Area Forecast Discussion
Issued by NWS Blacksburg, VA

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508 FXUS61 KRNK 060010 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 710 PM EST Mon Dec 5 2016 .SYNOPSIS... An area of low pressure in the Gulf of Mexico lifts northeast tonight and reaches the Appalachians Tuesday resulting in widespread rainfall overnight into late Tuesday. Moisture pulls off the coast Tuesday evening allowing weak high pressure to build in from the Midwest on Wednesday. A strong cold front will cross the region on Thursday. && .NEAR TERM /THROUGH TUESDAY/...
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As of 700 PM EST Monday... Some minor adjustments in mainly the temperature readings needed this hour to bring into line with current conditions. Dewpoints were on track. Light rain has already begun to spread into the southwest part of the CWA and it appears that the pop grids have a good handle on the current radar trends, so no adjustments were needed at this time. As of 230 PM EST Monday... In the wake of this morning shortwave, west winds were relatively quick in eroding the lower clouds out. High pressure over the Ohio Valley late this afternoon will build east this evening and reach southern New England tonight. Moisture will surge northeast tonight as low pressure over the western Gulf of Mexico strengthens and moves northeast along a stationary front. Despite the dry air across our region, WSR-88d regional radar images showed rain lifting into Tennessee and western North Carolina. Felt that the fastest models,including the HRRR, RAP and SREF were too quick with drier airmass in place. Used a NAM blend with GFS for pops tonight. Incorporated the temperatures aloft for weather type. The challenge tonight is whether or not clouds will trap any of the afternoon warmth permitting slightly warmer readings overnight before the rain arrives. Since some of the models are entertaining the idea of some light wintry precip for the VA and WV highlands, a 1 or 2 degree temperature flucuation could make the difference between a cold rain versus some light sleet/freezing rain for the Shenandoah valley and the VA/WV Highlands. No winter weather advisories for tonight because any sleet or ice accretion should be minimal, only adhering to elevated surfaces as opposed to the ground (ground temperatures still too warm to permit travel issues). Low temperatures tonight will vary from around freezing mark to the lower 40s in the Piedmont. A strong cold wedge east of the Appalachians will remain in place Tuesday morning as low pressure moves northeast with strong isentropic lift including upper dynamic support. A coastal low pressure center will develop off the North Carolina coast Tuesday afternoon. Widespread precipitation will continue through the first part of Tuesday before starting to taper off during the afternoon. There is enough warm air advecting in Tuesday to transition any light wintry mixed precipitation in Greenbrier Cty West Virginia and the Alleghany Highlands to all rain. A light glaze of ice or coating of snow may possible at the highest elevations. High temperatures Tuesday will range from the mid 30s in the northern mountains to the mid and upper 40s in the south.
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&& .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/... As of 215 PM EST Monday... Surface low along the coast should exit Tuesday evening ahead of a secondary trailing cold front that will help boot out residual deep moisture overnight. However enough lingering wrap around rainfall to keep chance to low likely pops going early Tuesday night espcly western third as well as the far northeast sections with rain tapering elsewhere. Will see a band of more shallow nature moisture along the cold front that looks to cross the far west after midnight and the Blue Ridge by early Wednesday. Appears enough to warrant a chance of added showers western slopes overnight with a gradual transition to higher elevation snow showers late as 850 mb temps fall below 0C. Otherwise clouds lingering mountains with downslope clearing east and lows mainly 30s. Wednesday into Wednesday night will be the in between period separating a more zonal trajectory from the impending digging upstream 500 mb trough associated with the upcoming Arctic cold shot. Models even suggest a period of warm advection in the afternoon under sunshine as the flow backs from a weak northwest trajectory to southwest later in the day. This should also push highs into the 50s from the Blue Ridge east and close to 50 west. Flow deepens from the south/southwest Wednesday night with guidance suggesting enough moisture return for a few late night showers south/west, that should transition to snow showers far west early Thursday when the Arctic front nears. Front then crosses from northwest to southeast Thursday with cold advection/upslope driven snow showers western mountains and possibly a few sprinkles east. However moisture appears much less today off models per overall westerly flow, so cutting back on pops, but leaving in some mention given light QPF showing up off ensembles. High temps Thursday tricky pending timing of the 850 mb front, with a possible range from 30s west to around 50 Piedmont, before falling with a slightly slower trend likely the best bet with the frontal passage. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... As of 130 PM EST Monday... Strong cold advection will be ongoing to start the period as Arctic air pours in ahead of high pressure that should shift east from the central U.S. early Friday to about overhead the region by Saturday morning. However latest models tending a bit weaker with the associated upper trough per less digging that likely will keep the core of the Arctic airmass to the north. This also along with very dry air advecting in should help limit the degree of upslope snowfall behind the front as the strongest northwest winds will only coincide with the coldest air aloft Thursday night into Friday at best before moisture fades. Thus going with only low pops northwest mountains Thursday night with coverage gradually decreasing Friday. Elsewhere should remain mostly clear but still cold and blustery into Friday evening with overnight wind chills near zero across the higher ridges. Still appears that Saturday morning will see the coldest temps as the high settles overhead, although increasing high clouds may curtail good radiational cooling per latest warmer guidance. Otherwise lows mostly teens to near 20 mountains both nights, and 20s east with highs 20s to low 30s mountains Friday, to mid 30s to lower 40s east. Trough lifts out later Saturday with fast zonal flow aloft developing Sunday into Monday as the surface high shifts east with warming/moistening return flow developing. This will allow pieces of energy ejecting out of the Rockies to perhaps bring some chance of precip by late Sunday into Monday at this point. However guidance shows considerable spread with consensus having a stronger northern stream wave push a weak surface front with faint southern connection toward the region by Monday. This could bring some chance of precip to mainly the western counties by Sunday evening per faster GFS despite residual low level dry air. Appears this would be mostly rain Sunday night into Monday given column warming with possibly some mix late Sunday pending timing. However quite iffy espcly given much slower timing off the 12z ECMWF. Should see temps rebound into the upper 30s/40s Sunday and 40s to mid 50s Monday as warm advection aloft reaches the surface. && .AVIATION /00Z TUESDAY THROUGH SATURDAY/...
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As of 700 PM EST Monday... Poor aviation conditions expected through the TAF valid period. Negatively tilted short wave will move northeast from the southeast U.S. Tuesday bringing widespread rain, fog, and low clouds to the area through much of the TAF valid period. Local radars indicate that light rain has already spread into northwest NC and far southwest VA. A very dry layer of air just above the surface to about 500mb will take a few hours to saturate, but this should occur in the 04Z-08Z time frame from south to north. At that times, ceilings will lower from just the mid/high cloud shield presently in place quickly to MVFR/IFR and to widespread LIFR by daybreak. Visibilities will quickly fall into the MVFR range, the settle into the IFR range in the 08Z-10Z time frame and hold there through much of the day Tuesday. Strong short wave will lift to the northeast of the area in the 20Z-22Z time frame, at which time the steady rain will end, but expect widespread low clouds, fog, and drizzle to remain in place into Tuesday evening, smilier to what we saw yesterday, until better drying can develop after daybreak Wednesday. Winds will come around to the northeast east of the Blue ridge and southeast to the west of the Blue Ridge, with the possible exception of KLWB, where a northeast wind is also possible. An 850mb southeast jet will traverse the region early Tuesday, which will likely equate to a period of gusty winds in the usual western locations, namely KBLF. Some low end gusty winds are also possible across the Piedmont later tonight into Tuesday morning. Otherwise, generally look for speeds of 7-10ts through the TAF valid period. Medium confidence in ceilings and visibilities through the TAF valid period. Medium confidence in wind direction/speed through the TAF valid period. Extended aviation discussion... Drizzle, low clouds, and fog will likely remain in place Tuesday night before drier air and westerly flow evolve across the area Wednesday. Wednesday will be the transition day ahead of an Arctic cold front that is expected to move through the region Thursday. Limited moisture may result in a few showers in advance of the front Thursday, but upslope conditions should support snow showers across the mountains Thursday into Friday along with a significant surge of cold air. This will lead to VFR conditions east of the Blue Ridge through much of the period beyond Thursday, but MVFR conditions may linger into Friday across the Alleghanys. West to northwest winds behind the front will be strong and gusty into Friday.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...KK NEAR TERM...KK/PM/RAB SHORT TERM...JH LONG TERM...JH AVIATION...KK/PM/RAB is the U.S. government's official web portal to all federal, state and local government web resources and services.