Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 300714 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 314 AM EDT Thu Jun 30 2016 .SYNOPSIS...
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High pressure will build over the area this morning and move offshore this afternoon and evening. A warm front across the Carolinas will lift north tonight into Friday. A cold front will move into the region Friday night. High pressure will return to the Great Lakes and northeast for the weekend. A frontal boundary is expected to hover over the Mid Atlantic Sunday night and Monday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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The synoptic pattern for the local area is quiet this morning, and is expected to remain that way for the day. The nearest frontal boundary is quasi-stationary across the Carolinas, and it may slowly sink a bit further south today. There is still sufficient positive vorticity advection across the Mid Atlantic early this morning. The impacts of these two features overlap across southern/central Virginia, manifesting in ample lower clouds. This cloud deck has spread as far north as DCA as of 3am. Do not anticipate these clouds to last locally much past sunrise as the vort and front both depart, to the east and south respectively. High pressure will be over the region today, but return flow will commence by late in the day. Anticipate dewpoints will start to creep up late in the day within a strengthening theta-e ridge, but will be more tangible (into the mid and perhaps upper 60s) tonight. As this occurs, instability and subsequently thunderstorm chances will follow. Am keeping 20-30 percent chance for late in the day today, generally southwest of the Potomac River, but have scattered precipitation spreading north in isentropic advection along the I-95 corridor in the evening. Will attempt to preserve a diurnal component as CAPE only a couple hundred joules; thus just have a few showers after midnight and a dry pre-dawn period. Clouds should be plentiful, but the increased humidity also makes fog a possibility by early Friday morning.
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&& .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
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Friday`s forecast will be a little bit complicated. There will be the isentropic ridge in the eastern (metro) part of the forecast area. Meanwhile, a shortwave and surface cold front will be crossing the northeastern US. Both features would be capable of supporting convection given daytime heating. The former will be pushing east ahead of the front, primarily affecting the Eastern Shore/Delmarva; while the latter doesnt arrive until Friday night. Area may end up in between the two, but wish to broaden the range in forecast grids to account for uncertainty. Highest PoPs will be in the east. Will introduce chances in the west for the afternoon and early evening. While shear will be present, the other factors either aren`t aligning well or are poorly timed. Thus, will be forecasting scattered, non-severe convection. Will need to monitor the strength of the shortwave/surface convergence though-- the setup is quite similar to Tuesday. The cold front forecast to push through Friday night. (How often can one say that that for successive cold fronts in the Mid Atlantic in early July?) Given support of numerous model runs, am fairly confident that this will lead to a partly-mostly sunny Saturday...even though this solution is a departure from climatology. The only wrinkle may be the extent of high level clouds, as the jet stream will be overhead.
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&& .LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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Zonal flow aloft is expected Sunday as a stationary front is located across the southern Mid-Atlantic region. The front will waver into Monday and showers will be possible in vicinity of the front... mainly across central VA and southern MD. The front is expected to retreat northward as a warm front into Tuesday. Showers may increase in coverage Monday night into Tuesday night. A shortwave trough may impact the region Wednesday and Thursday. Temps should stay below normal Sunday-Monday and near normal Tuesday-Wednesday.
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&& .AVIATION /07Z THURSDAY THROUGH MONDAY/...
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VFR flight conditions should generally prevail today. There may be a little early morning fog, but confidence in its extent/areal coverage low, and have omitted it from the TAFs at this time. A warm front will approach the terminals tonight. With it, thunderstorm chances increase. Scattered areal coverage preclude TAF mention at this time. However, the increase humidity may yield areas of fog. Will only be forecasting local predawn MVFR Friday morning, due in part to anticipated cloud cover. There is the potential for it to be worse. A cold front will approach the terminals on Friday, and cross Friday night. Scattered thunderstorms will be possible, especially during the afternoon and evening hours. Local IFR would be possible within any thunderstorms, but their coverage in doubt at this time. High pressure/VFR returns for Saturday. VFR conditions expected Sunday-Monday.
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&& .MARINE...
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Light winds (less than 10 kt) prevail on the waters at this time, and anticipate that will be the case for much of the day. Northerly flow will gradually become south by late in the day. The gradient will tighten on Friday as a cold front approaches. However, do not have much confidence in any strong winds outside of thunderstorms. There are a couple of possible period of storms... tonight as humidity returns, and Friday ahead of the cold front. The storms on Friday likely will be stronger, and may be capable of producing storms requiring Special Marine Warnings. The cold front will clear the waters Friday night, with high pressure returning Saturday. If mixing is strong enough, there may be a window of opportunity for Small Craft Advisory gusts behind the front Friday night-Saturday morning. Sub-SCA expected on the waters Sunday-Monday.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...HTS NEAR TERM...HTS SHORT TERM...HTS LONG TERM...HAS AVIATION...HTS/HAS MARINE...HTS/HAS

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