Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 181648 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1248 PM EDT Fri Aug 18 2017 .SYNOPSIS...
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A cold front will cross the mountains from the west late this afternoon before sliding southeast of the region later this evening into tonight. The front will remain over the Carolinas Saturday into Sunday before dissipating. High pressure works in from the Ohio Valley during the weekend, then overhead Monday resulting in drier weather across the region.
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&& .NEAR TERM /THROUGH TONIGHT/... As of 955 AM EDT Friday... Surface cold front over central Kentucky still on track to reach the western mountains later this afternoon before spilling east of the Blue Ridge this evening. Best support aloft will again shift just north of the region as a weak shortwave trough crosses in from the northwest. However degree of instability quite impressive with MLCAPE values near 3000 J/Kg espcly Blue Ridge east this afternoon given very moist dewpoints and strong heating. In addition, axis of 850 mb theta-e ridging forecast to enhance off the eastern slopes of the mountains where the increasing mid level jet likely supportive of a few stronger bands/clusters of storms as seen via the latest HRRR. This also where expect the overall deep westerly flow to back more southwest this afternoon as current convergence with western slope pre-frontal clouds/showers tends to jump toward the residual eastern lee trough this afternoon. Thus have bumped up pops a little mountains this morning and from the foothills east this afternoon where expect enough coverage for a ribbon of likely pops. Otherwise mainly chance to scattered pops as the transition in convection from west to east takes place. Think enough heating espcly central/eastern sections to bump up highs as thickness supports 90-95 east, and 80s west outside of the far western elevations where may stay in the 75-80 range. Previous discussion as of 341 AM EDT Friday... Quiet wx wise this morning aside from dense fog in the Greenbrier Valley, with a few low clouds across the piedmont/foothills. Today`s forecast centers on cold front located this morning from the Great Lakes, southward into west central Kentucky. Progression of the southern section of the front has slowed due to weaker flow aloft. This will be the challenge today where storms fire up and how much sun we get ahead of the front to allow for increased instability. Shortwave over southwest TN at 300 AM shifts northeast into NE TN by 12z, then slows as it shifts east of the Blue Ridge this afternoon. Further north several embedded vorts will shift east in faster flow. Sfc front will be located from the central Appalachians to mid TN by midday, then east of the Blue Ridge by early evening. Severe parameters are marginal for most of the forecast area, but SBCAPEs progged in the 2000-3000 J/KG range along/east of the Blue Ridge this afternoon with marginal shear mainly across the northern CWA may allow for some storms to bring gusty to damaging winds. At the moment best support per models for severe is much further northeast toward DC/MD/PA. SPC has marginal risk over most of the forecast area today, with slight risk northeast of Lynchburg, and low/no threat in the mountains of far SW VA/WV. Wind is the main threat, but low to medium confidence on the overall threat. Again, isolated threat. As for rainfall, showers/storms should move along fast enough to prevent any rainfall issues, so flood threat is low. As for rain chances, with some upper support and instability plus low level convergence per front and orographics, should see a coverage of 40 to 60 percent across the area this afternoon. Models have been too aggressive lately on coverage, but now with the front in play, should be a little better. Still not everyone will see rain. Staying muggy with highs a degree to several cooler than Thursday, thinking more clouds arriving and storms firing up by midday. Highs will range from the upper 70s to lower 80s mountains, to upper 80s to lower 90s east of the Blue Ridge, but including Roanoke/Lexington. Will see the southern half of the sfc front lag over NC this evening/overnight which should keep shower/storm threat in the NC piedmont and southside VA at least until midnight. Will keep high chance early evening from Farmville, VA to Danville/Reidsville, then dry it out by dawn Saturday. Lows staying muggy, except drier air advecting in late in the mountains allowing temps to fall into the lower 60s, while the piedmont drops to the upper 60s to lower 70s. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... As of 300 AM EDT Friday... Frontal boundary will stall Saturday to our south across the Southeast States. Upper trough will rotate east across the region Saturday afternoon into Saturday night. Drier weather will prevail with subsidence and northwest flow. The ECMWF tried to push the moisture associated with shortwave further south in southeast West Virginia. We will see if this trend continues. High temperatures Saturday will range from the mid 70s in the mountains to the lower 90s in the piedmont. In the wake of the shortwave, better subsidence arrives Saturday night. Low temperatures Saturday night will vary from the upper 50s in the northwest mountains to the upper 60s in the Piedmont. The upper flow will flatten under slowly building heights on Sunday with surface high pressure building in from the northwest. The low level flow turn more southeast Sunday afternoon and could return some instability into Sunday evening. An isolated shower or thunderstorm may be possible in the south. High Sunday in the upper 70s in the mountains to the lower 90s in piedmont. High pressure will slide eastward Sunday evening into Sunday night with low temperatures generally from around 60 degrees to near 70 degrees. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 300 AM EDT Friday... Flat upper ridging will remain in place to start next week as surface high pressure shifts from the Mid Atlantic on Monday to off the southeast coast by midweek. This along with warming aloft within a west to southwest trajectory should keep things quite warm and increasingly humid from later Monday into Wednesday. Models are coming in a little wetter for Monday with isolated to scattered showers and thunderstorms as higher PWATS begin to advect back north. On Tuesday, scattered showers and thunderstorms are possible with deeper moisture aided by orographics. Highs likely around 90 east Monday/Tuesday and 80s elsewhere except possibly a little lower Tuesday far west pending clouds/showers. 500 mb pattern will once again become more amplified as another upper low crossing north of the Great Lakes helps dig out an even deeper eastern trough by the end of the period. This feature will also propel a strong cold front toward the area Wednesday with the boundary likely just south of the region during Thursday. Lead pre-frontal trough axis likely to spark better coverage of showers/storms Wednesday afternoon/evening, followed by cooler/drier air during Thursday and Friday. Therefore will include higher chance to low likely pops Wednesday and then mostly residual showers southern sections Thursday pending timing of the frontal exodus by then. Should see some cooling due to clouds including more showers Wednesday, then cool advection behind the front resulting in highs only 70s mountains to lower to mid 80s east Thursday and Friday. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
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As of 1240 PM EDT Friday... Most showers with isolated storms continue to linger across the far western areas, mainly just west of KBLF-KLWB early this afternoon where seeing pockets of MVFR cigs. Latest models continue to develop showers/storms across and espcly east of the mountains this afternoon into early evening ahead of the upstream cold front. Appears most of this will initially be scattered in nature over the west before perhaps organizing into more bands/clusters along/east of the Blue Ridge by mid afternoon. Shra/tsra should then push into eastern sections by early this evening prior to exiting shortly after 00z/8 PM if not sooner. Thus running with mostly VCTS/VCSH at all locations with tempo groups for lower MVFR conditions in showers across the west, and deeper convection east where IFR vsbys may occur. Appears fog will be stuck in the valleys and eastern locations that see heavier rainfall this afternoon although uncertain given gradual advection of dry air late. For now included sub- VFR in fog at KLWB/KBCB/KLYH late tonight while leaving out elsewhere. Some stratus also possible with the fog and perhaps even at KBLF with weak upslope flow. Outside of convection, winds will be mainly west to southwest at 5-15 kts ahead of the front this afternoon before turning light/variable overnight. Weak high pressure will build into the area Saturday bringing VFR conditions under northwest winds at 5-10 kts. Extended Aviation Discussion... Mainly VFR conditions for the second half of weekend under weak high pressure. Scattered thunderstorms and periods of MVFR conditions return next week, with late night/early morning fog possible almost any day. Better potential for sub-VFR will come Wednesday when more widespread showers and storms arrive with the next cold front.
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&& .EQUIPMENT... As of 400 AM EDT Friday Aug 18th... KFCX doppler radar expected to be down for the rest of the month due to a failing bull gear. Technicians are working on the radar, through next week and the radar will most likely stay down completely as the repairs are being made. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...WP NEAR TERM...JH/WP SHORT TERM...KK LONG TERM...JH/KK AVIATION...JH/RAB/WP EQUIPMENT...RAB/WERT/WP is the U.S. government's official web portal to all federal, state and local government web resources and services.