Area Forecast Discussion
Issued by NWS Sterling, VA

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304 FXUS61 KLWX 090833 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 333 AM EST Sat Dec 9 2017 .SYNOPSIS...
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Low pressure will bring the first accumulating snow of the season to much of the area through 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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Clouds continue to lower and thicken as snow develops aloft and low levels saturate further. The going snowfall forecast appears on track, albeit a bit slower. Due to the slower onset, adjusted start time of WSW along PA border back a couple hours (starting around daybreak), and nudged snowfall amounts to tighten the gradient. There has been a lot of convection developing offshore of the Carolinas early this morning. Without a negatively-tilted upper trough and strong easterly 850 mb flow advecting in moisture, this convection may act to rob some precipitation (moisture used up in the convection further south instead of producing snow farther north). The latest HRRR/RAP subsequently have slightly lower QPF amounts but it is still within tolerance with the current forecast, so would rather stay the course for now and monitor trends rather than make any substantial changes. Northern and southern stream upper jets phase over the area this morning resulting in widespread light to at times moderate snow. A few bands of heavier snow are likely given the strong dynamics aloft. Low level thermal profiles show just enough warming for a little mix with sleet in parts of extreme southern Maryland, but considering they`ve already had a few inches of snow storm totals should still be in the 5 to 8 inch range. Generally 3 to 4 inches is anticipated in the metro areas, with 1 to 3 inches as far west as I-81. Overnight guidance hints at some lingering light snow/snow showers until around nightfall in the I-95 corridor. Precipitation should become more showery in nature by this time as well as more isolated and sporadic. 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/...
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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.
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&& .LONG TERM /TUESDAY THROUGH FRIDAY/...
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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.
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&& .AVIATION /07Z SATURDAY THROUGH WEDNESDAY/...
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IFR/possible LIFR likely by around 12z, 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...
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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.
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&& .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 MDZ006-011-013-014-503>508. Winter Weather Advisory until 4 PM EST this afternoon for MDZ004-005. Winter Storm Warning until 4 PM EST this afternoon for MDZ016>018. VA...Winter Weather Advisory until 4 PM EST this afternoon for VAZ025-026-029-030-036>040-050>054-501-502-506>508. Winter Weather Advisory until 4 PM EST this afternoon for VAZ505. Winter Storm Warning until 4 PM EST this afternoon for VAZ055>057. WV...None. MARINE...Small Craft Advisory from midnight tonight to 6 PM EST Sunday for ANZ530>533-535-536-538>542. Small Craft Advisory from 5 AM early this morning to 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...DHOF NEAR TERM...DHOF SHORT TERM...DHOF LONG TERM...HTS AVIATION...HTS/DHOF MARINE...HTS/DHOF

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