Area Forecast Discussion
Issued by NWS Sterling, VA

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793 FXUS61 KLWX 241402 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1002 AM EDT Sun Jul 24 2016 .SYNOPSIS... High pressure will be in control of the area`s weather today 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/... Quiet morning underway as the mid/upper level ridge nudges in from the west. The dew point gradient is near the Washington metro area as expected, with mid 60s to the north and around 70 in the south. A brief survey of obs suggests temperatures today are on a 2-degree slower pace than yesterday with similarly sunny conditions, which adds confidence to the slightly lower high temperature forecast. It will still be a hot day though with a heat index around 100, higher toward central VA. 12Z IAD sounding also shows potential CAPE is less today, and there will be few forcing features outside of the terrain. Previous discussion: There are some suggestions that shortwave energy will be riding atop the ridge, and will be entering the upper Ohio Valley by mid afternoon. This will need to be watched. Inherently, there is plenty of CAPE, and mid level lapse rates in the mountains are favorable to support convection. Add in terrain circulations, and there could be thunderstorms developing in the Potomac Highlands by mid afternoon. The concern is that the vort energy will feed off the instability, and organize a cluster of storms. A majority of the ensemble members of the NSSL-WRF suggest there will be something on our northwest doorstep by the end of the day. Have bumped up PoPs in western Maryland/adjacent West Virginia by early evening, and will be tracking this energy across the northern forecast area (mainly north of Winchester-Washington DC) through the evening and first part of the overnight. There are still placement/track/timing/intensity uncertainties, so don`t want to make too much of a commitment, but do believe its warranted to make some sort of a mention. && .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... After the lead shortwave exits early Monday morning, warmer air will once again advect into the area. Suggestions are that humidity would have increased overnight, leaving dewpoints in the lower 70s in place. Temperature forecasts will be at least as warm if not warmer than Saturday. That means records may be threatened. Therefore, the heat/humidity combo will once again push heat index values above advisory criteria. A cold front will plow into this airmass near the end of the day. Again, there are some timing/strength differences, and the front itself likely will be weakening as it interacts with the ridge. Although shear may be lacking, there will be plenty of energy to diffuse, and any storms that are able to develop will become strong quickly. Believe the presence of a boundary will be sufficient for scattered thunderstorms to develop along it, with the possibility for storms ahead of this boundary due to subtle height falls atop orographic forcing. That means the thunderstorms may develop by noon in the Potomac Highlands, with a boundary sinking across the forecast area through midnight. It remains to be seen if the boundary actually makes it through the area, or weakens and stalls nearby during the overnight hours. Regardless, anticipate some sort of diurnal weakening, but an isolated storm would still be possible pre-dawn Tuesday. With that stagnant boundary potentially in the area on Tuesday and more shortwave energy liable to pass through, believe there`s a decent potential for another round of thunderstorms. Strength, timing, and coverage are all uncertain...but believe there is enough potential to carry chance PoPs, especially south of the Potomac River and across southern Maryland. Have some sort of diurnal enhancement in grids right now, but ultimately the presence and strength of vort energy will dictate when and where thunderstorms will develop. And since there won`t be a significant chance in airmass, temperatures once again will be higher than normal, up to the mid 90s east of the Blue Ridge. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... An upper level ridge off the southeast coast will likely be suppressed to the south Wednesday and zonal flow will continue across the Mid-Atlantic for much of the week. A frontal boundary will be near the region while high pressure builds into the Ohio valley and northeast. The boundary will stay near the region for most of the week and showers and thunderstorms are possible each day. A shortwave trough is also possible through the end of the work week and may increase coverage of thunderstorms. Uncertaintyis high at this time. The heat continues through the week with above normal temperatures expected. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/... VFR conditions should prevail through much of the day. Thunderstorms will struggle to develop. However, there may be a little forcing late in the day through the evening and into the early overnight hours. There are quite a few uncertainties still, so there is low confidence at a direct strike. However, MRB/MTN and perhaps BWI have the highest potential for restrictions from storms tonight. DCA/IAD would be on the fringe. Fog may be a possibility early Monday. There should be enough low level moisture, and if there are any nighttime thunderstorms it will aide in fog development. After daybreak, any restrictions will erode. It will be a hot day, with an approaching cold front, which will assist in another round of thunderstorms, primarily (but not exclusively) during the afternoon and evening hours. Flight impacts possible, but coverage again a concern. The boundary will stall near the terminals Tuesday-Wednesday. Additional thunderstorms a possibility. && .MARINE... Light NE/E flow will become southerly by this evening. Primary marine hazard through Tuesday will come from thunderstorms. The most likely opportunity will be tonight for the northern waters, Monday afternoon and evening for all waters (although the threat will linger into the overnight for the lower Potomac and mid Bay) and again on Tuesday for the lower Potomac and mid Bay. Any of these storms may contain lightning and locally strong gusty winds. A frontal boundary may linger near the waters Wed and showers and thunderstorms are possible. && .CLIMATE... No records were broken or tied on Saturday. Here are the record highs and warm lows for the next three days. Monday`s numbers appear to be the most vulnerable. 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 Tuesday... DCA...103 in 1930...79 in 1987 BWI...101 in 1940...80 in 1940 IAD....98 in 2012...76 in 2005 && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. 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.