Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 202054 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 354 PM EST Mon Nov 20 2017 .SYNOPSIS...
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High pressure over the Carolinas will move east and off the southeast coast tonight. A cold front will move through the eastern United States Tuesday night, followed by high pressure on Wednesday and Thursday. Another cold front will cross the region this weekend bringing the next round of colder air.
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&& .NEAR TERM /THROUGH TUESDAY/... As of 200 PM EST Monday... Large high pressure area centered over the southern Appalachians is the main weather feature affecting weather across the Piedmont and into the Appalachians this afternoon. Temperatures have rebounded nicely back into the 40s to mid-50s under plentiful sunshine. A split flow regime was evolving aloft, with a southern stream trough over the southern Plains spreading an area of high clouds across the lower Mississippi Valley in a lower-tropospheric warm advection regime. Clear skies will initially predominate the region for the rest of the afternoon and into a good part of tonight. This should lead to rather strong radiational cooling areawide with a decoupled boundary layer leading to light surface winds. Radiational cooling should be enhanced in areas where light snowpack exists per satellite imagery across western Greenbrier County. Moderately strong surge of warm advection begins after midnight on a southwesterly 850 mb jet from 30-40 kts per 12z NAM-3km output, greatest south. Will also see an increase in high clouds across the southern third of the CWA. Given the low-level jet on the ridges and the aforementioned radiational effects, this should lead to low temperatures being the warmest at elevation with values in the mid 30s along the Blue Ridge and western side of the Appalachians, with lows in the lower valleys in the upper 20s to near freezing. For Tuesday, look for variable high cloudiness to exist early in the day. As the day progresses, weak shortwave energy rounding the base of the southern stream trough in diffluent flow should contribute to an increase in cloudiness mainly from the Blue Ridge foothills in NC and advancing into the Piedmont of NC and Southside VA. This appears to be more probable in the afternoon hours. Guidance has generally shifted the chances for (very) light rain into the Tuesday evening period, but left open the door for rain starting early as the afternoon with 15-20% probabilities along or south of Route 52. Rest of the region should be dry. Though temperatures should climb following typical diurnal trends pretty much everywhere, I`ve started to slow temperature climb by afternoon in areas under thickest cloud cover/chances for rain. A little less confident on temperature trends in these southern areas depending on timing thickest cloud cover. I may still not be low enough and the cooler 12z MET values (e.g. 51 at Winston-Salem, 54 at Mt Airy) may prove more correct if thicker clouds develop earlier in the day. Official forecast calls for highs in the 50s, in the lower 50s further south with warmer values in this range to the north and west. && .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY NIGHT/... As of 200 PM EST Tuesday... High pressure slides offshore Tuesday night in advance of both northern/southern stream shortwaves within the lingering split flow regime. These features still remain out of phase with most solutions showing only a few light showers across the south/east with the surface low offshore, as well as isolated rain/snow showers far northwest with the cold front. Thus will keep in low end pops mainly Tuesday night and dry elsewhere for now. High pressure following these systems to bring drier weather including a period of weak cool advection into Wednesday night and Thursday, but with less wind than previously thought given a weaker post frontal jet aloft. Otherwise after more clouds Tuesday night through midday Wednesday, should see skies clear Wednesday into Thanksgiving before more clouds shift north ahead of the next southeast offshore wave Thursday night. Low temperatures mainly Tuesday evening before rising along the ridges overnight per warm advection/mixing and clouds. This supports a range from low/mid 30s valleys and east to 40s at elevation. Cool pool at 850 mb looks to only bleed into the west Wednesday and across the area on Thursday. Latest guidance even giving around 60 out east Wednesday afternoon ahead of the lagging cool advection with mainly 50s east so trended warmer with highs. Overall thickness in line with cooler highs mostly in the 40s on Thursday with lows 20s to around 30 overnight. && .LONG TERM /FRIDAY THROUGH MONDAY/... As of 130 PM EST Monday... Mean upper troffiness to prevail through the period with reinforcing energy arriving late in the weekend that looks to carve out an even deeper 500 mb cold pool into Monday. Otherwise should see mainly dry weather under weak high pressure and slightly below seasonal temps until a strong cold front arrives later Saturday. This front will again be followed by colder temperatures Sunday with cooler air lingering into early next week under chilly high pressure. Appears a few rain showers will be possible northwest ahead of the front Saturday afternoon, followed by western snow showers early Sunday, but little more than continued very low pops for now. Highs mostly 50s Friday/Saturday, cooling to mainly 40s Sunday-Monday, with perhaps 30s in spots across the mountains behind the front similar to this past Sunday. && .AVIATION /21Z MONDAY THROUGH SATURDAY/... As of 1243 PM EST Monday... High-confidence VFR conditions through the 18z TAF period. Broad surface high pressure continues to migrate eastward to our south, leading to a light northwest component roughly east of the Blue Ridge contrasting with a light west to southwest component. Expect a subtle backing to west-southwest in the Piedmont with all TAF sites eventually going to a southwesterly wind 3-5 kt by 00z Tuesday. Clear skies should support an eventual decoupled boundary layer and lighter wind flow by overnight. May have some pockets of low-level wind shear as 925-850 mb flow increases to around 35 kts by early-morning Tuesday associated with warm advection. Accompanying that will be an increase in high clouds from southwest to northeast and predominantly over our Piedmont/Southside counties. Still could see some marginal low-level wind shear into the morning hours but less confident on this so left out of the TAFs for now. Expect more variable cloudiness into Tuesday, with the greatest cloudiness to the south and east. This will mostly affect Danville with VFR mid- clouds. It`s not until after 18z Tuesday when degradation in ceilings then becomes a possibility at Danville. Southwest winds increase Tuesday to around 6-8 kts. Extended Discussion... Look for lowering ceilings along the southern Blue Ridge foothills in NC and into the Piedmont Tuesday afternoon into the evening. Though VFR ceilings should be common outside of these areas, MVFR bases appear possible at Danville Tuesday afternoon and evening, along with limited chances for light showers. High pressure then establishes itself for the Wednesday through Friday Thanksgiving period, with an expectation of good flying conditions for Thanksgiving travel in the RNK airspace. Next chance for sub-VFR conditions is with a cold front on Saturday. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AMS/JH NEAR TERM...AL SHORT TERM...JH LONG TERM...JH AVIATION...AL

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