Area Forecast Discussion
Issued by NWS Sterling, VA

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455 FXUS61 KLWX 300125 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 925 PM EDT Mon May 29 2017 .SYNOPSIS... 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 Tuesday night and Wednesday night. Low pressure approaches from the west this weekend. && .NEAR TERM /THROUGH TUESDAY/...
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Primary forecast challenge tonight remains timing the return of marine wedge back into the area. This will occur as easterly onshore flow continues to be reestablished to the north of a quasi- stationary surface low across southern/northern VA/NC. While mid/high clouds are somewhat obscuring the western extent of the marine layer...advanced GOES microphysical products and ceilometer data indicate gradual westward motion...with marine layer beginning to impact portions of the western Chesapeake Bay coastline...even reaching BWI over the past few minutes. Low clouds will continue shifting westward overnight banking up against the Blue Ridge through morning. Some spotty drizzle is also possible overnight. Previous Discussion... Tuesday morning will be a struggle to erode marine air mass once again. That will impede afternoon thunderstorm development. However, the right rear quad of the upper jet, differential heating, and another surface boundary supported by a 500 mb shortwave should provide the catalyst for scattered if not numerous showers. Guidance generating enough CAPE west of the maritime layer to justify including chance thunder.
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&& .SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/... We will have one more front with shortwave support to contend with...this time late Wednesday. Have not made many changes to the database in association with this feature (still chance PoPs). In keeping with this pattern, have been reserved in use of thunder. Clouds should be numerous in advance of the front. One difference though is that this time it appears as though we will have a decent push of drier air behind the front, to support clearing skies Wednesday night. Decent consistency in guidance temps. Have not strayed far from an ensemble blend. && .LONG TERM /THURSDAY THROUGH MONDAY/... Thu is looking like a very pleasant day with Canadian high pressure settling in over the region for the first time in quite awhile. Plentiful sunshine and highs near or even slightly cooler than normal for the first day of June. It will be short lived however. By Saturday morning the next cold front is slowing as it approaches us from the north early Saturday. It will likely stall over or very close to us over the weekend, with low pressure riding along west to east across it and over us Sunday. && .AVIATION /01Z TUESDAY THROUGH SATURDAY/...
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Marine layer spreads inland overnight. MVFR has already returned to BWI/MTN...and have high confidence marine layer will continue westward...reaching most sites (though maybe not MRB/CHO). However, questions revolve around timing. Current TAFs a bit on the conservative side...currently have restrictions just for the morning push...though, it could be sooner. Erosion Tuesday will be a challenge as well. Restrictions will into mid morning if not later. Then, afternoon showers and thunderstorms a good possibility. These flight restrictions should be brief. Wouldn`t rule out more fog-related restrictions early Wed morning, followed by another round of possible thunderstorms Wed afternoon. VFR likely for rest of week.
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&& .MARINE... Flow pattern should be light for most of the week. Bigger concern would be fog tonight into Tue morning. Lesser chance Tue night into Wed morning. By late Wednesday, a cold front should dry airmass out. There could also be a few thunderstorms each afternoon through Wednesday, although not likely strong. && .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 are in place for Washington DC and St. Marys for the ongoing high tide cycle, which is the lower of the next two cycles. The advisory for St. Marys continues through Tuesday. A Coastal Flood Advisory will be issued shortly for Annapolis and Solomons (i.e., once they pass current high tide cycle)...as it appears likely they will reach minor flood stage for the morning high tide cycle. It is also likely that additional advisories will be needed for high tide cycles through Wednesday for the sensitive locations, and possibly elsewhere. 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 until 2 AM EDT Tuesday for DCZ001. MD...Coastal Flood Advisory until midnight EDT Tuesday night for MDZ017. VA...None. WV...None. MARINE...None.
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&& $$ SYNOPSIS...HTS NEAR TERM...MSE/HTS SHORT TERM...HTS LONG TERM...CAS AVIATION...MSE/HTS MARINE...HTS TIDES/COASTAL FLOODING...MSE/MM

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