Area Forecast Discussion
Issued by NWS Sterling, VA

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356 FXUS61 KLWX 231405 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1005 AM EDT Tue May 23 2017 .SYNOPSIS... 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. && .NEAR TERM /THROUGH TONIGHT/... Latest surface analysis indicates that high pressure remains over the northern Mid-Atlantic this morning. Yesterday`s cold front can be found just offshore. Previous low clouds and fog have shown a steady improving trend. Rain has made it to SHD-CHO associated with the first wave entering southeast VA. 12Z IAD sounding shows dry air in the mid levels and thus virga farther north. Am not sure if much rain reaches the ground in central/north portions of the area with the first wave, with better moisture sliding southeast of the area. The second shortwave trough is located in eastern TN. 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. Categorical PoPs will thus be focused during late afternoon/early evening. QPF will range between a quarter and half inch, although locally higher amounts likely, especially near the central VA Blue Ridge. They should be able to handle the rain through this evening even if amounts are on the upper end of the ensemble, with 6-hr FFG ranging from 2-2.5 inches. There is more uncertainty in amounts 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. Temperature forecast is on track. Highs will be trapped in the 60s today (matching nearby water temperatures) under mostly cloudy skies and onshore winds. Lows tonight will match the the 50s. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... 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. && .LONG TERM /FRIDAY THROUGH MONDAY/... 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. && .AVIATION /14Z TUESDAY THROUGH SATURDAY/... Clouds lifted into VFR range earlier this morning, although rain has started at CHO and MVFR is on the doorstep to the south. While some rain with this first wave may reach the northern terminals, it should be very light with minimal impact. However, onshore flow ahead of a developing coastal will allow low cloud deck to continue northward. 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. A better chance of rain will arrive into DC/Baltimore late this afternoon. 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. && .MARINE... 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. Latest guidance maintain these winds through the night, so am extending the SCA until 6 AM. 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. && .TIDES/COASTAL FLOODING... Water levels less than a 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. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 2 PM this afternoon to 6 AM EDT Wednesday for ANZ533-534-537-543. && $$ SYNOPSIS...HTS NEAR TERM...ADS/HTS SHORT TERM...HTS LONG TERM...HSK AVIATION...ADS/HSK/HTS MARINE...ADS/HSK/HTS TIDES/COASTAL FLOODING...HTS is the U.S. government's official web portal to all federal, state and local government web resources and services.