Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 211858 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 258 PM EDT SAT MAY 21 2016 .SYNOPSIS...
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Low pressure near the mouth of the Chesapeake Bay will move northeast toward the Canadian Maritimes through Sunday. A second area of low pressure will develop off Cape Hatteras Sunday; this too will head northeast through Monday. High pressure will build for the middle of the week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Stratiform rain ended across the area by midday. A shortwave/trough axis has been supporting a band of developing convection this afternoon-- mostly showers but there are a few areas of embedded thunder as well-- to the west of the Appalachians. As the shortwave nears, heights will drop, which will support a return of showery conditions late this afternoon into early this evening. In addition, still expect a reinvigorationof showers across northeast Maryland late today as easterly winds in advance of the coastal low bring moisture inland. Through sunset, likely PoPs will be confined to the Potomac Highlands (70 percent) and metro Baltimore (60 percent). Any rainfall totals will be less than this morning; generally 1/10 to 2/10th inch. As the trough axis crosses the area later this evening into the early overnight hours, it will bring sufficient forcing for likely if not categorical PoPs to the entire area. Lift most favorable in eastern West Virginia and northern Maryland.
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&& .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
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The 500 mb trough axis sharpens Sunday into Monday as shortwave energy continues to undercut the low. There will be enough energy to spawn a second coastal low by late Sunday. Unfortunately, have needed to make some pessimistic adjustments to the forecast. Cannot accurately time showers specifically in such a pattern, but am becoming increasingly convinced that there will be measurable rainfall each day. Have increased PoPs to likely most areas. Even have a bit of 80 percent probability in the Potomac Highlands Sunday. In regards to thunder chances...there will be a meager amount of instability in the mountains Sunday (no shear). The CAPE setup looks a little better areawide on Monday as heights will be lowest at that time. Am keeping thunder a slight chance at best (for one thing, dewpoints rather low), in the mountains with diurnal heating Sunday and areawide on Monday. Lowered high temperatures tomorrow given the expected cloud cover. Although Monday won`t be a sunny day, there should be at least a few glimpses. Rainfall won`t be all day, either. The added insolation may be enough to bump temps upward. Guidance fairly insistent on upper 60s/lower 70s. Low temperatures have been consistently on the higher side due to atmospheric moisture. No substantive changes made there.
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&& .LONG TERM /TUESDAY THROUGH SATURDAY/...
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Couple of quieter weather days Tuesday and Wednesday, as influence of vertically-stacked closed low shifts off to the NE and mid-level heights rise. Only real exception would be Tuesday morning, where a few lingering showers are possible (especially across eastern portion of CWA). Depending on how quickly we clear out, highs Tuesday might finally be near normal (M/U70s). With the help of downsloping westerlies we warm a few degrees Wednesday, though remain dry, with most places into the L80s by afternoon. By late Wednesday, developing Bermuda high offshore will begin to promote southerly flow over the area, which will increase low-level moisture rather quickly. Though, absence of large scale lift or significant upper level feature through at least Saturday suggest mesoscale forcing mechanisms will be primary driver of any diurnally driven thunderstorm activity through Saturday. As such, best thunderstorm chances will remain over the higher terrain...with lower chances to the east. While instability will be moderate-to- high, kinematic flow is poor (flow generally less than 30 kts at all levels), which would keep any pulse-type stronger storms generally isolated.
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&& .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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MVFR/IFR conditions persist this afternoon. Steady rainfall has pushed east, but there is plenty of low level moisture left behind. In easterly flow, wouldn`t rule out a little drizzle, although that`s not explicitly in the TAFs. There are also a plethora of showers west of the mountains. The overall trend will be for these to head east tonight. Timing problematic. Have forecast IFR ceilings. Wouldn`t rule out brief sub-IFR vsbys, but cannot skillfully forecast them at this time. There won`t be much change in the pattern Sunday-Monday. Numerous showers possible each day, but timing restrictions explicitly a challenge. Have MVFR forecast through the end of the TAF. Lower restrictions certainly possible. Further, ceilings should be on the rise, perhaps reaching VFR category. VFR conditions expected Tuesday and Wednesday with high pressure remaining in control. A few isolated thunderstorms possible Thursday afternoon as humidity increases over the area.
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&& .MARINE...
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Winds did increase earlier today, and have justified SCA in effect. Am keeping the current profile in place. After this evening, mixing looks poor with the upper low approaching. That covers the synoptic pattern for both Sunday and Monday. In terms of direction, these winds will mainly be coming from the north. With high pressure under control, westerly winds will remain light Tuesday and Wednesday. Winds remain sub-SCA Thursday, though they shift to southerly as Bermuda high develops offshore.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until midnight EDT tonight for ANZ530>534- 537>543. Small Craft Advisory until 7 PM EDT this evening for ANZ535- 536. && $$ SYNOPSIS...HTS NEAR TERM...HTS SHORT TERM...HTS LONG TERM...MSE AVIATION...HTS/MSE MARINE...HTS/MSE

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