Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 290043 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 843 PM EDT Wed Jun 28 2017 .SYNOPSIS... High pressure offshore will continue to drift east of the region overnight. This will allow a southerly flow of warmer and more humid air to return to the area starting on Thursday. Showers and storms will be on the increase Friday into the weekend as a weak cold front stalls out near the region into next week. && .NEAR TERM /THROUGH THURSDAY/... As of 840 PM EDT Wednesday... Evening raobs and surface obs show moisture increasing a bit across the region this evening as weak south/southeast flow develops in the wake of high pressure offshore. This should keep low temps warmer overnight despite mainly clear skies with only the deeper valleys dropping well into the 50s and perhaps upper 40s northwest. May even see parts of the east including near the Blue Ridge only fall to around 60 per latest guidance so bumped up lows a little. Otherwise another quite night in store under subsidence with only passing high clouds over perhaps a little trapped strato-cu in spots. Also should see river valley fog become a little more widespread with some bump in low level moisture as dewpoints/temps saturate by daybreak. Previous discussion as of 300 PM EDT Wednesday... High pressure was centered over our region. Most cloud cover was limited to cumulus production near and just west of the crest of the Blue Ridge from roughly Floyd, VA southwest into the Northern Mountains of North Carolina. These will dissipate with the approach of sunset. Other cloud cover was some dirty cirrus spilling into the region in association with a storm system moving through the Midwest. The vast majority of this cirrus is on the thin side, and its presence will continue through the overnight hours. The center of the high pressure will head east overnight. This will have the impact of a trend of low level flow becoming more southerly, and allowing for higher dew point values to creep back into our area. Currently values in the upper 40s to lower 50s are common. By tomorrow morning we expect readings in the lower 50s to upper 50s. This will have the impact of preventing temperatures from falling as much as they did last night. Lows tonight are forecast to range from the full range for 50s across the mountains to the mid 50s to near 60 across the Piedmont. With the increase in moisture, light winds, and a best thin cirrus, fog will be more abundant late tonight into early Thursday morning as compared to this morning. On Thursday, the trend towards increasing low level moisture will continue with gradually increasing dew points. Today the atmosphere was conducive for some cumulus development along the southern half of the area near the crest of the Blue Ridge. Tomorrow, with the increased moisture and instability, this same region will be a target for actual isolated shower development, and will be reflected as such in the forecast. High temperatures are expected to be at least a category warmer than those of today. Anticipate highs around 80 to the lower 80s across the mountains with mid to upper 80s across the Piedmont. && .SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... As of 315 PM EDT Wednesday... Southerly surface winds on the backside of a large high pressure system well off the mid Atlantic coast expected to slowly replace very dry airmass currently over the forecast area (dew points in the 40s) with dew points more typical of early summer (mid 60s to lower 70s) by the end of the short-term period. The threat for at least widely scattered mainly diurnally-driven shower/thunderstorm activity will also increase through the period as a weak disturbance drifts northeast from out of the northern Gulf Coast states toward and through the mid-Appalachian region by late in the period. This system will likely significantly weaken and/or shear out by the time it arrives on Friday or Saturday, but weak forcing combining with increasing lower and mid tropospheric moisture supports most mountain areas receiving at least some rain Friday into Saturday, with a slightly lower threat across the Piedmont which will be a bit further removed from the weakening system. With relatively lackluster thermodynamic instability and only weak dynamic forcing, the threat for severe weather Friday into Saturday appears to be minimal at the present time. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... As of 315 PM EDT Wednesday... Although initial weak short wave trof will be moving away from the forecast area on Saturday night, additional upstream weak disturbances and associated weakening fronts are expected to move east through nearly zonal upper flow during the long-term period. First wave expected to move into/through the area on Sunday, which may lay down a weakening cool front that will stall out over/near the area by Sunday - with additional weak disturbances moving east and through the area on almost a daily basis through mid week. Exact timing of each wave is still somewhat problematic, but daily proximity of waves combining with a moist airmass and an old frontal boundary warrants a continuation of at least a widely scattered mainly diurnally-driven shower/thunderstorm threat throughout the long term period. Overall forcing and thermodynamics will remain lackluster through this period, so severe weather threat expected to remain extremely limited. && .AVIATION /01Z THURSDAY THROUGH MONDAY/... As of 120 PM EDT Wednesday... Expecting generally quiet weather this period at TAF sites as high pressure moves off to our east and return flow on the back side starts to bring increasing moisture up from the south as seen in satellite images of Cu field this evening. Expect the Cu to have a strong diurnal component and mostly dissipate with loss of heating tonight, though some remnant patches may stew around overnight. We should be able to radiate reasonably well with lower clouds decreasing and just some patchy cirrus above, and with dew points a bit higher than last night expect a little more fog/stratus development toward daybreak. Believe it will be mostly confined to the traditional valley locations and expect some obvis at KLWB and KBCB. Any fog/stratus will dissipate Thursday morning and expect VFR conditions through the end of the period with some Cu redeveloping especially from the southern Blue Ridge down through the mountains of NC where an isolated shower is possible. Expect generally light winds through the period. Extended Aviation Discussion... Friday, the flux of moisture into the region from the south will continue. Anticipate even better chances of showers and storms with localized, brief sub-VFR vsbys/cigs under the heavier cells. Friday night, the potential for IFR/MVFR fog will be greater, along with some areas with sub-VFR stratus formation. Saturday into Sunday, coverage of showers and storms increase with the approach and arrival of a cold front. Sub-VFR conditions will be more probable for a longer period of time, especially during the overnight hours. By Monday, the bulk of the precipitation will be south of the region. However, a wet surface and boundary layer will more easily prompt overnight fog and stratus. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...DS/JH NEAR TERM...DS/JH SHORT TERM...WERT LONG TERM...WERT AVIATION...DS/MBS

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