Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 271425 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1025 AM EDT Sun May 27 2018 .SYNOPSIS...
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A cold front moving south out of Pennsylvania will cross the Mid-Atlantic later this afternoon, then stall over the region before dissipating by the middle of the week. The remnants of Alberto may pass west of the region later in the week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 10am, a few showers are trying to develop across extreme western Maryland. These showers appear to be along a weak frontal boundary, which lies west- southwest to east-northeast across parts of eastern West Virginia, northern Virginia and northeast Maryland. Additional showers and some heavy thunderstorms are anticipated as we progress into the afternoon. The main concern this afternoon will be flooding. A small/weak area of low pressure at the triple point between a surface trough extending north to south just east of the Blue Ridge and a cold front extending west to east dropping southward out of Pennsylvania will act as a focus for surface convergence. 850-700 mb flow is slightly more convergent over these same areas. This coupled with weakly diffluent flow aloft and PWATs approaching 2 inches is expected to result in scattered to widespread showers and thunderstorms again this afternoon and evening. Mid-level steering flow will be parallel to the cool front, so training convection seems likely. For these reasons, have gone with a Flash Flood Watch for the I-95 corridor from NE MD to Fredericksburg, including the DC/Baltimore metros and central VA where regenerative convection seems most likely amidst aforementioned forcing mechanisms. Convection should gradually wane with the loss of heating this evening.
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&& .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... High pressure building in from the north behind the cold front is only about 1015 mb, which this time of year is usually not enough to force the front fully south of our area. The front will likely stall in the I-64 corridor up into southern MD while weakening early in the week, with scattered showers and thunderstorms possible each afternoon near and south of the boundary. Further north, the airmass will become slightly cooler and more stable, but the boundary is weak so appreciably lower dew points never really make it. As such, an isolated shower is possible each afternoon even as far north as the PA border, but should be spotty given weaker forcing. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... Per latest National Hurricane Center forecast, the remnants of Alberto will be moving west of the area Wednesday and Thursday. As high pressure moves off the New England coast during the same time, flow will become southerly with deep moisture returning to the area. The high may retain some dry influence on Wednesday across northern/eastern portions of the area, but precipitable water values will rise back close to 2 inches by Thursday. Ensembles suggest moisture and instability will peak on Thursday, and heights will begin to fall as the upper trough approaches. While this will result in the highest chance of rain area-wide during the extended forecast, it will be tough to rule out showers and storms any day. Given the jet stream will be well to the north, it appears upper level troughing will linger over the area Friday and Saturday, with perhaps a surface low crossing the area at some point. Showers and storms will likely follow a diurnal pattern, but there could be some activity at night as well given the amount of moisture in place. Specific impacts, particularly any areas or times of heavier rain, remain uncertain and will be dependent on how the pattern evolves. Temperatures will likely remain above normal through the period, especially at night due to dew points in the upper 60s to lower 70s. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/...
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MVFR to IFR cigs at BWI and MTN terminals late this morning with gradually join the other terminals with VFR conditions this afternoon. BKN to OVC conditions expected this afternoon with scattered to numerous showers and thunderstorms 18-00Z ahead of approaching cold front (dropping south out of PA). Heavy rain and brief vsby restrictions possible. Light SW flow this afternoon. We may occasionally encounter a light or briefly moderate NW flow at a few terminals through the day due to the stationary front sliding just to the south or weak outflow boundaries generated by dissipating convection. As the front stalls near CHO to southern MD, IFR CIGS likely tonight again. Light/variable to light NE flow. Lower CIGS could linger well into Monday with low level moisture firmly in place. Otherwise, a shower or thunderstorm is possible INVOF the front stalled over central VA Mon-Tue. Showers and thunderstorms will be possible at times Wednesday afternoon through Thursday as low pressure moves west of the area. Sub-VFR clouds and/or fog could occur during the overnight periods as well.
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&& .MARINE...
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Flow is light from the SW across the central Chesapeake Bay and Tidal Potomac and more SE across the northern Chesapeake Bay. Winds are 5 to 10 knots with gusts around 15 knots, mainly in the central Chesapeake Bay. A weak front should stall nearby through early this week. Showers and thunderstorms with frequent lightning and perhaps a few wind gusts are likely this afternoon and evening, less likely Mon-Tue but non-zero chance with the front stalled nearby. Southeast winds may near SCA thresholds on Wednesday as high pressure moves off the New England coast and low pressure moves into the Ohio Valley. Thunderstorms will be possible from Wednesday afternoon through Thursday as the low remains to the west and draws moisture into the area.
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&& .HYDROLOGY... Have issued a Flash Flood Watch for the I-95 corridor from NE MD to Fredericksburg VA including the Baltimore/DC metros and central VA. Training convection seems most likely in these areas given enhanced surface convergence, with high rain rates expected due to PWATs near 2 inches and warm cloud layers of 12+ kft. Convection should gradually wane this evening with the loss of heating. && .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Flash Flood Watch until 10 PM EDT this evening for DCZ001. MD...Flash Flood Watch until 10 PM EDT this evening for MDZ006-011- 013-014-016-503>508. VA...Flash Flood Watch until 10 PM EDT this evening for VAZ038>040- 051>055-501-502-506-507. WV...None. MARINE...None.
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&& $$ SYNOPSIS...DHOF NEAR TERM...KLW/DHOF SHORT TERM...KLW/DHOF LONG TERM...ADS AVIATION...ADS/KLW MARINE...ADS/KLW HYDROLOGY...DHOF/KLW

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