Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 220845 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 345 AM EST Wed Nov 22 2017 .SYNOPSIS...
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A cold front will pass through the area early this morning and high pressure will build overhead later today through Friday. High pressure will move offshore Friday night and a cold front will pass through late Saturday into Saturday night. High pressure will return for early next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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A cold front is located over southwest Pennsylvania into West Virginia as of 3 am this morning. The cold front will pass through our area over the next few hours from west to east. At the same time...southern stream moisture is advecting north into our area. The southern stream moisture is most likely to make it into areas near and east of Interstate 95. Therefore...a period of rain is most likely across these areas early this morning with the frontal passage. The front will move off to the south and east by mid-morning and high pressure will approach from the Midwest and Great Lakes. A potent shortwave will pass by to our north during this time...but with most of the dynamics remaining to the north it should not have an impact on our weather. Therefore...increasing sunshine and gusty northwest winds are expected ahead of the building high. Max temps are a little tricky to forecast since there will be cold advection behind the cold front...but that will be offset by increasing sunshine and mixing along with a downsloping flow. Did tweak the hourly trend in temps a little away from the typical diurnal trend. Feel that max temps will occur late this morning to around noon before holding steady and even falling during the middle afternoon hours. Max temps are most likely to range from the lower to middle 30s along the ridge tops of the Allegheny Highlands...to the mid and upper 40s for most of northern Maryland into the Shenandoah Valley...to the middle 50s near Washington and Baltimore...to the upper 50s and even near 60 in central Virginia into southern Maryland. Also of note...there will be a period of snow showers through mid-morning for locations along/west of the Allegheny Front. Moisture will be limited...but an upslope component along with a higher subsidence inversion will cause some accumulation above 2000 feet. Accumulations around an inch or less are most likely before snow showers taper off by late morning. High pressure will build overhead tonight and winds will decrease around sunset for most areas. Winds will become light overnight and with dry air and mainly clear skies this will provide a great setup for radiational cooling. Min temps will range from the upper teens and lower 20s in sheltered valleys and rural areas west of the Blue Ridge Mountains...to the mid and upper 20s across most other locations to the lower and middle 30s in downtown Washington and Baltimore.
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&& .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
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High pressure will remain overhead Thanksgiving Day and Thanksgiving night...bringing dry and chilly conditions along with mainly clear skies. High pressure will shift to the east coast Friday and move offshore Friday night. A southerly flow will return during this time. Max temps will be a bit warmer Friday...but still near or even slightly below climo. More dry conditions are expected during this time. Friday night will be seasonably chilly...but not as cold as Thanksgiving night.
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&& .LONG TERM /SATURDAY THROUGH TUESDAY/...
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Long term period starts out Saturday with a ridge west/trough east pattern, but by the end it completely flips, with trough west/ridge east by Tuesday. At the surface, this will translate into a cold frontal passage on Saturday, followed by high pressure progressing slowly east across the region Sunday through Tuesday. In many ways, surface features Saturday will mirror what is happening tonight. As with tonight, one thing to watch Saturday is a coastal low pressure which will also be moving northeast along the Carolina coast. Should this system end up a bit further northwest, it could spread some rain into the region, but unlike tonight, most guidance keeps this system well to the southeast, completely separated from the approaching northern stream shortwave and surface cold front. This means that right now, we think the cold frontal passage should be dry for most of the CWA. We will likely moderate thanks to southwesterly flow just ahead of the cold front, resulting in Saturday being rather like yesterday was... breezy with some clouds and temps well into the 50s, perhaps even low 60s. Along and behind the front, some showers will affect the northwestern and western portions of the CWA, primarily the higher terrain, and much like today, these will change to snow showers behind the front, which in this case will occur on Saturday night. However, the system is fast moving, so do not expect significant accumulations even in the favored upslope regions. Chilly and breezy but mostly dry Sunday with high pressure starting to build in...any lingering upslope snow showers will be winding down. Most areas will not get out of the 40s. On Monday, high pressure crosses the region, but temps remain chilly with the trough just starting to exit aloft, so only slight warming. By Tuesday, however, southwest flow will start again in earnest as the high slides off the coast, and moderation will become more apparent, with highs returning to the 50s most likely.
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&& .AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
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A period of rain is possible across the eastern terminals...KDCA...KBWI and KMTN through about 12z. Brief MVFR conditions are possible during this time...but a majority of the time should be VFR. Gusty northwest winds will develop behind a cold front between 13z and 15z. The gusty winds will continue through the afternoon with frequent gusts around 20 to 25 knots with a few gusts up to 30 knots. The winds will diminish around sunset. High pressure will build over the terminals tonight through Thursday night...bringing VFR conditions and lighter winds. Winds will turn to the south later Friday and Friday night as high pressure shifts offshore. Saturday and Sunday look mainly VFR for now, with a gusty wind (perhaps up to 30 knots at times) starting southwest, then switching to northwest behind a cold front later Saturday. These winds likely persist Sunday. A coastal low will be sliding northeast off the coast on Saturday, but right now effects look minimal to non-existent. If it ends up closer, a period of sub- VFR would be possible on Saturday due to rain, but odds of this look low right now.
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&& .MARINE...
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A cold front will pass through the waters early this morning. High pressure will approach from the north and west today before building overhead tonight through Thanksgiving night. Gusty northwest winds are expected behind the cold front today. Gusts of 20 to 30 knots are most likely. An SCA is in effect for the waters. The winds will gradually diminish tonight...but an SCA is still in effect for most of the Bay and lower Tidal Potomac River. Winds will continue to diminish for Thanksgiving as the high settles overhead. High pressure will shift offshore Friday through Friday night and a southerly flow will return...but winds should remain below SCA criteria for most of the time since the gradient will be weak. Cold front crossing the area Saturday will bring southwest winds which then switch northwest by Saturday night, lingering Sunday. These are likely to reach SCA criteria and could be near gale, but confidence is not high on this.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 6 AM this morning to 6 AM EST Thursday for ANZ531>534-537-539>541-543. Small Craft Advisory from 6 AM this morning to 6 PM EST this evening for ANZ530-535-536-538-542.
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&& $$ SYNOPSIS...BJL NEAR TERM...BJL SHORT TERM...BJL LONG TERM...RCM AVIATION...BJL/RCM MARINE...BJL/RCM

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