advisory until 4 PM EST this afternoon for DCZ001. MD...Winter Weather Advisory until 4 PM EST this afternoon for MDZ004>006-011-013-014-016-018-503>508. Winter Storm Warning until 4 PM EST this afternoon for MDZ017. VA...Winter Weather Advisory until 4 PM EST this afternoon for VAZ025-026-029-030-036>040-050>057-501-502-505>508. WV...None. MARINE...Small Craft Advisory from midnight tonight to 6 PM EST Sunday for ANZ530>533-535-536-538>542. Small Craft Advisory until 4 PM EST this afternoon for ANZ533- 541-542. Small Craft Advisory until 6 PM EST Sunday for ANZ534-537-543.
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&& $$ SYNOPSIS...BKF/DHOF NEAR TERM...BKF/DHOF SHORT TERM...DHOF LONGTERM...HTS AVIATION...HTS/DHOF MARINE...HTS/DHOF

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