Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 192001 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 301 PM EST Sun Nov 19 2017 .SYNOPSIS...
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High pressure will build to our south tonight through Monday night. The high will move offshore Tuesday and a cold front will pass through Tuesday night into Wednesday. High pressure will build back overhead later Wednesday through Thanksgiving Day. Coastal low pressure will likely move well out to sea Friday and a cold front will pass through Saturday.
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&& .NEAR TERM /THROUGH MONDAY/...
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Current surface analysis reveals deepening low pressure over the lowe St. Lawrence Valley heading northeast, with the cold front which passed through the area early this morning now well off the coast. High pressure remains centered well to the southwest over Texas, however. Aloft, a deep longwave trough dominates the eastern United States, with one shortwave crossing New England and a second approaching our area from Ohio. The approaching shortwave aloft and departing surface low will continue to dominate our weather tonight. As the surface low continues moving away, the gusty winds will continue slowly diminishing, but we are unlikely to really get light overnight as cold advection continues. As the shortwave passes, the snow showers over the Allegheny Front are likely to reintensify, so the winter weather advisory remains in effect for the higher elevations of western Pendleton and western Grant counties. The shortwave may in fact be enough of a forcing mechanism to get a few flurries or sprinkles to travel east of the mountains, most likely towards Martinsburg and Hagerstown, but possibly approaching the metro itself. There should be no impact from these except for a little wonder at seeing snowflakes for the first time this season, if it happens. Overnight, snow showers along the Allegheny Front should quickly wind down behind the shortwave. Temps will bottom out below freezing in most of the area, except the warmest urban centers and along the warmer large bodies of water (Potomac/Chesapeake). On Monday, high pressure should continue building eastward, reaching eastern North Carolina by day`s end. This will cause winds to shift from northwest to southwest, but warm advection will be slow, so we still expect Monday to be chilly, with highs struggling to reach 50. The lighter winds and greater sunshine under a shortwave ridge will likely make Monday feel a little more comfortable, especially in the afternoon.
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&& .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/...
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High pressure will shift east off the coast Monday night and Tuesday, allowing southwesterly flow to dominate. While Monday night looks radiational thanks to diminishing winds and clear skies near the center of the high, by Tuesday sunshine and warmer air aloft will translate to warmer surface temps, with most places rebounding 5-10 degrees above Monday`s expected highs. Tuesday night, a cold front will approach from the northwest while a shortwave will spawn a coastal low near the Outer Banks. Still some question regarding how far west the rain shield from this low makes it, but most guidance now shows rain reaching southern Maryland, and a fair number have rain reaching Baltimore and Washington proper. Thus, have chance pops for the big cities and likely in southern MD. Further west, odds are considerably lower. It should overall be a milder night thanks to the warm advection ahead of the cold front combined with increased cloud cover, with lows mostly above freezing.
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&& .LONG TERM /WEDNESDAY THROUGH SUNDAY/...
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High pressure will build into the region Wednesday through Friday night. Dry conditions expected with plenty of sunshine each day and mainly clear skies at night. Despite the daytime sunshine, temperatures each period will be about 5 to 7 degrees below average with highs in the upper 40s to near 50 and lows in the upper 20s to near 30. A storm system over eastern Canada will bring a cold front toward and across the mid-Atlantic Saturday and Saturday night. There is a chance for a few rain showers along the Mason-Dixon region Saturday with a chance of rain and snow showers in the Appalachians and along the Mason-Dixon regions Saturday night. Temperatures will be closer to average. A gusty northwest wind will usher in cold air into the region Sunday. With that, a chance of snow showers are expected in the Appalachian Mountains. Below average temperatures expected to retake the region.
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&& .AVIATION /20Z SUNDAY THROUGH FRIDAY/...
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VFR overall through the TAF period. There could be a stray sprinkle or flurry, but really, the main concern tonight is the slowly diminishing wind gusts. Gusts should mostly be below 30 knots this evening and may drop below 20 knots late tonight. Winds may return to the 20 knot range during the day Monday. Tuesday night is the only real chance of sub- VFR as a low pressure system could bring a bit of rain, which may reduce cigs and vis for a time. Still uncertain just how far west this precip gets. VFR conditions Wednesday through Thursday night. Winds northwest 5 to 10 knots Wednesday. Winds northwest around 5 knots Wednesday night. Winds becoming light and variable Thursday and Thursday night.
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&& .MARINE...
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Gale warning remains in effect through 6PM. Some uncertainty for the overnight given continued cold advection over warmer waters, which could enhance mixing and bring some higher gusts back to the surface. This might mean we need to extend the gale, but with high uncertainty, have left it SCA for the time being to give the evening shift one last look. SCA continues most waters through Monday as high pressure shifts east to our south. Winds probably go sub SCA on Monday night but may go back to SCA Tuesday ahead of the next cold front, which passes late Tuesday night. SCA possible behind cold front Wednesday, may be diminishing Wednesday night. Winds light and variable Thursday and Thursday night.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...Winter Weather Advisory until 11 PM EST this evening for WVZ501-505. MARINE...Small Craft Advisory from 6 PM this evening to 6 PM EST Monday for ANZ530>534-537>543. Gale Warning until 6 PM EST this evening for ANZ530>543. Small Craft Advisory from 6 PM this evening to 10 AM EST Monday for ANZ535-536.
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&& $$ SYNOPSIS...RCM NEAR TERM...RCM SHORT TERM...RCM LONG TERM...KLW AVIATION...RCM/KLW MARINE...RCM/KLW

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