Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 151025 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 525 AM EST Fri Dec 15 2017 .SYNOPSIS... A weak disturbance will pass over the northern Mid-Atlantic with the potential for light wintry precipitation from northern Virginia into Pennsylvania. High pressure then returns tonight with blustery Northwest Winds, followed by fair weather Saturday into early Sunday. && .NEAR TERM /THROUGH TONIGHT/... As of 400 AM EST Friday... Broad northern stream trough over the eastern U.S. will eject one more short wave this forecast period before the entire pattern changes this weekend. The final shortwave is weak and pretty strung out in terms of vorticity. Dynamic lift is weak, thus relying on isentropic lift in the low levels to generate any precip, and that alone is pretty anemic. The actual surface front which passed through the forecast area yesterday has stalled over the eastern Carolinas. As the upper disturbance swings through this morning, a surface low is forecast to develop along the old frontal boundary over eastern North Carolina then move northeast off the Mid-Atlantic Coast. The airmass north of the boundary and developing low pressure is quite dry, so not expecting much in the way of precip. Initially the lift will go into the development cloud cover with cloud bases of 3-5kft. Only elevations above 3kft in VA and eastern WV would be subject to any rime ice and or very light snizzle (freezing drizzle and or ice crystals (very light snow)) this morning...and this would be primarily along and north of Interstate 64 where models indicate slightly deeper moisture profiles. Farther south the lower clouds have been struggling to develop. The only other cloud element visible is coming from the cirrus shield across the Southeast which is forecast to remain primarily south of the VA/NC border today. Forecast for our CWA will reflect slight chance light snow or light freezing drizzle mainly north of the Roanoke valley this morning, but confidence very low that anything materializes much below 3000 ft in elevation in terms of the freezing drizzle. In order to get precip into the valleys, the moisture profile would have to get deeper, but if it becomes too deep, it would favor ice crystal growth vs. supercooled water thus permitting very light snow instead of drizzle. Either way, impact not meaningful and will not introduce any accumulations of ice or snow to the forecast. The only exception might be a dusting in the mountains north of Lewisburg WV and VA highlands north of Covington, but thats about it. By 2PM this afternoon, the upper level disturbance passes east of the area. This will allow winds to become more westerly, ending the isentropic lift and transition to some upslope clouds/flurries in the west and downslope clearing in the east. Temperatures today will struggle per presence of cloud cover, remaining in the 30s. Readings in the 40s may be obtained from the piedmont southward into NC where clouds will thin for the afternoon. Cold air advection will bring one more cold night before the pattern changes Saturday. The cold air advection and pressure rises will also result in blustery conditions this evening and overnight with gusts of 20-35 mph. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... As of 415 AM EST Friday... The weather pattern over the continental U.S. is forecast to change this weekend...the broad eastern U.S. trough exiting the region with flow becoming more zonal. Heights slowly build Saturday into Sunday and this will be reflected in a gradual moderation of the temperatures. There is an upper disturbance that will move northeast out of Texas, but it is forecast to become weaker with time as the building heights over the Southeast CONUS dampen the effects of the short wave. Models remain inconsistent in just how much moisture will make it into the area given westerly flow and ridging aloft, but it is looking like anything measurable at this point would be a tenth of an inch or less. As such will continue to advertise just low shower chances into the western part of the CWA later Sunday and chance pops most sections Sunday night. Should be warm enough for all rain given late day arrival with temps in the 40s Sunday, and only falling into the 30s to around 40 Sunday night. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 301 PM EST Thursday... Appears mild period to maintain itself for much of next week as flatter upper flow around ridging across the southeast states remains in place for most of the period. Will see some digging northern stream shortwave energy make for a brief return to passing eastern troffiness around midweek but overall weaker now with each run. Other than for much warmer temperatures, main concern will be with shower chances that could linger into Monday, and then perhaps expand again Tuesday with a residual shearing upper system from Mexico that will attempt to undercut the ridge. Guidance spread remains quite large in northward extent of deeper moisture into westerly flow aloft as well as timing through Tuesday night. Appears initial system much weaker so only running with low pops into Monday night with somewhat higher chances Tuesday pending added support coming out of the southwest. Should finally see a cold front cross from the northwest Wednesday allowing for drier but only slightly cooler weather before temperatures rebound again Thursday as high pressure slides offshore. Highs to rebound to well above normal levels for early next week with mostly 50s to near 60 east before cooling some to 40s/near 50 mountains to 50-55 east Thursday. && .AVIATION /10Z FRIDAY THROUGH TUESDAY/... As of 525 AM EST Friday... Upper level disturbance passing across the northern Mid-Atlantic will result in MVFR cigs from central WV/VA northward into PA. VFR ceilings will continue elsewhere. Several models suggest that low clouds/stratus and potentially light snow and freezing drizzle could develop along especially north of Interstate 64. Moisture is very shallow, so confidence is low with respect to any meaningful accumulation or icing. Aside for light icing threat, no significant weather hazards of note. Passage of the upper trough this afternoon will bring a period of gusty winds, mainly from a westerly direction with surface gusts of 20-30 kts likely vcnty of the ridge crests developing after 20Z/3PM. Upslope clouds will linger west of the Blue Ridge tonight, dispersing Saturday with widespread VFR anticipated Saturday into Sunday. Medium confidence in cloud bases and visibilities through the TAF valid period. Low confidence on any precip. Medium to high confidence for wind speed and direction. Extended Discussion... Generally VFR conditions are expected Saturday and Sunday. A chance for sub-VFR conditions from a weather system forecast to emanate from the mid-Mississippi Valley Monday into Tuesday. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...PM NEAR TERM...PM SHORT TERM...JH/PM LONG TERM...JH AVIATION...PM

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