Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 291841 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 241 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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The marine wedge has eroded across the forecast area, and resides just across the eastern shore at this time. The surface boundary extends down I-95. There is not much thermal contrast anymore, but there is still weak surface convergence. That, combined with modest instability (MLCAPE around 500 j/kg) and good shear (40-50 kt effective bulk) has resulted in widely scattered showers across southern Maryland and the Fredericksburg area of Virginia. Would not rule out a thunderstorm or two, but conditions do not appear favorable for anything strong. These should dissipate by sunset. With ample low level moisture (dewpoints in the 60s) along the metro corridor and without anything impeding it, the marine wedge should return tonight. The only questions are how soon it will return and how far west it will make it. Current NAM/GFS plus latest RAP/HRRR suggest at least to the Blue Ridge if not I-81. Am taking an overcast layer to CHO-MRB. Believe the PoPs in MOS data drizzle. Have not yet added it to weather, but will advise evening forecaster. 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/...
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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.
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&& .LONG TERM /THURSDAY THROUGH MONDAY/...
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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.
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&& .AVIATION /18Z MONDAY THROUGH SATURDAY/...
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VFR for now, but aob IFR (low clouds and vsbys) should return tonight as marine layer spreads inland again. High confidence in its occurrence; questions revolve around timing. Current TAFs on the conservative side, taking restrictions just for the morning push; 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...
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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.
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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 are in place for Washington DC and St. Marys for this evening`s high tide cycle, which is the lower of the next two cycles. The advisory for St. Marys continues through Tuesday and additional advisories may be necessary at other sites for Tuesday morning`s 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 from 11 PM this evening to 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...RCM NEAR TERM...HTS SHORT TERM...HTS LONG TERM...CAS AVIATION...HTS MARINE...HTS TIDES/COASTAL FLOODING...MM

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