Area Forecast Discussion
Issued by NWS Blacksburg, VA

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392 FXUS61 KRNK 201419 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1019 AM EDT Wed Jul 20 2016 .SYNOPSIS... A weak cold front will remain to the south of our forecast area today with high pressure in control through the remainder of the week. This will result in warm and drier weather, but an isolated shower or storm still cannot be ruled out. && .NEAR TERM /THROUGH TONIGHT/...
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As of 1015 AM EDT Wednesday... Latest satellite images show that fog/stratus west of the Blue Ridge has just about burned off, leaving mostly sunny conditions across the region. Morning sounding is quite dry but some short wave energy rotating through the northwest flow aloft will help generate some Cu this afternoon. Expect CAPE values to be modest at best but orographic effects along the Blue Ridge may be able to squeeze out a few showers/rumble of thunder so POPs of 15 to 20 percent range look good. The NAM is a persistent outlier solution in generating convection across the piedmont late today into early this evening. Other meso models do not support this solution and previous runs of the NAM too moist based on latest RNK sounding and satellite imagery. Will restructure POPs a bit to reflect this thinking and go with essentially a dry forecast with POPs of only 10 to 12 percent for the piedmont into this evening. previous discussion as of 405 AM EDT Wednesday... Difficult to find exact location of diffuse cold front any more, but based on mean sea level pressure trough it may be from far southeastern VA into northern NC and then southwest into far northern GA. Surface dew point front may still lie just to the north of our CWA. Still, it is clear much drier air has moved in aloft behind last evenings upper short wave and with mixing during the day should expect the dew point front to shift to the south of the area as well. SBCAPE values may be moderate during much of the morning, but with daytime mixing of drier air these should drop during the afternoon. Therefore, would expect a drier day, but with more sunshine still quite warm, or at least seasonable daytime temperatures. Some very weak convergence in the NW NC mountains and perhaps still close to the frontal boundary may be just enough to squeeze out an isolated storm or two, but anything trying to form will be battling entrainment of drier air aloft so not especially confident in anything forming. Hardly any hi-res models suggest this either. One small concern is that by evening as dew points increase again with loss of mixing that surface- based instability increases again. While there is hard to find any kind of triggering mechanism, any lingering outflow boundary may be enough to pop something but would tend to be weak and short- lived. The NAM is suggesting scattered storms across central part of forecast area by evening but think its low- level moisture is overdone. However, one or two other hi-res models or ensemble members suggest some signals similar but not as agressive as NAM. Based on this idea, have added a slight chance PoP in basically from ROA southeastward where these models suggest the best potential is, and only from about 6pm through 10pm. There may end up being nothing at all, but feel this pattern can produce isolated overnight weak convection and since just a couple of models hinted at it, felt it worthy of a very small PoP. Otherwise clearing skies again overnight with light winds under influence of surface high pressure. Patchy early morning fog a decent possibility in favored locations and especially if there are any pop-up evening showers which help to re-moisten the ground.
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&& .SHORT TERM /THURSDAY THROUGH SATURDAY NIGHT/... As of 200 AM EDT Wednesday... During this portion of the forecast a broad upper ridge will be centered over the central portion of the country. The center of this high will very gradually retrograde westward. The result will be a trend towards better chances of disturbances across the Great Lakes region heading southeast towards our region. This looks to happen on Saturday when a cold front works its way into the area and brings isolated to scattered showers to the region Saturday into Saturday night. Prior to this time, precipitation on Thursday and Friday will be limited to primarily isolated orographical activity along the crest of the Blue Ridge. Temperatures will trend warmer through the period with readings on Saturday averaging five to ten degrees above normal. Also on Saturday, the heat index is expected to be in the 100 to 105 range generally along and east of a Lawsonville, NC to Lynchburg, VA line. && .LONG TERM /SUNDAY THROUGH TUESDAY/... As of 230 PM EDT Tuesday... By Sunday the upper pattern will have transitioned to a quasi zonal regime with the jet stream riding well to our north. This will keep us in the heat and humidity through the first part of next week. The zonal flow will result in little push to get fronts trough the region and as a result, a washed out frontal boundary combined with a lee/thermal trof will keep an elevated chance for thunderstorms across the region on Sunday, followed by another weak front moving in from the northwest and continuing our chances for thunderstorms as it washes out over the area Tuesday. Air temperatures and heat index values will remain at hazardous levels on Sunday especially across the piedmont, with only a slight tempering of the heat through the first part of next week. && .AVIATION /14Z WEDNESDAY THROUGH SUNDAY/... As of 740 AM EDT Wednesday... Weak and slowly moving cool front near the VA/NC border this morning and continues to drop southward. Patchy IFR fog mainly impacting KLWB this morning but should lift by 14Z. Otherwise clear skies under ridge and north of frontal boundary will give way to some scattered cumulus clouds by midday, closer to blue ridge and just east this deck may be broken at times and an isolated early evening shower or storm is not out of the question but confidence way too low to consider any mention at TAF sites, but if it were to occur most likely from KROA to KLYH to KDAN through 02Z or so. With upper ridge dominating today, expect light winds of less than 7 kts from a generally northerly direction. VFR the rule for the day and this evening unless an isolated storm does happen to move over a TAF site. Late tonight toward Thursday morning patchy IFR fog is likely again at KLWB under the higher pressure and clearing skies, but less confident in fog at other locations. Brief MVFR vsbys would likely be the lowest at KBCB and KLYH. Extended aviation discussion... Strong high pressure aloft will move east across the region Thursday into next weekend. Cannot totally rule out an isolated storm each afternoon/evening through the period but appears overall VFR outside of the typical late night valley fog in spots. Hot and humid conditions east of the Blue Ridge this weekend which may impact density altitude, and also slightly better thunderstorm chances across the Appalachians. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH/SK NEAR TERM...MBS/SK SHORT TERM...DS LONG TERM...MBS AVIATION...JH/RAB/SK

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