Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 300803 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 403 AM EDT Tue May 30 2017 .SYNOPSIS...
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Upper level low pressure will drift east across the north side of the Great Lakes this week. In the meantime, weak high pressure will prevail over the Mid Atlantic outside of weak cold frontal passages tonight and Wednesday night. Another cold front will slip south into the region later Friday and stall over the region this weekend.
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&& .NEAR TERM /THROUGH TONIGHT/...
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It`s a little difficult to discern in surface obs, but a boundary resides south of the area with a weak impulse around it. This impulse combined with weak elevated instability has allowed isolated showers to develop in southern MD. Onshore flow is also causing low clouds, fog, and perhaps a little drizzle to ease westward. It appears this will make it to the northern Shenandoah Valley, and perhaps barely to Charlottesville. Once it reaches its westward extent, it appears it will be very slow to erode today as onshore or southeasterly flow continues. HRRR has been the most aggressive model spreading showers with southern impulse northward during the midday hours. Gradually have POPs ramping up, but feel most precipitation through midday will be drizzle. Weak front approaches from the west this afternoon and evening. SBCAPE of 1000-1500 J/kg will build west of the maritime layer amidst deep layer shear of 40-50 kt. A few cells may develop ahead of the boundary on the terrain, but it looks like most of the activity would be near the actual front. With these parameters, at least a few strong to severe storms will be possible. Strong winds would be the main threat, though some hail is also possible. What remains uncertain is the position of the stable airmass. Majority of models keep the I-95 corridor locked in with clouds until at least early afternoon. The Baltimore area may struggle to reach 70 (areas which see sun will reach the 80s). So while elevated instability may help maintain storms into the I-95 corridor, their intensity should wane. Sunset will also help the cause. Certainly a situation that will need to be monitored today. Low clouds and fog look probable to redevelop tonight, especially from DC north and east. Lows will range from the mid 50s west to mid 60s metros.
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&& .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
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Clouds and fog should be quicker to erode Wednesday with less of an onshore component. Final front and shortwave will be approaching, but likely not until late in the day into the evening. Have timed precipitation chances accordingly. Instability looks shallower and lower in magnitude, so thunder may be more difficult to come by. However shear will be plentiful so the situation will need to be monitored. With the front clearing the area late at night, drier air will arrive and likely last through Thursday night as surface high pressure slides across the area. Temperatures will be seasonable.
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&& .LONG TERM /FRIDAY THROUGH MONDAY/...
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Next cold front will be approaching from the northwest on Friday, with southwesterly winds bringing warmer and more humid air northward into the region. Some guidance has slowed the approach of the front, keeping the region dry thru the day, while others bring the front close enough to get convection going in the afternoon and evening. Regardless, it will feel more summerlike. Saturday looks cooler as the front settles into the area, but the same split in the agressiveness of the front continues, with some guidance having it stall in the area while others pushing it south. Either way, chance of showers and thunderstorms will be present. Sunday, wave of low pressure approaches from the west, with increasing risk of showers and t-storms later in the day and at night. By Monday the speed of this low is in question, with some guidance pushing it east of us, while others keeping it nearby, so Monday could still be wet. Either way, late weekend into early next week does not look particularly warm.
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&& .AVIATION /08Z TUESDAY THROUGH SATURDAY/...
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Marine layer has been working inland with IFR to perhaps LIFR conditions. MRB/CHO will have the shortest duration of reduced conditions. However, it appears low clouds could hang on into the afternoon across the metros, especially Baltimore area, which may never see VFR conditions as low clouds are expected to develop again tonight. Baltimore area is most likely to have near-IFR conditions tonight. Showers/storms develop around MRB by mid afternoon and push eastward, although their intensity should decrease as they move toward the I-95 corridor. Coverage is also uncertain. Low clouds should be quicker to erode Wednesday. A front will move through late in the day with a chance for showers and perhaps a thunderstorm. VFR conditions are expected late Wednesday night through Thursday night as high pressure builds across the area. VFR likely Friday, but showers and t-storms will be possible later in the day thru Saturday as a front drops into the region.
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&& .MARINE...
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We saw an enhancement of onshore flow in southern MD during the evening, but that seems to be diminishing. Low clouds and fog will be the story this morning and again tonight with light onshore flow. A few showers or storms may reach the waters late in the afternoon to evening. though intensity should be decreasing. The upper Potomac has the best chance for gusty storms. A cold front will move through Wednesday night with another chance for showers and perhaps a storm. At this time it appears post frontal westerly flow remains below SCA. Light winds with high pressure Thursday and Thursday night. SCA not likely Friday or Saturday, but shower and thunderstorm risk grows as we head into the weekend with another cold front dropping down.
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&& .TIDES/COASTAL FLOODING...
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Water levels remain high and should continue to be above normal for the next couple of days due to the persistent onshore flow. Coastal Flood Advisories for most of our tidal shorelines with the exception of the far northern part of the Chesapeake Bay, where the easterly flow is actually keeping water levels a little lower. Some will likely need to be extended further given expectation of continued onshore flow into tonight. By Thursday, northwesterly flow as high pressure builds in should finally allow anomalies to drop.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Coastal Flood Advisory from 10 AM this morning to 4 PM EDT this afternoon for DCZ001. MD...Coastal Flood Advisory until 10 AM EDT this morning for MDZ018. Coastal Flood Advisory until 10 AM EDT this morning for MDZ016. Coastal Flood Advisory from 8 AM this morning to 2 PM EDT this afternoon for MDZ011. Coastal Flood Advisory until 3 PM EDT this afternoon for MDZ014. Coastal Flood Advisory until midnight EDT tonight for MDZ017. VA...Coastal Flood Advisory until 10 AM EDT this morning for VAZ057. Coastal Flood Advisory from 10 AM this morning to 4 PM EDT this afternoon for VAZ054. WV...None. MARINE...Small Craft Advisory until 5 AM EDT early this morning for ANZ533-534-536-537.
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&& $$ SYNOPSIS...RCM NEAR TERM...ADS SHORT TERM...ADS LONG TERM...RCM AVIATION...ADS/RCM MARINE...ADS/RCM TIDES/COASTAL FLOODING...MSE/MM/RCM

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