Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 112026 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 326 PM EST Mon Dec 11 2017 .SYNOPSIS... Weak high pressure will slide offshore tonight. An arctic cold front will pass through the area Tuesday and weak high pressure will build overhead Wednesday. Weak low pressure will pass through Thursday and another one will pass by to the east Friday. High pressure will return for Saturday. && .NEAR TERM /THROUGH TUESDAY/... High pressure is overhead at present but we have mid clouds moving overhead as warm advection gets underway aloft. This will continue tonight. We may have a brief period of clear skies allowing temps to radiate down near or just below freezing, but after that clouds should become abundant enough (if they aren`t abundant enough through the night) to keep temps from falling further, and in fact they may rise through the night. A south wind, though light, will dominate. A few models still generate some weak warm advection precip late tonight and early Tuesday as the warm front ahead of the approaching system lifts north. The atmosphere is dry and most models show no measureable precip east of the Allegheny Front, however. So, have kept POPs low, in the chance to slight chance category. Given the dry profiles, frozen precip could try to make it to the ground (sleet or snow), but with the warm actual temps, its not impossible something melts and then tries to refreeze at the surface if it remains cold enough overnight. Therefore, have allowed for all p-types where temps may be below freezing, and rain/sleet/snow showers elsewhere. All that having been said, given the low probability, the most likely outcome of tonight still looks like dry conditions with temps, if they do fall below freezing early, rising back above freezing later on, with no travel issues east of the Allegheny Front. On Tuesday, the warm front lifts northward east of the mountains and we could get pretty mild ahead of the cold front. Raised our highs a bit and they may need to come up a bit more given how mild it got today. East of the Allegheny Front, precip should be spotty and light, with any frozen transitioning to rain showers as temps warm during the AM. However, during the afternoon the strong cold front will blast across the area, with temps likely falling during the afternoon. With strong forcing and a little instability, would not rule out some showers with or just behind the front itself, so maintained some slight chance pops, with any rain showers transitioning back to snow. Winds will blow at least up to 30 mph with the front, and its not impossible we touch wind advisory criteria, though at this time its not our official forecast. Will let next shifts re- evaluate this potential. Along the Allegheny Front itself, the story will be quite different. Upslope flow will allow snow showers to develop late tonight and continue through the day on Tuesday. Accumulations during the day look like a general 3-6 inches, so have issued a winter weather advisory for the most likely locations (western Pendleton and western Grant). This may be a bit conservative as accumulations could touch advisory levels in western Mineral, western Alleghany and western Highland, and next shift will need to re-evaluate. Its also possible that given the strong upslope flow on Tuesday, we could reach warning criteria in western Pendleton and western Grant, particularly our sweet spot in Bayard. Next shift will look hard at this potential as well. Final issue which occurs mainly Tuesday night but starts late Tuesday afternoon is the wind chill. Most of Tuesday will stay above criteria all zones, but by late Tuesday afternoon, the strong gusty winds plus falling temps well into the teens will bring wind chills down into the single digits along portions of the Allegheny Front, so started the advisory for wind chills late in the afternoon in these areas. && .SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/... The cold is the main story for the short term period as a brutal air mass moves down from Canada. Winds will gust all night long and into Wednesday morning as high pressure briefly builds into the region. Probably stay shy of wind advisory most areas, but it may be close. Either way, with the strong cold advection, wind chills will fall into the single digits across the board, and across most of the far western counties it will reach wind chill criteria, so expanded wind chill advisory across western zones. May need to add Blue Ridge zones as well. Other story is the continued upslope snow along the Allegheny Front, which will continue through the night into early Wednesday morning before tapering off as high pressure builds. As mentioned above, we may approach warning criteria in our advisory zones, and advisory criteria in some of the zones adjacent to the current advisory, so future shifts may further expand/upgrade the existing headlines. As weak ridge of high pressure builds across Wednesday, winds will slack off and upslope snow will end by afternoon, but cold air mass will keep temps mostly below freezing through the day - coldest day of the year. Then a clipper system starts moving in Thursday night. Still some uncertainty with this system as it croses the area, but with the low expected to track south of at least part if not most of the CWA, the odds of some snow is certainly significant. Accumulations however will likely not be much. Best odds of snow however are northern areas near PA, locations where the low is most likely to stay south of. Lows will be in the 20s Wednesday night. && .LONG TERM /THURSDAY THROUGH MONDAY/... Not as chilly Thursday with a chance for snow showers, mainly in the west and north, as the next low pressure system arrives from the west. The chance for snow showers linger Thursday night and Friday, as well, due to the upper level cold air support behind the surface storm. A modifying area of high pressure is expected to build eastward behind the low pressure system Friday night through Saturday night, bringing milder temperatures and temperate southwesterly breeze. By Sunday, the next threat for rain or snow showers will come with a cold front sagging southward across the region. High temperatures, next weekend, could reach the 50s. A low pressure system could attach to the cold front and bring added lift of support for rain or snow showers Sunday and Monday. && .AVIATION /20Z MONDAY THROUGH SATURDAY/... VFR expected overall through Thursday night. However, it may not be completely so. Light precip is possible at times through the next 24 hours, which could be mixed rain/sleet/snow later tonight into Tuesday morning, and snow showers/squalls later Tuesday into Tuesday night. Any of these can reduce cigs and vis below VFR. Additionally, clipper system may bring light snow to the region late Wednesday night into Thursday, which would do likewise. Otherwise, main story is the northwest wind later Tuesday into early Wednesday, with 30+ knot gusts likely at times. A few 40 knot gusts can`t be ruled out either. VFR conditions Thursday night through Friday night. Winds southwest at 5 to 10 knots Thursday night, becoming northwest around 10 knots Friday and Friday night. && .MARINE... Quiet on the waters now but it won`t stay that way. SOutherly flow ahead of approaching cold front will increase tonight, with SCA slowly spreading across the waters as we head into Tuesday. Then strong cold front is likely to bring Gales late Tuesday into early Wednesday, and a Gale Warning is in effect for this period. Gales may linger through Wednesday morning before diminishing, but SCA may be needed through Thursday as another, much weaker low moves across the region. No marine hazards Thursday night through Friday night. Winds southwest around 10 knots Thursday night, becoming northwest around 10 knots Friday and friday night. && .TIDES/COASTAL FLOODING... Elevated water levels from earlier have droped some today as the flow goes west to northwest...but anomalies will likely increase as a southerly flow picks up for tonight into Tuesday. At this does not appear that there will be minor flooding but it will have to be monitored closely for sensitive areas. Strong northwest winds are expected later Tuesday and Tuesday night. Tidal blowout conditions are possible Tuesday night into Wednesday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Wind Chill Advisory from midnight Tuesday night to 6 AM EST Wednesday for MDZ501. VA...Wind Chill Advisory from midnight Tuesday night to 6 AM EST Wednesday for VAZ504. Wind Chill Advisory from 6 PM Tuesday to 6 AM EST Wednesday for VAZ503. WV...Wind Chill Advisory from midnight Tuesday night to 6 AM EST Wednesday for WVZ503-506. Winter Weather Advisory from 7 AM Tuesday to 7 AM EST Wednesday for WVZ501-505. Wind Chill Advisory from 4 PM Tuesday to 6 AM EST Wednesday for WVZ501-505. MARINE...Small Craft Advisory from midnight tonight to 4 PM EST Tuesday for ANZ531>533-537-539>543. Gale Warning from 4 PM Tuesday to 6 AM EST Wednesday for ANZ530>543. Small Craft Advisory from noon to 4 PM EST Tuesday for ANZ535- 536-538. Small Craft Advisory from 6 AM to 4 PM EST Tuesday for ANZ530. Small Craft Advisory from 9 PM this evening to 4 PM EST Tuesday for ANZ534. && $$ SYNOPSIS...RCM NEAR TERM...RCM SHORT TERM...RCM LONG TERM...KLW AVIATION...RCM/KLW MARINE...RCM/KLW TIDES/COASTAL FLOODING...RCM is the U.S. government's official web portal to all federal, state and local government web resources and services.