Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 102019 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 319 PM EST Tue Jan 10 2017 .SYNOPSIS...
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A warm front will gradually approach from the south this evening and tonight as high pressure slides east off the coast and a storm system passes to the northwest Wednesday. The warm front will cross the region Wednesday. A cold front will then drop back south across the region Friday. The front will stall to the south and a wave of low pressure will ride east along the front this weekend.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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High pressure resides off the Delmarva/Hatteras coast this afternoon. In spite of south flow, and wedge of subfreezing air remains between I-95 and the Appalachians. Further, dewpoints are in the teens and ground temperatures are in the 20s. These are all likely to play a role in tonight`s weather. Shortwave energy associated with cyclone over the western Great Lakes will head toward the forecast area this evening. There is ample lift ahead of the best vorticity advection, mainly upglide associated with a developing 925-850 mb low level jet, forecast to reach 50-60 kt. The chance of precipitation begins after dark (6- 9pm) with the best forcing/highest PoPs late evening west of the Blue Ridge, and just after midnight to the east. The main forecast challenge revolves around what form the precip will take. Guidance seems to be running curiously warm in light of ground temps/wet bulb temps and precip onset timing for the middle of January. Climo suggests that cold air will linger longer than guidance, especially in the Shenandoah Valley and across northern Maryland. Further, reports from Ohio have indicated that freezing rain can form even if surface air temperatures are above 32F due to the cold ground. Bottom line, believe that the warm up will be delayed. Have taken that into account for p-type forecast. So, low level profiles would be supportive of freezing rain when the rain does arrive. Have issued a Freezing Rain Advisory where the risk is the highest. Its possible that onset precip along I-95 may be freezing rain as well, but confidence in that is lower. In addition, the 1am end time is the best estimation right now based on current guidance. That may need to be adjusted as the event unfolds.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH THURSDAY NIGHT/...
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The first round of rain will pull out by early Wednesday morning. The middle portion of the day may even be partly sunny. Am still on the cool side of guidance, with high temperatures in the 40s. The next round of precip will approach late in the day, and overspread the area during Wednesday evening. This should be just rain. Main frontal boundary will remain positioned out to our west Thursday before settling towards the region Thursday night. Deep southwest flow out ahead of the front on Thursday will promote well above normal temperatures with highs likely reaching into the 60s. Thursday night will stay mild ahead of the front, but will see increasing chances of rain showers from the west.
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&& .LONG TERM /FRIDAY THROUGH TUESDAY/...
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A cold front will push south of the area Friday into Friday night. Meanwhile, a large area of high pressure will build over northern New England. This sets the stage for what looks like an ideal overrunning wintry precipitation event for at least the northwestern half of the CWA during the first part of the weekend. High pressure at the surface will wedge down the eastern slopes of the Appalachians reinforcing low-level cold air while warm advection precipitation moves in from the west and southwest. Models are in decent agreement on this setup so confidence is a bit higher than is typical for a day 5 forecast. A wave of low pressure passes south of the area Saturday night which should cause precipitation to end. Another system will approach the area early next week while a gradual warming trend ensues.
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&& .AVIATION /20Z TUESDAY THROUGH SUNDAY/...
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VFR conditions prevail across the terminals this afternoon. Rain will overspread the area early this evening. Ground temperatures likely to be cold enough to cause freezing concerns, especially at MRB/IAD/CHO. Warmer air will slowly work its way northwestward. Have greater confidence that there will be a changeover to rain at CHO/IAD near midnight. Confidence at MRB is substantially less. Precipitation should pull out before the morning push, and temperatures should be above freezing by then too. Anticipate that ceiling will drop rapidly as the rain arrives, in light of cold air and warmer air attempting to move in. Am forecasting the lowest fringe of MVFR (cigs 010); IFR entirely plausible. In addition, a 50-60 kt low level jet will prompt low level wind shear concerns, mainly east of the Blue Ridge. Once this first wave departs, VFR should prevail for much of the day Wed. Winds will be lighter, and periods of sun likely. Another round approaches late Wednesday, and will affect the terminals Wed night. This should be just rain, but will offer up ceiling and vsby restrictions...at least MVFR if not IFR. Mainly VFR Friday as high pressure moves just to the north, then sub-VFR becomes increasingly likely Saturday as wintry precipitation moves into the region. Winds should be light out of the northeast during this time.
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&& .MARINE...
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Mixing has been poor today, so winds have struggled to achieve Small Craft conditions. However, guidance suggests the best opportunity comes late this afternoon and this evening. Obs match this forecast, so have left Small Craft Advisory in place, for all waters through this evening til 3am and then just the Maryland Bay and lower Potomac past sunrise. The pressure gradient relaxes substantially tomorrow, and a 50-60 kt low level jet passes. Therefore surface winds will be much less. No additional Advisories will be necessary during the day. It`s possible again at night. Confidence not high, and will hold off for now. Winds diminish Friday and remain low into Saturday as a wedge of high pressure noses south over the waters.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...Freezing Rain Advisory until 1 AM EST Wednesday for MDZ003>006- 501>503-505-507. VA...Freezing Rain Advisory until 1 AM EST Wednesday for VAZ025>031- 036>040-501-503>508. WV...Freezing Rain Advisory until 1 AM EST Wednesday for WVZ050>053- 055-501>506. MARINE...Small Craft Advisory until 7 AM EST Wednesday for ANZ530>534- 537>543. Small Craft Advisory until 3 AM EST Wednesday for ANZ535-536.
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&& $$ SYNOPSIS...HTS NEAR TERM...HTS SHORT TERM...MM/HTS LONG TERM...DFH AVIATION...HTS/DFH MARINE...HTS/DFH

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