Area Forecast Discussion
Issued by NWS Sterling, VA

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366 FXUS61 KLWX 230746 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 346 AM EDT Tue May 23 2017 .SYNOPSIS...
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Low pressure will develop over eastern Carolina today and move northeast off the DelMarVa tonight. Upper-level low pressure will move toward the area Wednesday through Thursday, and then depart on Friday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Latest surface analysis indicates that broad high pressure remains across the eastern third of the United States early this morning. Yesterday`s cold front can be found just offshore. Drier air really hasn`t infiltrated too far east, with dewpoints along and east of the Blue Ridge in the lower and mid 60s. This has resulted in fairly widespread low clouds and/or fog. In those locations experiencing fog, visibility hasn`t been consistently reduced to quarter-mile, although am seeing some indications that may change. May need to launch a Dense Fog Advisory for a few counties northeast of the Potomac River. Will be monitoring obs and act accordingly. Whatever does develop will lift after sunrise. There has also been a stray shower or two in central Virginia. Mesoscale guidance hasn`t been maintaining much. Moisture and overrunning are both present though, so its worth an isolated PoP through dawn. HRRR/RAP/NAM/GFS all have slowed down precip onset today, and paint a sharp northwest edge just inside of the forecast area boundary. The responsible shortwave (there isn`t a distinct surface reflection as of yet) is approaching the Tennessee Valley as of 3am. There is copious moisture and vertical velocity across the southeastern United States. As the moisture advects northeast, the warm conveyor will encounter upglide from east/northeasterly surface flow. As the shortwave hits the coast, cyclogenesis will develop, supported by jet dynamics as well. That deformation band will be the feature responsible for the rapid end to the precip. Have categorical PoPs late afternoon/early evening for central Virginia and southern Maryland. QPF will range between a quarter and half inch, although locally higher amounts likely. There is substantial uncertainty along the Maryland/Pennsylvania border. The overnight hours will be dry in the wake of this low. But, residual low level moisture, wet ground and rather high dewpoints suggest that fog could once again be a concern. Have not gone far from a model blend for temperatures forecasts today. Highs will be trapped in the 60s today (matching nearby water temperatures) under mostly cloudy skies and onshore winds. Lows tonight will match the dewpoints...in the 50s.
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&& .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
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For Wednesday and Thursday, the forecast area will be affected by a deep trough axis and closed upper low pivoting across the eastern United States. The area will start out in a lull Wednesday morning, then height falls, shortwave energy, and a 700-500 mb speed max will pave the way for showers to overspread the area late afternoon and evening. The nighttime hours will be wetter than the day, as that will be when the aforementioned forcing features will be at their max. Would not rule out elevated thunder either, but mean layer instability will be minimal so doubt that there will be any storms rooted in the boundary layer. A second round of showers and thunderstorms will cross the area Thursday afternoon and evening. The 500 mb low will be passing through about this time, so there is a better chance at developing instability and thus stronger storms. That said, stronger is a relative term here. Daytime heating will be fleeting, so its unclear whether stronger storms actually will manifest as strong storms. Again, not straying too far from model consensus for temperatures through the period.
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&& .LONG TERM /FRIDAY THROUGH MONDAY/...
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Closed upper low will be moving off the southern New England coastline Friday morning. Post frontal conditions expected across the Mid-Atlantic region Friday with partly sunny conditions expected. A few showers are possible Friday as upper level energy rounds the backside of the upper level trough. Amts will be scattered and light. High pressure will briefly move overhead Fri night-Saturday and dry conditions are expected. Unsettled conditions return Saturday afternoon as fast moving shortwave trough moves across the region. A warm front will also approach the Mid-Atlantic during this time and rounds of showers are likely Saturday afternoon/evening through Memorial Day. Temperatures will be near normal Memorial Day weekend.
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&& .AVIATION /08Z TUESDAY THROUGH SATURDAY/...
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Low clouds and fog (sub-IFR) have been lurking east of the terminals early this morning. The next cloud layer is mid deck (around 8000 ft). The low deck has been wavering across DCA/BWI/CHO, resulting in highly changeable conditions. Mesoscale guidance have been struggling to capture current conditions, so there is substantial uncertainty in its output. Drier air should be filtering east, which may be why guidance hasn`t been exhibiting its usual bias of outright tanking, especially at CHO. Whatever does develop should lift in the morning, before onshore flow ahead of a developing coastal low brings in another low cloud deck. There is uncertainty at the timing and height of today`s cloud. Am going on the more pessimistic side of options, which introduces MVFR by early afternoon and IFR this evening. Rainfall associated with this low will pull out this evening. But believe that low clouds and fog will be left behind, and am taking rather substantial flight restrictions (aob IFR) through the night. These will gradually improve Wednesday. The cloud deck Wednesday morning will be on the thin side. So, if it breaks apart, that rapid VFR could result. Do not have high confidence in this solution. Two additional periods of showers and thunderstorms anticipated though midweek. Flight restrictions will be numerous, but there should be some periods of embedded MVFR if not VFR. VFR conditions expected Fri-Sat. Periods of rain moves back into the terminals Sat-Mon.
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&& .MARINE...
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Low clouds will result in poor momentum transfer though the midweek. Coastal low pressure will move past the Delmarva this afternoon and tonight. Am maintaining Small Craft Advisories for the mid Bay and lower Potomac. There will also be opportunities for higher winds in southerly flow Wednesday night (mixing will be in question) and on Thursday in vicinity of thunderstorms. NW winds expected Friday behind a cold front. SCA condtions are likely. Winds diminish into the weekend and rain returns.
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&& .TIDES/COASTAL FLOODING...
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Water levels between a quarter and half foot above astronomical normals. No issues expected today or tomorrow. However, deep low pressure will cross the area Thursday, with southeast winds picking up Wednesday night. The early Thursday morning tide cycle will need to be monitored, as the preponderance of evidence suggests that minor flooding will be a concern. Still considerable spread in model solutions though.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 2 PM this afternoon to midnight EDT tonight for ANZ533-534-537-543.
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&& $$ SYNOPSIS...HTS NEAR TERM...HTS SHORT TERM...HTS LONG TERM...HSK AVIATION...HTS/HSK MARINE...HTS/HSK TIDES/COASTAL FLOODING...HTS

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