Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 290810 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 410 AM EDT Wed Mar 29 2017 .SYNOPSIS...
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High pressure will build into our region from the north, then slide east Thursday. The next storm system arrives from the west triggering showers and thunderstorms on Friday. Mainly dry weather is expected over the weekend in the wake of this system.
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&& .NEAR TERM /Through Tonight/ ...
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As of 300 AM EDT Wednesday... At 06Z (2 AM EDT)...a deep northwest flow was firmly established across the forecast area in the wake of a cold front that extended from eastern NC southwest into SC and west into Texas. Upslope stratus has spread south from the mountains well into the New River Valley. Models are in very good agreement that a wedge of high pressure builds into our area today from the north which allows sfc winds to veer to the east by late afternoon/early evening. Where the models struggle today is with the extent of cloud cover which will have an impact on the temperature forecast. Leaned toward the more pessimistic guidance for clouds based on current location of low clouds compared with short range models. This solution allows for clearing late morning into early afternoon before weak low level convergence and easterly upslope flow generate clouds along and near the Blue Ridge by late afternoon. With most guidance running too warm on high temps over the past few days...leaned toward the slightly cooler guidance with highs in the lower-mid 70s in the south to 60s in the mountains. Forecast soundings show a fairly deep column of saturated air (800mb and lower) developing late tonight as the wedge of high pressure strengthens. Confidence continues to grow that light precipitation and widespread low clouds will develop as southerly winds above the wedge transport higher PWAT values and the western portion of the forecast area falls under the nose of a surging theta-e ridge. With widespread cloud cover expected...leaned toward the warmer guidance numbers for lows.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY NIGHT/...
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As of 400 AM EDT Wednesday... A continuation of dreary, cloudy conditions driven by easterly flow associated with a cold air damming signature will continue into much of Thursday. Interestingly both the 00z NAM, GFS and 21z SREF Mean all point to modest/light QPF values along the southern periphery of the Blue Ridge (from Roanoke southwestward to the NC mountains and into a portion of the foothills). This may be driven by shallow upslope flow. However, large scale regime points to strong mid-level height rises/implied subsidence above shallow stable layer so this may in fact be something more along the lines of drizzle. Challenge will be temperatures and both 00z MOS output are only slightly warmer than their prior runs. Offered highs from the New River Valley eastward in the 50s due to the overcast, but west of the Blue Ridge (eastern TN Valley counties in VA northeast into southeast WV) may be some 5-10 degrees warmer due to southerly flow. Region will still be dominated by cold air damming, however its influence should begin to weaken after midnight owing to increasingly stronger forcing/precipitation moving in from the west. PoPs gradually increase as the nighttime progresses, highest by early Friday morning. NAM-based wind fields are quite strong above stable layer (nearing 50 kts). That is a setup for potential mountain waves and associated breezy to at times gusty southeast/south winds at higher elevations in our southwestern CWA (Smyth and Tazewell Counties southward into the Grayson Highlands/NC high country). Will include mention of gusty conditions in the HWO. Accompanying the southerly flow will be an increase in PWAT values to around 1" by morning. Should really only see temperatures fall a few degrees at most to lows in the upper 40s to lower 50s. That sets the stage for Friday, which stands to be the most active day in the short-term period. Vertically stacked low should be near the western Ohio Valley by 12z Friday, with an associated mid-level trough extending southeastward into GA and coastal SC. That trough will likely be the focus of what should be a band of rain and embedded thunderstorms, the thunderstorms driven primarily by elevated instability/mid-level lapse rates around 6.5-7 C/km. How strong any thunderstorms develop can become would be contingent on surface-based instability, and that should really be nil given the weakening CAD which may only lead to isothermal low-level profiles at best. Effective-layer shear is likely to be rather strong at values around 50 kts though, and the SHERB parameter supports a potential high-shear/low-CAPE setup. Certainly is something that will need to be watched in the coming days. Only kept thunder mention at slight chance at this point with elevated CAPE values only 250 J/kg off the 00z NAM; also note SPC`s SWODY3 which reflect general thunder for most of our CWA except for far eastern counties (5% severe/"Marginal"). Categorical PoPs are indicated for Friday as we should see at least a period of steady rain, primarily focused in the late morning to mid/late afternoon. Highs should be in the low to mid 60s for many on Friday. Rainfall amounts of around a half to three-quarters of an inch, highest along and in the lee of the Blue Ridge into the VA/NC piedmont. In the wake of the trough, the parent upper low begins to deamplify as it makes its way into West Virginia Friday evening. Generally diminishing PoPs in central sections, although does stay high chance to Likely in southeast West Virginia counties due to the deamplifying upper low track and northwest upslope flow. Precipitation character should also trend more showery in these areas. Should see lows in the mid 40s to low 50s Friday evening. For Saturday...upper low shifts off the coast with upslope rain showers in eastern West Virginia coming to an end early in the day. Mostly clear skies then predominate Saturday afternoon and evening for all areas. Highs in the mid 50s/low 60s across southeast WV, the lower 60s for the New River Valley/Alleghany Highlands to the Blue Ridge, and downslope-driven low 70s east of the Blue Ridge. Cold advection aloft (850 mb temps falling to +4 to +6C) along with clear skies due to building surface ridging supports lows Saturday night into the upper 30s to mid 40s.
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&& .LONG TERM /SATURDAY THROUGH TUESDAY/...
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As of 400 AM EDT Wednesday... Conditions remain dry into Monday morning, then we wait on the next system tracking across Texas to the Tennessee Valley Tuesday. Precipitation pattern may be similar to Thursday- Friday`s event with light rain over a weak wedge followed by short waves from an upper level low/trough to our west. Temperatures trend above normal again for Sunday. Temperatures cool down Monday with rain falling into a wedge. Tuesday will be wet with slightly above normal temperatures as wedge erodes.
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&& .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/... As of 125 AM EDT Wednesday... Post frontal upslope winds have generated low clouds across the mountains early this morning with IFR to MVFR cigs reported at 05Z (1 AM EDT) at KBLF and KLWB. Satellite imagery indicates this blanket of low clouds continues to expand across the mountains and expect to see IFR to MVFR cigs reach KBCB, but confidence lower these cigs reach KROA. Meanwhile east of the Blue Ridge, skies are clear but patchy fog is possible in the valleys with light winds and ground moisture from showers/storms that passed over this region on Tuesday afternoon. VFR clouds from northern VA may slip south into KDAN/KLH before dawn. High pressure slowly builds in from the north on Wednesday with a low level wedge of cooler air developing east of the mountains. Should see some drying under the increasing subsidence from the mid morning to the early afternoon...but also some uncertainty with the degree of residual cloud cover given heating and flow turning light north . Confidence low that KBLF and KLWB will see clearing. The threat for IFR and MVFR ceilings and visibility increase late in the TAF period tonight with widespread stratus along with light rain and drizzle, especially along the Blue Ridge. Extended Aviation Discussion... High pressure moves offshore Wednesday night into Thursday with the Mid Atlantic region remaining in the wedge. The next storm system moves in the Mississippi Valley Thursday night into Friday. The highest probability of rain will be on Friday along with sub-VFR cigs/vsbys with the showers. Once the low passes, northwest flow sub-VFR ceilings will remain into Saturday across the mountains with gusty northwest winds possible. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...PH NEAR TERM...JH SHORT TERM...AL LONG TERM...AL/RCS AVIATION...AMS/PH

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