Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 150014 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 814 PM EDT Mon Aug 14 2017 .SYNOPSIS... The approach of an upper level disturbance through Tennessee will help draw a stalled frontal boundary back into our region overnight. The interaction of the front and system will allow for increasing coverage of showers, and a few storms, across mainly western sections of the region. On Tuesday, the front will buckle back south with the departure of the upper system, returning the focus for additional showers, and a few storms, to the southeastern sections of the region. && .NEAR TERM /THROUGH TUESDAY/...
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As of 800 PM EDT Monday... A stationary front extended from Georgia to the eastern North Carolina. An area of low pressure was moving east through Tennessee. This low will help buckle the stationary front back north as a warm front this evening as the low makes progression east toward our region. Have focused the highest probability of precipitation across southern Virginia and northwest North Carolina between 11PM and 8AM based in the timing of the precipitation currently in the Tennessee Valley as depicted by the last few HRRR runs and other high resolution guidance. More fog is expected for tonight due to the light surface winds, dew points in the mid 60s to lower 70s and the expected precipitation. The fog is expected to dissipate by mid- morning Tuesday. During the day Tuesday, the front will have slipped back south a bit, but not too far outside of our region that it won`t act as a focus for for additional showers and storms to develop along it during the heating of the day. Anticipate the southeast sections of the forecast area to have the best coverage of precipitation on Tuesday. Low temperatures tonight will be comparable to those of this morning given limited change in our airmass.
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&& .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY NIGHT/... As of 312 PM EDT Monday... We`ll finally be able to rid ourselves of cloudy, rainy wedge conditions Tuesday evening. With mid-level heights generally rising and a steady stream of southwesterly warm and moist advection Wednesday night into Thursday, it portends a return to warmer and more humid conditions. As far as rainfall chances go, though a limited chance should exist Wednesday, the better chance for showers and thunderstorms is later Thursday and Friday associated with a upper-level disturbance and related surface low/cold front, each of which passes well to our north. First in a series of weak disturbances in broad mid-level southwest flow advances eastward before midnight Tuesday evening, with clearing expected from northwest to southeast. Lingering periods of showers, with a couple embedded thunderstorms should remain possible through the evening with the best chance in the east and south early. Steady NW -> SE decrease in PoPs after midnight. As clearing begins after midnight, should also see periods of radiation fog in the western river valleys with weak ridge of surface high pressure gradually nosing southeastward. All in all still a rather mild night with lows in the mid to upper 60s, though lower 60s in the Greenbrier Valley and along the southern Blue Ridge into the NC high country. For Wednesday, at least the first part of the day should be on the dry side. Today`s 12z guidance does show a mid-level vorticity max embedded in broader mid-level height rises that rotates from the Cumberland Plateau into western NC. The NAM shows a weaker disturbance and also less of an impact for our area, while the GFS and the ECMWF would warrant at least slight to lower end Chance PoPs Wednesday afternoon and early evening across the southern half of the county warning area for showers and garden-variety storms. Sided toward the global models in showing PoPs in the 20-30% range from a line along and south of Bluefield to Rocky Mount to Appomattox, highest furthest south. With weak wind flow leading to slow storm motion, could see some heavier downpours in spots. Coverage should diminish by mid-evening with forecast trending dry for Wednesday night. In the wake of this disturbance, warm advection commences Wednesday night leading the first of a couple mild and muggy nights. Highs mainly in the 80s with lows in the mid 60s to lower 70s. Today`s model guidance continue to show a frontal system and associated upper-level disturbance passing across the Ohio Valley and into the northern mid-Atlantic Thursday afternoon into Friday. On Thursday, the best co-location of stronger convective instability/deep-layer shear is more in IN/OH, which suggests a more organized scattered severe threat away from the Appalachians and Piedmont. Under modest height falls aloft and model-derived surface winds indicating surface convergence axis/pre-frontal trough along the western Appalachians, surface-based CAPEs of under 1500 J/kg with surface heat should generate scattered thunderstorms during the afternoon primarily in that area, with more isolated coverage elsewhere. Greater potential for more widespread showers and isolated embedded thunderstorms for the evening, this being essentially remnants from Ohio Valley storms. On Friday, a considerable amount of early-day cloudiness casts enough question on convective instability - if not limiting it entirely - ahead of the cold front aloft/850 mb wind shift. With limited deep-layer shear to values under 30 kts, thunderstorms would not likely be strong based on current indications from guidance. Warmest day would be Thursday with highs in the mid 80s to low 90s, and with dewpoints in the 60s to low 70s it leads to heat indices in the upper 90s in the Piedmont away from Smith Mountain Lake. More cloudiness supports highs in the 80s. && .LONG TERM /FRIDAY THROUGH MONDAY/... As of 312 PM EDT Monday... Cold front finally moves across the forecast area Friday night. A considerable amount of variance exists in the 12z GFS and ECMWF solutions for the weekend, particularly on if the front makes a full clear across VA or if it hangs around/stalls and picks up tropical moisture keeping our eastern and southern counties wet. Adding to this uncertainty is the timing of a mid-level disturbance in the northern stream across the Great Lakes region. Therefore, kept at least 15-30% PoPs east of the Blue Ridge for the weekend until less variability in guidance becomes apparent. Stayed close to a blend of guidance for highs and lows which keep temperatures near typical mid- August levels. A potentially warmer period is indicated by the GFS and ECMWF early next week as a broad heat ridge dominates much of the Plains and into the Ohio Valley. && .AVIATION /00Z TUESDAY THROUGH SATURDAY/...
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As of 800 PM EDT Monday... Stratus in the foothills and piedmont of Virginia and North Carolina dissipated late today for a brief break in sub VFR conditions at KLYH and KDAN but a short wave tracking out of the Tennessee Valley will spread numerous showers and scattered thunderstorms across southwest Virginia, northwest North Carolina and southeast West Virginia overnight. MVFR ceilings will fill back in across the region before midnight with IFR to LIFR clouds over the mountains and in any pockets of very heavy rain. Medium confidence that a majority of the precipitation will cross through the region between 03Z/11PM and 12Z/8AM. South to southeast winds will remain light at the surface. Combined with dew points around 70 and all the precipitation high confidence that areas of fog will develop this evening. Visibilities will be in the MVFR range. On Tuesday, medium confidence of slow improvement of ceilings and visibility. Once a weak surface low crosses into eastern Virginia, and surface winds become more west and northwest, conditions will improve, especially east of the Blue Ridge. This may not be until after 18Z/2PM at KROA, KLYH, and KDAN. Extended Aviation Discussion... Widespread fog is again expected on Tuesday night. Wednesday into Thursday the frontal boundary to our south will meander north and south for a bit until such time the departure of T.S. Gert allows the front to be shunted to the southeast. While the front is in our proximity, daily scattered precipitation and rounds of sub-VFR conditions are probable. Late night/early morning mountain and river valley sub-VFR fog will be possible. Friday into Saturday, we look to our northwest for our next approaching upper level system. This one will bring a cold front into and through the area Friday night. Look for an increase in precipitation on Friday with storms possible during the afternoon and evening. Sub-VFR conditions will accompany the heavier showers and storms, and also persist in the form of late night and early morning fog heading into Saturday morning.
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&& .EQUIPMENT... As of 755 PM EDT Saturday Aug 12th... KFCX doppler radar will continue to be down for the rest of the month due to a failing bull gear. It will only be operational during this time frame for brief periods if an imminent significant widespread severe weather and/or hydrologic event occurs. Operating the system for any amount of time until the bull gear is replaced risks a catastrophic failure that would further extend system downtime. From the 17th through the 27th the radar will most likely be down completely as the repairs are expected to be made during this period. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...DS NEAR TERM...DS SHORT TERM...AL LONG TERM...AL AVIATION...AMS/DS EQUIPMENT...RAB/WERT

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