Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 110722 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 322 AM EDT Wed Oct 11 2017 .SYNOPSIS...
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A frontal zone will remain nearly stationary across the Mid-Atlantic through this evening. An area of low pressure will track along the front tonight and push it to our south and east late tonight into Thursday. High pressure will build north and east of the area later Friday through Saturday before moving offshore Sunday. A stronger cold front will pass through the area early next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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The stalled front has become a lot easier to locate overnight thanks to modest instability and isentropic lift triggering a line of showers and thunderstorms located just south of Fredericksburg and Charlottesville...or just south of the forecast area. Since the air mass contains anomalously high precipitable water, rainfall rates have been impressive...locally exceeding 2 in/hr per dual- pol estimates. While instability will be easing slightly toward dawn, the zone of best lift...and the front...will both be lifting north to the aformentioned cities. Therefore, anticipate an uptick of precipitation across central Virginia toward dawn. Meanwhile, the shortwave trough will be passing eastbound just south of the Great Lakes today. So, not only will there be isentropic lift, but also ascent due to positive vorticity advection and the right rear quadrant of the upper jet. As such, a line of showers should overspread the region today. Anticipate that modest instability will remain, especially south of the boundary...which should stall somewhere close to the Potomac River. Rainfall should continue to be efficient, with rates moderate to locally heavy. And, instability will regenerate too with daytime heating, so have added at least a slight chance of thunder. Guidance sugesting that central Virginia should see MUCAPEs exceeding 1000 j/kg, so have a higher order (chance) thunder there. The trough axis/occluded front and triple point low should cross the area this evening. Precipitation chances will diminish. However, ample low level moisture will remain along with easterly winds. Believe any overnight precipitation will be either light rain or, more likely, drizzle.
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&& .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
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The pattern through the end of the week (Thursday and Friday) will be similar, marked by east/northeasterly flow around building high pressure over New England. Model guidance exhibiting a pattern of cold air damming, with light precipitation lingering along the Mid Altantic. Forecast will be for ample clouds, chance of rain, areas of drizzle, and patchy fog. The mountains will be preferred for precipitation due to upslope, but chances reside just about everywhere. Temperatures will be substantially cooler, likely struggling to reach 70 degrees during the day, and staying near the dewpoints (in the 50s) at night.
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&& .LONG TERM /SATURDAY THROUGH TUESDAY/...
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A strong mid-upper level ridge will remain anchored to the south on Saturday with a return to above normal temperatures on Sat after a couple of cool days Thu and Fri. The ridge will be pushed to the east Sunday as a shortwave-trough and associated cdfnt move across the Great Lks region Sun night. There could be a few showers Sun night with the frontal passage but the westerly sfc wind trajectories ahead of the front and lack of instability at night suggest any rainfall will be very light at best. The trof axis will move across the area Mon night with a taste of fall weather middle of next week as sfc high pressure builds in from the northern Rockies.
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&& .AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
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TAFs will be a challenge for the morning, as conditions will be deteriorating as showers overspread the area. In the big picture, flight conditions will decline from VFR to IFR. The challenge will be to pinpoint when those threshold crossings will occur. DCA has already reached IFR, and CHO looks like its about to. Am uncertain whether DCA will remain constant after daybreak, as the source causing the showers still far removed. Am more confident with respect to CHO; believe that IFR will be in place by the morning push. TAFs have prevailing IFR after the morning push, during the midday hours, just ahead of a band of showers and perhaps a thunderstorm. Caution that there likely will be an error of several hours, and anticipate updates as these trends become more certain. Flight conditions likely won`t improve once the showers pass east of the terminals tonight. Instead low clouds and some fog will linger due to east flow. MVFR-IFR likely to linger for a majority of the time through the remainder of the week with a real taste of fall weather for the middle of next week. Cdfnt will be crossing the area late Sun night and early Mon with only a few showers but no flying restrictions anticipated at this time.
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&& .MARINE...
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Flow across the waters light from the east at this time, but winds should be increasing this afternoon and tonight. A Small Craft Advisory in effect for the Bay and lower tidal Potomac after midnight, and have extended it through Thursday. It likely will need to be extended further. Small craft advzy conditions likely Sun into Tue with a small chance of gale conditions Mon night due to cold air advection over warm water.
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&& .TIDES/COASTAL FLOODING...
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Water levels close to normal early this morning. Believe that water will increase as prolonged east winds will push water into the Bay. Issues may arise as soon as tonight or Thursday near St George Island, although have a conservative forecast of Thursday evening there. Complications will become more widespread on Friday.
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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 tonight to 6 PM EDT Thursday for ANZ530>534-537-539>541-543.
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&& $$ SYNOPSIS...LFR NEAR TERM...HTS SHORT TERM...HTS LONG TERM...LFR AVIATION...LFR/HTS MARINE...LFR/HTS TIDES/COASTAL FLOODING...HTS

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