Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 231433 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1033 AM EDT Sat Jul 23 2016 .SYNOPSIS... A weak cold front will dissipate north of the area today. High pressure will then be in control of the area`s weather Sunday before another front enters the area Monday into Tuesday. This front may stall across the region during the middle of next week. && .NEAR TERM /THROUGH TONIGHT/... Update: expanded Heat Advisory to Calvert and St. Marys Counties as reworked temperature grids produce 105 heat index there. Previous discussion: Synoptic features across the eastern portion of the United States early this morning are quite weak. The most noticeable item is a trough axis across New England, which extends southward along the Mid Atlantic coast. There is also a dewpoint gradient across the eastern Great Lakes to New York City. These two features will play a small role in today`s forecast. The only clouds of note this morning are located in western MD, so temperatures have been able to rise quickly to near 90 at 10 AM. Deep mixing and a W/NW low level flow will support highs in the mid-upper 90s, and am thinking a few spots (such as urban DC) have a real shot at 100. While the westerly flow supports higher temperatures, it also would dry dewpoints out to the mid-upper 60s. Have continued a middle-of-road solution to dew points in the upper 60s-around 70, which yields a Heat Index on the threshold of advisory criteria of 105. In terms of impact, the heat and sun will be an issue for anyone outdoors. For thunderstorm chances, the dewpoint boundary, which will be sinking south late this afternoon and evening, may provide a zone of weak forcing. But the westerly flow likely will act to suppress anyactivity. While plenty of instability abounds, there is little forcing to utilize that CAPE. In addition, the 12Z IAD sounding shows a strong stable layer between 600-450 mb. With 500 mb forecasts changing little through the day, deep convection may be thwarted. There is some signal in guidance that there may be a slightly better chance east of I-95 this evening, perhaps due to the boundary from the north interacting with bay breezes, along with slightly lower heights. Benign weather will be in place overnight, with perhaps patchy fog in climo-favored locations. That will be especially true in the higher dewpoint air if any storms do cross those areas. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... High pressure will return for Sunday, with ridging in place aloft too. Temperatures at 850 mb will still be with a degree or two of 22C, but dewpoints should be in or near the mid 60s, which means that the heat index should remain under 100F for most. Temperatures may be a degree or two cooler as well since downsloping is lacking, further supporting the feeling that an Advisory won`t be warranted. If any thunderstorms develop, they would be initiated by terrain circulations, and have relegated low end PoPs to the mountains. Monday looks to be the most active day of the three. First of all, return flow stands a chance at pumping 70F dewpoint air north across the forecast area once again. The airmass could certainly support temperatures that rival Saturday, so headlines will once again be possible. In terms of storms, shortwave energy tracking across the northeastern states will drop a front southward, which would provide forcing. While instability across the forecast area once again remains ample, the better shear will be found north of us. Thus its not certain if something more organized can develop, and if it does would it survive the trip. Regardless, believe the setup would support at least scattered convection. Given heat of the airmass, storms could become strong assuming they can penetrate the warm mid-levels. PoPs have been lowered from previous forecast given these uncertainties, but still have 30-40% PoPs from midday through the evening. && .LONG TERM /TUESDAY THROUGH FRIDAY/... High pressure will continue to move into the Mid-West Tuesday while a cold front will be located near the Mid-Atlantic region. Multiple waves of low pressure are expected to move along the frontal boundary and could potentially lead to an active weather pattern for the region. Showers and thunderstorms will be possible Tue-Fri with the best chance in the afternoon and evening. Warm and humid conditions are expected to continue through the end of the work week. && .AVIATION /15Z SATURDAY THROUGH WEDNESDAY/... VFR conditions will largely dominate through the valid TAF period. The only exception may be localized fog at climo-favored terminals... MRB/CHO. Even there am carrying only a couple hours of MVFR. The only other risk today will be thunderstorms, which should be isolated at best late this afternoon and evening. This risk much too low to carry in a TAF at this time. A similar setup carries through Sunday...VFR with a low end risk of patchy fog. Thunderstorm chances even less than today. Out of the next three days, the risk of storms highest on Monday. Coverage would most likely still be scattered. Gusty winds/brief restrictions a possibility. SHRA/VCTS possible Tue-Wed. && .MARINE... Flow this weekend will generally be light and from the W/NW today...10 kt or less. An isolated thunderstorm would be possible late this afternoon-evening. Gradient winds will likely increase Monday ahead of a cold front. Do not have high confidence on reaching Small Craft conditions, but its within the realm of possibilities. Will leave the wording in the synopsis at this time. Thunderstorm chances increase Monday as well, which may be the greater risk of locally high winds. A cold front may linger near the waters Tue-Wed and showers and thunderstorms are possible. && .CLIMATE... Record highs and warm lows for the upcoming weekend... Saturday... DCA...102 in 2011...84 in 2011 BWI...102 in 2011...79 in 1978 IAD... 99 in 2011...76 in 2011 Sunday... DCA...101 in 2010...84 in 2011 BWI...101 in 2010...82 in 2010 IAD... 99 in 2010...78 in 2010 Monday... DCA...100 in 1930...79 in 1965 BWI...100 in 2010...77 in 1887 IAD... 98 in 2010...76 in 1965 && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Heat Advisory until 8 PM EDT this evening for DCZ001. MD...Heat Advisory until 8 PM EDT this evening for MDZ011-013-014- 016>018-503>506-508. VA...Heat Advisory until 8 PM EDT this evening for VAZ050>057-502- 506. WV...None. MARINE...None. && $$ SYNOPSIS...HTS NEAR TERM...ADS/HTS SHORT TERM...HTS LONG TERM...HAS AVIATION...ADS/HAS/HTS MARINE...ADS/HAS/HTS CLIMATE...HTS is the U.S. government's official web portal to all federal, state and local government web resources and services.