Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 300201 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1001 PM EDT Sat Apr 29 2017 .SYNOPSIS...
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A front will sag south into the area through early Sunday...before lifting north as a warm front late Sunday. A cold front will then cross the mid-Atlantic Monday night. A secondary cold front will push southeast Tuesday night. Low pressure from the Tennessee Valley will head toward the Mid- Atlantic Thursday and Friday.
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&& .NEAR TERM /THROUGH SUNDAY/...
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Guidance has consistently overdone convection today. While there are still suggestions of thunderstorms developing as the backdoor front drops down overnight, have overall cut POPs somewhat, though still maintain chance showers and slight chance thunder for a descent portion of the region. That having been said, if little to nothing occurs, would not be shocked, but there remains plenty of instability out there, which if triggered could still result in thunderstorms. Low clouds and reduced visibility remain an issue overnight, with low clouds more of a threat north of the front, and reduced vis from fog an issue more likely to the south. Lows will be in the 60s for most areas. See climate section for record information.
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&& .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/...
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Backdoor front will likely stall out across the area on Sunday and be slow to lift back north. At the minimum, this results in a very tricky temperature forecast with 60s to the north and upper 80s to the south. The boundary will snake back north over the mountains, and this will be the most likely place for instability to combine with terrain forcing to produce a few thunderstorms during the afternoon. Shear will still be strong enough that gusty winds could be a threat with stronger storms. Absent of other forcing, these storms should end quickly with sunset. Temperatures will once again remain in the 60s overnight. On Monday, a deep low pressure system will work from the Plains to the Great Lakes, which will drag a cold front toward the forecast area (after ensuring the warm front lifts back to the north). Timing of the front (or prefrontal trough which could be what actually initiates convection), appears slightly slower, but the focus remains on the afternoon and evening, with rain quickly departing after midnight. Shear will remain strong, but instability should be lower, with hints of a lingering wedge over the region. The result may be linear storms which pose a locally gusty wind threat. Given slower timing and potential for lingering wedge/marine influence, best chance of severe would appear to be west of the metros. Highs likely reach the upper 70s to 80s, with lows Monday night falling into the 50s to lower 60s.
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&& .LONG TERM /TUESDAY THROUGH SATURDAY/...
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Weak high pressure will build in behind the cold front Tuesday. A gusty westerly breeze will usher in drier air. Temperatures will not be as warm as recent days. As the high moves east, a secondary cold front will move across the region from the northwest Tuesday night. This front should be a dry front. The front could sag into the Carolinas Tuesday night before starting to move northward as a warm front on Wednesday. A low pressure system is expected to develop along the western end of the warm front over the Lower Mississippi Valley Wednesday night and early Thursday. We introduce a slight chance of showers or a rumble of thunder Wednesday night with a higher chance to likelihood of encountering showers and thunderstorm Thursday through early Friday. At the tail-end of an associated cold front with the storm system, another storm system could develop and deepen over the interior Southeast U.S. Friday and Friday night. The chance of showers and thunderstorms will linger during the period. Temperatures will also be closer to normal Friday and Friday night. Brief high pressure could build into the mid-Atlantic region Saturday ahead of a cut-off low pressure system over the Southeast U.S. Dry conditions will be follow by increasing clouds later in the day then a chance of showers Saturday night.
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&& .AVIATION /02Z SUNDAY THROUGH THURSDAY/...
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At the moment, VFR conditions exist across the area with gusty S-SW winds as high pressure to the southeast exerts its influence. Have removed mention of thunder from TAFs since odds now appear significantly lower than earlier, but the chance for a storm is not zero overnight as a backdoor cold front slides south across the area, which could still trigger something given the unstable air mass in place. Winds become NE to E behind the front, and some low clouds and/or light fog could develop. Uncertain if front reaches CHO. While guidance indicates IFR potential, have limited to MVFR for now. Front likely bisects area on Sunday, with low clouds potentially persisting into the afternoon. Thunderstorm development may be limited to MRB vicinity. Gusty south winds develop Monday ahead of cold front. Showers/storms will likely affect the area during the late afternoon and evening, with the cold front moving east overnight. VFR conditions Tuesday through Wednesday night. Winds west- southwest 10 to 15 knots Tuesday, west 5 to 10 knots Tuesday night, becoming light and variable Wednesday, and northeast 5 to 10 knots Wednesday night. Conditions deteriorate on Thursday as low pressure moves toward the area.
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&& .MARINE...
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Thunderstorm potential has lowered significantly, but a stray storm could still develop overnight as backdoor front slides south into the unstable air mass. The front will stall out across the waters late tonight and Sunday with east winds north and southerly to the south. Winds may approach SCA criteria across the far southern waters on Sunday. Think thunderstorms chances are fairly low on Sunday. Gradient increases Sunday night into Monday, with SCA likely all waters on Monday. Potentially gusty showers and storms will arrive ahead of a cold front Monday evening. SCA conditions will continue behind the front. Small craft advisories likely continue into Tuesday before abating Tuesday night into Wednesday. Winds west- southwest 10 to 15 knots Tuesday, west 10 knots Tuesday night, becoming northwest 5 to 10 knots Wednesday, and northeast 5 to 10 knots Wednesday night.
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&& .TIDES/COASTAL FLOODING...
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An onshore flow will develop Sunday as a backdoor cold front stalls overhead. A stronger cold front will approach from the west Monday before passing through Monday night. Southerly winds will strengthen ahead of the front during this time. Elevated water levels are expected Sunday due to the onshore flow and minor tidal flooding is possible for sensitive areas Monday into Monday night as the southerly flow strengthens. By the end of the week though, a coastal low is likely to cause elevated water levels once again.
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&& .CLIMATE...
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Record highs were set at DCA and IAD today, but it fell short at BWI. Record high mins are possible at DCA, IAD and BWI for the day if it stays warm enough through 1AM (Midnight standard time). Records are less likely Sunday as a backdoor front drops down. Here are the record highs/warm lows through the weekend: Washington DC... Saturday 29 April...91 (in 1974)/68 (in 1956) Sunday 30 April...92 (in 1942)/67 (in 1983) BWI Airport... Saturday 29 April...91 (in 1974)/67 (in 1956) Sunday 30 April...92 (in 1910)/63 (in 1983) Dulles Airport.. Saturday 29 April...87 (in 1996)/62 (in 1996) Sunday 30 April...86 (in 2007)/64 (in 1983) Also of note...the warmest night time low at DCA in April is 69 degrees. That is being challenged as DCA only dropped to 70 this morning.
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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 midnight Sunday night to 6 AM EDT Monday for ANZ533-541. Small Craft Advisory from 6 PM Sunday to 6 AM EDT Monday for ANZ534-537-543.
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&& $$ SYNOPSIS...ADS NEAR TERM...ADS SHORT TERM...ADS LONG TERM...KLW AVIATION...ADS/KLW MARINE...ADS/KLW TIDES/COASTAL FLOODING... CLIMATE...

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