Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 140836 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 336 AM EST Tue Nov 14 2017 .SYNOPSIS...
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High pressure will move across the area through Wednesday. A cold front will cross the region late Wednesday night into early Thursday. High pressure will return briefly Thursday night into Friday before a strong cold front moves through during the first half of the weekend.
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&& .NEAR TERM /THROUGH TODAY/...
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Surface high pressure of about 1030 mb is drifting east out of Ohio into western West Virginia this morning. Any stratus or patchy fog trapped underneath the subsidence inversion should burn off by late morning as temperatures rise. A mostly sunny afternoon is anticipated, but despite the sunshine high temperatures will only be around 50 degrees, nearly 10 degrees below normal for mid-November.
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&& .SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/...
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Transient ridging at the surface and aloft glides directly over the area tonight. With light winds and clear skies, anticipate patchy fog in the typically favored locations (river valleys, central VA piedmont). Low temperatures will drop close to freezing in many locations outside the major cities with a favorable radiational cooling setup. Wednesday will be a touch warmer than today but still a few degrees below normal for high temperatures despite mostly sunny skies for much of the day. The high moves offshore by late afternoon, though, so high clouds should start filling in pretty quickly from west to east by sunset. Various GEFS/SREF/EC members are in good to very good agreement on the timing of a cold front Wednesday night. There is only a narrow zone of moisture in the low-levels, and that should cause most showers to dwindle as they cross the mountains, though some sprinkles or showers are still possible east of the higher terrain (particularly near the Mason-Dixon line). Rainfall amounts should be very light, generally a tenth of an inch or less, and model guidance is in very good agreement on this as well. Temperatures should be above freezing through the low levels which will allow any precipitation to fall as liquid. The surface front departs east of the area Thursday, but the upper trough axis lingers. This will likely result in a mix of sun and clouds, a few lingering upslope showers (rain, possibly mixing with snow by afternoon) and perhaps a sprinkle east of the mountains along the PA/MD border. With downsloping NW flow and cooler air lagging a bit behind the front, temperatures should be noticeably warmer Thursday as compared to previous days (mid 50s to near 60).
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&& .LONG TERM /FRIDAY THROUGH MONDAY/...
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Surface high pressure and a shortwave ridge aloft will be traversing the area on Friday. While it will be cooler than Thursday, temperatures should remain within several degrees of normal. The weekend will become more active as a strong low pressure system lifts northeast from the Great Lakes and the trailing cold front pushes toward the east coast. There is still some uncertainty in the timing, but showers -- perhaps moderate or briefly heavy -- will become likely sometime Saturday or Saturday night. Thunder chances are low but not zero. This system will have plenty of wind energy, although the strongest winds (low level jet) prior to the front will not be able to mix completely to the surface. The timing/magnitude of the winds will need to be monitored, but breezy conditions are likely behind the front in a cold air advection regime. Upslope snow showers will be possible, and it`s not out of the question some flurries could spill east of the higher terrain on Sunday. Model spread in timing continues on Monday with differences on how quickly high pressure builds to the east coast. Given a cutoff upper low developing over eastern Canada, this process probably won`t happen quickly, making chilly and breezy conditions the more likely forecast outcome.
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&& .AVIATION /07Z TUESDAY THROUGH SATURDAY/...
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Some patchy fog has developed across the central VA piedmont as well as northeastern Maryland early this morning. This fog should abate as some stratus sneaking in from south- central/southeastern Pennsylvania advects south, but levels should be low-end VFR (3500-4500 feet or so). VFR as high pressure crosses today with generally light north winds. These winds will go calm tonight as the high moves overhead. Patchy fog is possible after midnight for MRB/CHO/IAD, perhaps BWI/MTN depending on how low level moisture profiles evolve over the next 12-24 hours (NAM, as is typical, is more aggressive with fog potential. MVFR most likely, but spotty IFR can`t be ruled out toward daybreak Wednesday). VFR returns Wednesday into Wednesday night. A spot shower or two can`t be ruled out Wednesday as a cold front crosses, but probability of flight restrictions appear minimal at this time. Mainly VFR expected Thursday and Friday with winds around 10 knots or less. A strong cold front will bring showers and potential for sub-VFR conditions sometime Saturday or Saturday night. Gusty winds will also be possible with and following this system.
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&& .MARINE...
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Narrowed down SCA timing to 4-10 AM this morning with just enough of a gradient/mixing combo around daybreak to probably get a few gusts to 20 knots near Point Lookout/Tangier Sound areas. Otherwise winds will be light through Wednesday night as high pressure moves overhead. SCA will likely be needed Thursday into Thursday night in the wake of a cold front (NW flow). Winds will diminish Friday as high pressure crosses the area. A strong cold front will sweep through on Saturday or Saturday night. SCA will likely be needed (at a minimum), with strong winds possible both before and (especially) after the front. There is still a wide range of possibilities regarding the magnitude of the winds.
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&& .TIDES/COASTAL FLOODING...
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Tidal anomalies of around one foot are being observed near the mouth of the Chesapeake Bay as of early this morning. Light northerly flow should keep this excess water to our south through much of the day today, but as winds go light tonight into Wednesday, some of this may work its way back north. Currently looks like action stage at Straits Point/Annapolis during the middle of the week, but typically more aggressive CBOFS has minor at both sites and can`t discount that completely.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 11 AM EST this morning for ANZ534- 543.
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&& $$ SYNOPSIS...DHOF NEAR TERM...DHOF SHORT TERM...DHOF LONG TERM...ADS AVIATION...ADS/DHOF MARINE...ADS/DHOF TIDES/COASTAL FLOODING...DHOF

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