Area Forecast Discussion
Issued by NWS Blacksburg, VA

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793 FXUS61 KRNK 250139 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 839 PM EST Fri Feb 24 2017 .SYNOPSIS...
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A strong cold front will approach from the west overnight before sliding across the mountains early Saturday, and east of the region Saturday afternoon. High pressure follows the front Saturday night into Sunday bringing cooler air on gusty northwest winds for the remainder of the weekend.
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As of 825 PM EST Friday... Cold front now crossing east of the Mississippi River will slide east overnight, likely reaching the far western mountains around 12z Saturday. Evening soundings show quite a bit of dry air aloft ahead of the preceding pre-frontal bands of convection that look to reach the far western zones around daybreak. Guidance does show some weak instability late as well as moistening ahead of the showers under increasing southwest flow as forecast PWATS increase to around an inch or so by morning. Thus appears some of this upstream convection may hold together long enough to reach the I-77 corridor Saturday morning per steep lapses and latest HRRR. Otherwise slowed down clouds/pops a little given latest trends with overall clear to partly cloudy overnight until the leading edge of the solid cloud deck to the west arrives late. Also bumped up southwest winds along the western ridges just ahead of the boundary as expect decent mixing to persist as the gradient just ahead of the shra/tsra tightens. This may keep lows from dropping much at elevation far west so bumped up temps on the ridges, while leaving elsewhere similar to before, mainly 50s. Surface front should pivot east through the region by early to mid afternoon Saturday with the actual 850 mb boundary lagging a few hours behind. This along with the best dynamics passing north and more westerly flow, as perhaps the old outflow zips east, should clip off coverage nearing the Blue Ridge Saturday morning. However guidance shows some increase in instability east/southeast sections by early afternoon where likely to see showers redevelop along the previous outflow. Timing and fast movement would suggest a very limited severe threat unless things slow down more at this point. Otherwise other concern with post frontal winds as much colder air behind the 850 mb front arrives late Saturday when subsidence increases and the jet aloft remains marginal. Latest local guidance suggests some potential for a period of wind advisory criteria mainly Blue Ridge from ROA to TNB so included mention mountains in the earlier updated HWO. Previous discussion as of 320 PM EST Friday... High pressure and upper level ridge along with abundant sunshine pushed the temperatures into the 70`s across most of the area today. Warmer than normal temperatures will continue into tonight as southerly flow is maintain ahead of an approaching cold front. Overnight lows will generally be in the 50s with mid to upper 40s along ridgetops. Showers associated with the cold front will enter southeastern West Virginia and southwestern Virginia around sunrise tomorrow, then move to the Blue Ridge by 10 AM. Some gusty winds and a rumble of thunder may accompany these showers. These showers will move over the foothills and piedmont counties during the early afternoon. Increase instabilities from daytime heating may allow some of these showers over the piedmont to intensify during the afternoon. The main threat from these storms will be straight-line winds. Latest models are showing the line of showers redeveloping and intensifying east of route 15 (Farmville Road). If this is the case, the wind threat for the piedmont will be greatly reduced. The best threat and higher confidence area for strong storms Saturday afternoon and evening will be north of I64 and moreso north of the Mason-Dixon line. Rainfall amounts from showers Saturday will generally be up to a quarter of an inch (0.25). The upper level trough of this system will track across the Ohio Valley Saturday. This trough will bring a round of light precipitation to the mountains during the late afternoon and overnight hours to western slopes of southeastern West Virginia and southwestern Virginia. These showers will start out as rain, then transition over to snow as colder air filters in behind the Theta-E boundary late in the day. Trajectories from the Great Lakes are brief and along with the warm ground, only light accumulations of snow are possible at elevations above 3000 feet into the evening. With the slow arrival of cold air, Saturday`s temperatures will warm above normal with readings ranging from the mid to upper 50s west to the 60s east of the Blue Ridge. Southside Virginia may peak in the lower 70s Saturday afternoon.
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&& .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/... As of 330 PM EST Friday... Shot of cold air will cross the forecast area Saturday night into Sunday with temperatures tumbling back to near seasonal norms with highs in the 40s/50s and lows in the 20s/30s. Blustery northwest winds Saturday night will gradually diminish Sunday. Model soundings indicate some very shallow moisture across the Mountains west of the Blue Ridge Saturday night, so can`t rule out a few flurries, but the overall trend will be for drying and clearing with Sunday turning out to be Mostly Sunny. Zonal flow will develop across the CONUS by Monday, cold shot of air over our forecast area short lived, and replaced with warm air advection from the southwest. Southwest winds will also bring a return of increasing moisture and threat for showers. Short wave trough within the southern stream will bring increasing cloudiness on Monday, with potential for scattered rain showers Monday night. With moderating temperatures, not expecting any p-type issues...temperatures warming back above freezing and above normal. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 345 PM EST Friday... Zonal flow will promote warming temperatures for mid week with main focus for precip centered around noisy southern stream short wave energy which models are having difficulty timing. Through Wednesday, the region is expected to remain in a deep southwest flow in advance of a developing trough across the Central Plains states. The GFS continues to be more progressive in depicting pieces of energy streaming across the area, each with the potential for additional rounds of precipitation, especially on Wednesday. CAPE is also forecast to peak on Wednesday, and with trough generally forecast to come through during the day this suggest potential for thunderstorms. Since models have not quite nailed down the timing will list the threat as slight chance attm. Temperatures will also peak Wednesday followed by lowering numbers for Thursday behind the trough, dry weather and near seasonal temperatures returning Thursday night into Friday. && .AVIATION /01Z SATURDAY THROUGH WEDNESDAY/... As of 725 PM EST Friday... Guidance is depicting a scenario similar to last night which was a bit too aggressive with development of low clouds/fog so will shade conditions away from this and go more optimistic with generally VFR all sites tonight. The biggest challenge will be timing the cold frontal passage west to east across the region tomorrow. Expect fropa early morning at KLWB and KBLF, mid/late morning at KBCB and KROA, then early afternoon KLYH and KDAN. while best dynamic forcing will be shearing off to our north, expect there will still be thunder with the front. Winds behind the front become west to northwest and quite gusty as cold air advection generates good downward momentum transfer and the low level wind field amplifies late in the TAF period. Extended Aviation Discussion... Showers and isolated thunderstorms will be east of KLYH and KDAN by 00Z/7PM. Drier weather and increasing northwest winds follow behind the front on Saturday night through Sunday morning. High pressure will cover the region behind the front for late Sunday into Monday with lighter winds. A warm front is expected to stay near us Monday night into Tuesday keeping a threat of showers around with sub-VFR possible. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...JH/RCS SHORT TERM...PM LONG TERM...PM AVIATION...MBS/RCS is the U.S. government's official web portal to all federal, state and local government web resources and services.