Area Forecast Discussion
Issued by NWS Sterling, VA

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311 FXUS61 KLWX 221001 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 601 AM EDT Tue Aug 22 2017 .SYNOPSIS... High pressure will redevelop over the mid-Atlantic region today. A strong cold front will drop southeastward across the Lower Great Lakes today, before pushing across the mid-Atlantic tonight into Wednesday morning. The front could stall just offshore the Delmarva Peninsula Wednesday afternoon through Wednesday night. Reinforcing upper level energy will help to push the front farther offshore and allow for high pressure to build in from the northwest Thursday into Friday. && .NEAR TERM /THROUGH TONIGHT/... Radar as of 3am is much quieter to nil than 12 hours ago across the region in part due to radiational cooling at night and yesterday`s disturbance moving well to the east. We are still dealing with the warmth and humidity through sunrise. Some patchy fog could form in spots through sunrise, mainly in parts of the Virginia Piedmont and Shenandoah Valley. The heat and humidity will carrying on well into this afternoon. High temperatures are expected to reach the middle 90s in most places east of the Blue Ridge Mountains and away from the coastal areas of the Chesapeake Bay. Dewpoint temperatures in the lower to middle 70s today indicates rich tropical moisture across the region and when combined with the heat and plenty of sunshine, we will see heat index values near 100 to lower 100s in a large area from just east of US 15 to just inland of the Chesapeake Bay region. Use caution with outdoor activities and drink plenty of fluids to aid in conquering this heat. Along with the heat and humidity comes increasing clouds from northwest to southeast later this afternoon through tonight as a strong cold front pushes across the eastern Ohio Valley and into the mid-Atlantic region. Although we have a slight chance of showers and a thunderstorm along the Mason- Dixon region and northeast Maryland early this afternoon, we anticipate the showers and thunderstorms to increase in coverage and intensity from western Maryland toward northern Virginia and the I-95 corridor late this afternoon and through the evening. There is a slight risk for severe thunderstorms in western Maryland and parts of the eastern West Virginia Panhandle. A marginal risk for strong to severe thunderstorms extends from the mountains of Highland County in western Virginia to the metropolitan areas of the District of Columbia and Baltimore. The main threats with the strongest thunderstorms will be damaging winds and large hail and would be more likely to accompany such thunderstorms between 3pm and 8pm this afternoon into this evening. As we get into this evening and lose the daytime heating, the damaging wind threat should diminish in the thunderstorms. The biggest uncertainty later today and tonight is the timing of frontal passage, along with the coverage of the thunderstorm activity tonight as many of the models are hinting that a wide line of severe thunderstorms spread out and dissipate as it crosses the I-81 corridor. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... For the most part, Wednesday begins with a cold frontal passage across our region. As we get into the midday hours, the front could show signs of stalling just to our southeast as clouds and widely scattered showers linger along the front and just to the north. With added daytime heating and some backing of winds with height with frontal passage, additional showers and a few thunderstorms could form and be scattered throughout the eastern Piedmont of Virginia, southern Maryland and a large portion of the Chesapeake Bay. At this time, there is a marginal risk of severe thunderstorms across lower Southern Maryland and south into the Tidewater of Virginia Wednesday afternoon and evening. High temperatures, along and north of the front, will be cooler with highs in the lower to middle 80s. Humidity will be lower as well with high pressure to the northwest building southward and ushering in lower dewpoint temperatures. Throughout the overnight of Wednesday, the front should move far enough southeast to allow for conditions to dry out with the building high pressure. High pressure will continue to build into the region Thursday and Thursday night. Some upper level energy will try to scoot over the ridge of high pressure but should be unsuccessful of producing any precipitation Thursday into Thursday night. && .LONG TERM /FRIDAY THROUGH MONDAY/... The weekend forecast still looks quiet as high pressure passes north of the forecast area. One minor change is that guidance suggesting there may be a few terrain circulations, for a stray shower or thunderstorm, coincident with daytime heating in the mountains. Otherwise, sunshine should be plentiful and temperatures should be slightly below normal...with dewpoints in the 50s. && .AVIATION /10Z TUESDAY THROUGH SATURDAY/... VFR conditions expected at all terminals through Thursday night. Showers and any severe thunderstorms could reduce conditions briefly to MVFR at the MRB and perhaps IAD terminals later this afternoon into this evening. Also, showers or a thunderstorm could bring down conditions to MVFR near the CHO and DCA terminals Wednesday. Winds southwest 10 knots today, shifting west-northwest 5 to 10 knots tonight with frontal passage. Winds northwest around 10 knots Wednesday and Wednesday night. Winds generally northwest 5 to 10 knots Thursday and Thursday night. VFR conditions anticipated Fri-Sat under high pressure. && .MARINE...
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Wind gusts still on the cusp of Small Craft Advusory criteria in the mid bay/lower Potomac at this time. Have extended Advisory into mid morning. Small craft advisories are also in effect from Noon today through 11pm tonight. No marine hazards expected Wednesday through Thursday night. North/northwest winds will be in the 5-10 kt range Friday- Saturday under high pressure. A few gusts in the mid- teens possible.
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&& .TIDES/COASTAL FLOODING...
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Water levels remain around 1/2 to 2/3rds of a foot above astronomical normals early this morning. Overall, do not anticipate many inundation problems. At the current pace, Annapolis will just touch minor criteria; Advisory was previously issued and will let that hold. The departure at Southwest DC waterfront, which was under a quarter foot, has returned to 0.6 ft above normal. If that remains consistent, minor flooding would result. Have opted to issue Advisory for DC. Northwest will will arrive by Wednesday morning. We shall see if that will be soon enough to preclude another borderline situation at Annapolis tomorrow.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Coastal Flood Advisory until 11 AM EDT this morning for DCZ001. MD...Coastal Flood Advisory until 9 AM EDT this morning for MDZ014. VA...None. WV...None. MARINE...Small Craft Advisory until 10 AM EDT this morning for ANZ531>534-537-539>541-543. Small Craft Advisory from noon today to 11 PM EDT this evening for ANZ530>543.
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&& $$ SYNOPSIS...KLW NEAR TERM...KLW SHORT TERM...KLW LONG TERM...HTS AVIATION...HTS/KLW MARINE...HTS/KLW TIDES/COASTAL FLOODING...HTS

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