Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 072337 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 637 PM EST Wed Dec 7 2016 .SYNOPSIS... Weak low pressure will slide by to our south after midnight shifting off the coast Thursday. A strong push of arctic air arrives Thursday as a strong high pressure system sinks southward into the the Plains. This colder airmass will stick around for the remainder of the week. && .NEAR TERM /THROUGH THURSDAY/... As of 622 PM EST Wednesday... Going to see some increase in cloud cover this evening as sfc low over western NC allows for some northern push of moisture. An upper vort streams across as well overnight and may be enough to produce isolated precip. No changes were made concerning precip threat after midnight, but looks low. Previous afternoon discussion... Weak high pressure will provide us with pleasant weather into this evening. Clouds will increasing tonight ahead of an approaching arctic cold front. This front is the leading edge of some very cold air, originating from Canada. It is time to find your winter gear for cold weather. The 12z models continued the trend of going drier with the front. WPC has cut back their qpf for us. But with such a cold air airmass, what little moisture there is associated with the front will all get squeezed out over the mountains. Incorporated a blend of the NAM and GFS for POPS tonight into Thursday. In any case, we are only talking about some very light precipitation, mainly trace amounts, associated with the arctic front with light snow showers western slopes and potential for a few light showers, sprinkles and/or virga along leading edge of the airmass change for the Piedmont toward daybreak Thursday. Low temperatures tonight will range from the mid 20s in the northern mountains to the mid 30s in the Piedmont. Cold front cross our area on Thursday with limited moisture. Expect cloud cover to erode rather quickly in the post-frontal airmass except in the favored southeast West Virginia counties where stratocu will linger. By Thursday afternoon, period of strong cold air advection arrives as 850 mb temperatures fall to -6 to -12C. It will become windy Thursday afternoon with strong pressure rises and northwesterly 850 mb winds around 35 knots. Wind speeds will remain below wind advisory levels. However, wind gusts as high as 35 to 40 mph are possible across the highest terrain. High temperatures Thursday will vary from the upper 20s in the northwest mountains to the lower 50s in the Piedmont, Temperatures will fall behind the 850 mb front Thursday afternoon. Wind chills will vary from the teens in the northern mountains to around 40 degrees in the Piedmont Thursday afternoon. && .SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY NIGHT/... As of 322 PM EST Wednesday... On Thursday night, gusty northwest flow will continue in the wake of the departing cold front. Across southeast West Virginia, and perhaps the first tier of neighboring counties in southwest Virginia, will experience upslope cloud cover and some isolated to scattered snow showers or flurries Thursday night into and through the day on Friday. Coverage will gradually decrease during this time frame, with ultimately western parts of western Greenbrier County being the last to see and end to the activity as flurries Friday evening. Any snowfall will be on the light side with western Greenbrier County experiencing the most snow with an amount by Friday evening around one inch. The remainder of the region will experienced limited or no cloud cover, especially in locations east of the crest of the Blue Ridge during this same time period. On Saturday, high pressure will build across the region and allow for weakening winds and very little cloud cover. Temperatures will trend colder heading into the weekend. Friday night the area will experience the coldest night of the next seven nights. Expect lows to be in the teens across the mountains with reading around 20 or the lower 20s across the Piedmont. The combination of gusty winds and lower than average low temperatures will allow the wind chill readings to reach the single digits across the mountains and teens across the Piedmont late Thursday night into early Friday morning. At elevations above 3500 feet MSL, wind chills may dip to around five degrees below zero for a few hours. Friday night, despite colder temperatures, wind chills will be slightly milder thanks to weaker wind speeds. Saturday night, temperatures will be a bit milder than the previous two nights thanks to the center of high pressure shifting to our east. This will will allow for an increasing southwest flow across the area and an increase in cloud cover associated with our next approaching system. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... As of 322 PM EST Wednesday... During this portion of the forecast, our region, and much of the central and eastern parts of the U.S. will be within a pattern of zonal flow or general troughiness, model dependent. Each of which offers a series of shortwave troughs moving through the overall pattern that will impact our region. Timing of the features differ, with GFS and Canadian solutions about twelve hours quicker regarding arrival and departure as compared the ECMWF - Sunday afternoon through Monday morning versus Sunday night through Monday evening. Low level thermal profiles differ among the guidance with specifics very questionable this far in time, especially regarding the notable differences in timing. Given the area will be within a region of warm air advection in advance of the system, regardless of timing, is seems reasonable that some location with the forecast area will be experiencing some for of wintry mix besides a rain versus snow forecast. However, given the uncertainties at this time, a rain versus snow, or a rain/snow mix, are the only two p-types being offered in the forecast at time time regarding this system. The forecast for Wednesday is equally, or perhaps a bit more, uncertain. Where the GFS has a substantial amount of coverage of precipitation from the Gulf to the Great Lakes in association with the passage of another shortwave trough, ECMWF has a much weaker system traversing our region. The GFS wants to tie both a northern stream wave with a piece of energy ejecting out of the western Gulf. The ECMWF keeps any wave in the Gulf disconnected, the thus has the drier solution for our area. Our forecast will reflect a rain versus snow forecast with more weighting placed on the wetter GFS solution. Temperatures will trend warmer Sunday into Monday with some cooling Tuesday, before another slight warm-up in Wednesday. As a whole, this time period will average a few degrees plus or minus of normal temperatures for this time of year. && .AVIATION /00Z THURSDAY THROUGH MONDAY/... As of 624 PM EST Wednesday... Terminals will stay VFR through the period. An increase in lower cigs will occur later this evening from south to north as weak low pressure over the NC mountains shifts east to the coast through Thursday morning. Appears cigs should stay VFR around 4-6kft. Arctic air pours in Thursday as a 1045 mb high slides into the Plains. A tighter gradient develops between this high and the low off the coast where gusts from the west to west-northwest occur. Most will see around 7-14kts sustained with gusts to 21-30kts mainly mountains and Roanoke/Blacksburg, with less wind LYH/DAN. Confidence high in all aspects of the forecast. Extended aviation discussion... Continued cold blustery conditions Thursday night into Friday, with upslope snow showers along areas west of BLF/LWB though some snow may tease BLF. MVFR cigs will be found across the mountains with VFR east of the Blue Ridge. West to northwest winds behind the front will be strong into Friday with gusts likely topping 25-30 kts at times. Winds and any residual low level cloud cover should quickly fade by Saturday as high pressure builds overhead making for overall VFR conditions this weekend. Next weather system to follow will be a cold front, models suggesting a Sunday Night arrival. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...KK/WP NEAR TERM...KK/WP SHORT TERM...DS LONG TERM...DS AVIATION...AL/KK/WP

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