Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 170747 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 347 AM EDT Wed May 17 2017 .SYNOPSIS...
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High pressure will remain near Bermuda through Friday. A cold front will pass through the area late Friday into Friday night before stalling well to our south on Saturday. The boundary will return north as a warm front Sunday before a stronger cold front from the west passes through Monday into Monday night. High pressure may briefly return for Tuesday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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High pressure will remain near Bermuda through tonight. An upper-level ridge will also build overhead during this time. The subsidence from the upper-level ridge combined with a south to southwest flow around the Bermuda high will usher in unusually hot and humid conditions for this time of year. Max temps today are expected to range from the 70s and 80s along the ridge tops to the lower 90s across most other locations. Guidance is in good agreement that near record heat will take place this afternoon. See climate section below for details. Plenty of sunshine is expected today due to the upper-level ridge building overhead. However...the heat and humidity will lead to an unstable atmosphere. Isolated showers and thunderstorms are possible later this afternoon into this evening. Best chance for this to occur will be along a weak pressure trough across northern Maryland into northern Virginia and eastern West Virginia. Rising heights associated with the upper-level ridge will keep convection isolated at best across these areas and much of the time will be dry. Elsewhere...no significant lifting mechanism and a capping inversion should keep dry conditions in place. A south to southwest flow will continue around the Bermuda high tonight. Unusually warm conditions are expected with min temps ranging from the upper 50s and lower 60s in the Allegheny and Potomac Highlands...to the middle and upper 60s in the suburbs of Washington and Baltimore...to the lower and middle 70s in downtown Washington and Baltimore. Min temps this warm are more comparable to July and August instead of the middle of May.
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&& .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
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Bermuda high will remain in place for Thursday. More hot and humid conditions are expected along with sunshine. Max temps will be in the lower 90s across most locations. Cannot rule out an isolated thunderstorm...but current thinking is that a capping inversion will keep most places dry. Did allow for the slight chance of thunderstorms Thursday afternoon near the higher terrain of the Blue Ridge Mountains and Potomac Highlands in central Virginia. Terrain circulation may provide the lift needed across these areas but confidence is low. A cold front will drop through the Ohio Valley into and Pennsylvania Thursday night. The boundary should remain far enough away from our area to have any significant impact. A southwest flow ahead of the cold front will allow for more unusually warm and humid conditions. The cold front will move into the area Friday afternoon before eventually passing off to our south Friday night. A few showers and thunderstorms are possible ahead of the cold front. Confidence is lower in thunderstorms at this time since there will be a downsloping westerly flow ahead of the cold front. This may limit coverage of convection. The best chance for thunderstorms will be across central and southern portions of the CWA where a pressure trough may back the surface winds a bit. More unusually warm and humid conditions are expected...but it may not be quite as hot as today or Thursday due to lower heights. Max temps well into the 80s are expected for most areas and some locations will top off near 90...especially across the Washington Metropolitan area into central Virginia. Noticeably cooler conditions are expected behind the boundary later Friday night due to a north to northeast flow.
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&& .LONG TERM /SATURDAY THROUGH TUESDAY/...
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High pressure over southeast Canada will build southward along the eastern slopes of the Appalachian Mountains Saturday and Saturday night. There is a slight chance of rain showers in the Potomac Highlands and portions of the Shenandoah Valley. Elsewhere will be cooler and dry. As the high pressure weakens and moves offshore of southern New England Sunday and Sunday night, humidity will increase across the region with a southerly flow. A chance for rain showers and perhaps a rumble of thunder lingers in the Potomac Highlands. A cold front will move across the region Monday bringing increased chances of thunderstorms that could produce heavy downpours. Dry air returns with a separate area of high pressure building in from the west Monday night through Tuesday.
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&& .AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
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VFR conditions are expected through Thursday night along with south to southwest winds. An isolated thunderstorm cannot be ruled out near KMRB later this afternoon into this evening but confidence is low. A cold front will pass through later Friday into Friday night. A few showers and thunderstorms are possible ahead of the boundary. Winds will shift to the north and northeast behind the boundary Friday night. High pressure behind a cold front may bring VFR conditions Saturday. The boundary will return north as a warm front Saturday night through Sunday. SubVFR conditions are possible during this time.
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&& .MARINE...
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Bermuda high will setup through Thursday. A south to southwest flow is expected during this time. The wind field aloft will strengthen a bit later this afternoon. This may cause wind gusts to approach SCA criteria for a brief period...but confidence is too low for a Small Craft Advisory since the mixing layer will be decreasing during this time. However...southerly winds are expected to channel up the middle portion of the Bay and lower Tidal Potomac River tonight into Thursday. A Small Craft Advisory is in effect for these areas during this time. The wind field aloft will decrease a bit later Thursday...but the mixing layer will increase. Winds may approach SCA criteria during this time...but confidence is too low for an advisory due to the relatively cooler waters. More southerly channeling is possible Thursday night into Friday morning. A cold front will approach the waters Friday before passing through Friday night. A Small Craft Advisory will likely be needed for the waters during this time. An easterly flow is expected behind a cold front Saturday. The boundary will return north as a warm front Sunday. A Small Craft Advisory may be needed for portions of the waters during this time.
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&& .CLIMATE...
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Unusually hot conditions are expected today and Thursday. Record heat is possible during this time. Below are the record highs and high min temps for today through Thursday night... May 17th and 18th. Today`s Record Highs: DCA: 92 in 1974. IAD: 89 in 1986. BWI: 93 in 1896. Thursday Record High Minimums: DCA: 72 in 2015. IAD: 68 in 2015. BWI: 68 in 1900. Thursday Record Highs: DCA: 96 in 1877. IAD: 91 in 1987. BWI: 97 in 1962.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...BJL NEAR TERM...BJL SHORT TERM...BJL LONG TERM...KLW AVIATION...BJL/KLW MARINE...BJL/KLW CLIMATE...BJL

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