Area Forecast Discussion
Issued by NWS Sterling, VA

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769 FXUS61 KLWX 250759 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 359 AM EDT Tue Jul 25 2017 .SYNOPSIS... A cold front will pass through the area tonight. High pressure will build into the Mid Atlantic Tuesday before moving offshore Wednesday. Another cold front will approach the area Thursday before passing through Friday into Friday night. High pressure is expected to return for the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Cold front has pushed southeastward of the area as of early this morning with north/northwest flow taking hold. This has ushered in a much less humid air mass with dew points as of 3AM in the mid 50s north and west of the metros. This air mass will continue overspreading the region through the morning with dew points by the afternoon from the mid 50s to mid 60s area-wide. In addition, temperatures will be a good 5-10 degrees cooler than yesterday, with highs generally in the low to mid 80s. Otherwise, early morning high clouds will give way to a developing cumulus deck during the day with more clouds north than south. An upper level trough moving off the northeast coast may even get close enough to help spark an isolated sprinkle or shower this afternoon across far northeast Maryland in conjunction with a developing westward push of marine air. && .SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY/... As the upper trough pulls away tonight and surface high translates eastward across New England, push of marine- influenced air will gradually move westward and towards the metropolitan areas. Westward extent by morning is still in question, but there may be some low stratus that move across portions of the region by sunrise. There may even be a few isolated light showers that develop and move into central/eastern Maryland. In addition, some patchy fog is also possible. Lows tonight 60-70F. On Wednesday easterly flow will turn southeasterly as surface high moves offshore. Broken cumulus deck expected again with temperatures similar to today, mainly low to mid 80s. Could also be an isolated shower or thunderstorm around, with highest chances near the Chesapeake Bay and near the Allegheny Front. Flow will then turn more southerly Wednesday night as the surface high moves further away and a low pressure system begins to make its approach from the west. There is potential for some low clouds/fog again late Wednesday night into early Thursday morning. South/southwest flow returns for Thursday and with it, the return of heat and humidity. Highs will rebound back to near 90F with dew points rising back to near 70F. As the frontal system approaches late in the day, chances for showers/thunderstorms increase, and SPC has placed the region in a Slight Risk for severe thunderstorms. The greatest risk of severe thunderstorms and heavy rain at this time appears that it may come Thursday evening/night into Friday morning. See long term discussion below for more information. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... Low pressure and its associated cold front will move into the area Thursday night before passing through Friday. A southerly flow ahead of this system will usher in plenty of moisture. Latest guidance shows PWATs close to 2 inches. An upper-level low associated with the cold front will also approach Thursday night before passing through Friday. Deep layer shear profiles should strengthen during this time. The forcing from shortwave energy associated with the upper-level low...the surface low and the cold front should trigger showers and thunderstorms. Some thunderstorms have the potential to be severe due to the strengthening shear profiles. Damaging winds seem to be the primary threat...but the threat for isolated tornadoes cannot be completely ruled out with the surface low nearby. There is also a threat for locally heavy rainfall as well. There is the opportunity for multiple rounds of thunderstorms Thursday night into Friday...and with PWATS around 2 inches this poses a threat for torrential downpours and localized flash flooding. Confidence in timing of heaviest thunderstorms and location remain low this far out...but there is an elevated threat for severe thunderstorms with localized flash flooding Thursday night into Friday. The cold front will drop to the south Friday night and high pressure will build overhead for the weekend. Drier....cooler and less humid conditions are likely along with sunshine. The high will likely move offshore early next week. && .AVIATION /08Z TUESDAY THROUGH SATURDAY/...
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VFR expected through today. Early morning high clouds will give way to a broken cumulus deck of clouds this afternoon with cloud bases generally 3500-5000 feet. May be an isolated shower at MTN. Winds today out of the northwest 5-15 knots. For tonight, flow will gradually turn light northeasterly as low level marine flow develops. This may bring a period of low ceilings/fog with MVFR/IFR possible late tonight into Wednesday morning. Confidence in placement/duration is low at this time. Conditions improve by Wednesday afternoon, with a returning chance later Wednesday night into Thursday morning. Winds turn southeast by later Wednesday. By Thursday, south/southwest flow returns along with increasing chances for showers/thunderstorms late in the day. Showers and thunderstorms are possible Thursday night into Friday. Some storms may be severe and vsbys may be reduced to IFR levels in heavy rain. High pressure will return for the weekend with sunshine and VFR conditions.
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&& .MARINE... SCA is in effect until 10 AM this morning as surge of northwest winds occurs behind frontal passage. Winds should gradually subside during the mid/late morning hours and remain generally sub-SCA through Thursday morning. Wind direction will turn northeast tonight, southeast on Wednesday, and south/southwest later Thursday. Winds will increase again by late Thursday, which should be our next chance of SCA conditions. Showers and thunderstorms are possible Thursday night into Friday. Some storms may be severe with damaging winds possible. Storms may also contain torrential rains that reduce vsbys below one mile at times. High pressure will return for the weekend. A Small Craft Advisory may be needed for portions of the waters Thursday through Saturday. && .TIDES/COASTAL FLOODING... Minor flooding is expected early this morning near Straits Point in St. Marys County. A Coastal Flood Advisory is in effect until 5 am. A northwest flow will cause anomalies to drop a bit today. However...an onshore flow will develop tonight and persist through Thursday. Elevated water levels are expected during this time. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Coastal Flood Advisory until 5 AM EDT early this morning for MDZ017. VA...None. WV...None. MARINE...Small Craft Advisory until 10 AM EDT this morning for ANZ531>534-537-539>541-543. && $$ SYNOPSIS...BJL NEAR TERM...MM SHORT TERM...MM LONG TERM...BJL AVIATION...BJL/MM MARINE...MM TIDES/COASTAL FLOODING...

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