Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 170808 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 308 AM EST Sun Dec 17 2017 .SYNOPSIS... High pressure will build across the Southeast U.S. tonight and Sunday. Lows pressure over the Southern Plains will move northeast toward the Ohio Valley and weaken tonight and Sunday. Another low over the Southern Plains and lower Mississippi Valley on Wednesday will move east through the end of the week. && .NEAR TERM /THROUGH TONIGHT/...
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As of 300 AM EST Sunday... High pressure was along the southeast Atlantic Coast. This feature will remain relatively stationary the next few days, blocking any significant weather systems from impacting the area. There is an impressive looking short wave coming out of the Arklatex with precip advancing northeast through the Mid/Lower MS valley, but this feature will encounter this high pressure road block. As such, anticipating bulk of the rain associated with the trough to get steered around the periphery of the Atlantic Ridge, any encroachment of precip drying up as it enters the dry air and subsidence associated with the anticyclone. Sensible weather today will reflect increasing cloudiness from the approaching short wave disturbance. Some light rain is possible west of the Blue Ridge during the late afternoon and evening, but not looking for anything measurable to make it east of the Blue Ridge, although can`t rule out some sprinkles. In spite of increasing cloud cover, temperatures will climb above freezing, although any significant rise of temperature will be dampened per increasing cloud thickness. Most areas should reach the 40s today, and around 50 Southside into the Piedmont of NC where clouds will be thinner...values close to the seasonal norm. Considerable cloudiness early tonight will give way to partial clearing as the system fractures. Westerly winds will permit continuation of upslope low level cloudiness along the western slopes, cloud depth shallow. With no dendritic growth forecast in the cloud, p-type would be limited to mainly drizzle. Temperatures tonight will be dependent on cloud cover, any clearing allowing readings to slip close to freezing, but if clouds persist then lows will be in the mid-upper 30s.
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&& .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
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As of 200 AM EST Sunday... The medium range models are in good agreement, especially early in the period that our region falls under a zonal flow with weak ribbons of elongated vorticity skirting to our north, especially Monday night and early Tuesday. Meanwhile, a progressive upper trof moving through the northeast U.S. Tuesday night/early Wednesday helps push a cold front through our area during this timeframe. The third feature of interest during the short term forecast is a cut off low currently near Baja California, which ejects northeast and weakens as it moves over our area Wednesday. The 00Z ECMWF is farther north with the track of a sfc low associated with this feature, and as a result, brings precip farther north into our southern zones (NC/southern VA) Tuesday night into Wednesday. For Monday, lingering moisture and upslope flow may generate some light precip in the mountains, and the GFS still hints at some light freezing drizzle/freezing rain in the Greenbrier Valley to Bath county early Monday morning. Most of the SREF members keep precip liquid or do not develop precip at all. Even if the worst case scenario unfolded with spotty freezing drizzle, amounts would be extremely light and no adverse impacts are expected. Continued to lean toward the warmer side of guidance for highs, especially in the east where more sun is expected under downsloping winds. An increasing low level jet Monday night, should keep ridges mixed resulting in warmer temps for lows Tuesday morning. Downsloping winds persist into Tuesday, which have the potential to bring very warm conditions to our region. This may be mitigated, however, by increasing high/mid level clouds. Therefore, made little adjustments to highs Tuesday which still fall on the warmer side of most guidance. Precipitation associated with the ejecting upper level storm system could arrive as early as Tuesday night, but GFS ensemble means indicate best chances for rain (south of Highway 460) would be Wednesday into Wednesday evening. Coldest air remains to the north, with just a glancing shot, so temps Wednesday will still be near to slight above normal.
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&& .LONG TERM /WEDNESDAY THROUGH SATURDAY/... As of 210 PM EST Saturday... The second upper low also weakens as it moves northeast but takes a more southern track through the Tennessee Valley. This short wave crosses the Mid Atlantic region on Wednesday and Wednesday night. A third upper low reaches the Southwest United States late in the week with a larger spread in models solutions concerning location and intensity. The main track of the vorticity maximums will depend on the strength of the digging trof in the central United States and the upper ridging over the Southeast. Models were showing a spread of solutions for Thursday through Sunday. In-situ type wedge of high pressure may develop over the area on Thursday. Models keep the bulk of any precipitation south of Virgina. Will keep better probability of precipitation mainly on Friday night and Saturday. Decent plunge of much colder air advertised by the GFS and ECMWF on Friday and Saturday for the central and eastern United States. Air mass will be cold enough to support snow in the mountains on Saturday. && .AVIATION /08Z SUNDAY THROUGH THURSDAY/... As of 1105 PM EST Saturday... VFR conditions are expected during the extent of the valid TAF forecasts, with mainly mid clouds increasing Sunday. May start to see clouds lowering toward low end VFR in the mountains Sunday evening with a few showers possible. Extended Discussion... A weak frontal system approaching from the west may induce a brief period of MVFR with light rain showers across the mountains through Sunday night. This may impact KBLF and KLWB. VFR cloud bases anticipated elsewhere. Little or no impact is expected to aviation for the central Mid-Atlantic through mid week as weather features remain well north and south of the geographic region. The greatest potential will be Wednesday night, but even here, guidance is still not consistent with its various solutions. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...PM NEAR TERM...PM/WP SHORT TERM...PH LONG TERM...AMS/PH AVIATION...DS/WP

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