Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 151702 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 102 PM EDT Tue Aug 15 2017 .SYNOPSIS... Upper disturbances move over the area through this afternoon, with a stalled frontal boundary in place. This front shift southeast tonight, with high pressure shifting over the Mid- Atlantic Wednesday. A cold front moves from the Midwest Thursday into the Appalachians Friday. && .NEAR TERM /THROUGH TONIGHT/...
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As of 100 PM EDT Tuesday... Early afternoon update will reflect a bigger distinction between the broad region of showers that is progressing through the western half of the area, as compared to the limited coverage, and breaks in the cloud cover, over eastern sections. As the coverage in the west heads eastward, and more surface heating takes places thanks to gaps in the cloud cover, look for coverage in the east to increase. Until then, do expect about another two or three hour break in the precipitation in the east. Have also adjusted hourly temperature, dew point, sky cover, winds/gusts based upon the latest observations and expected trends through the afternoon. Have lowered forecast high temperatures over several portions of the mountains thanks to plenty of cloud cover and rain over the mountains and current temperatures lower than earlier anticipated. As of 945 AM EDT Tuesday... Tropical moisture across the region this morning with observed PWATS around 2 inches and supported via latest blended TPW. This in advance of a wave axis aloft that will cross the region from the west by late afternoon, allowing the flow to turn more from the northwest into this evening. Seeing one batch of showers exiting to the northeast with one weak shortwave attm, with another along a trailing impulse near the cusp of the buckled surface boundary over the far west. Latest HRRR advances these showers east/northeast across the mountains through midday before fading as the low level wedge and overrunning start to break down this afternoon. This should allow some breaks to develop with latest guidance indicating some decent instability espcly southern half just ahead of the trough aloft pivoting across. Otherwise trends off most guidance suggest a decreasing trend to showers this afternoon, so after bumping up coverage/pops this morning, left an overall chance coverage scenario with more thunder/locally heavy rain later on. Temps/clouds remain tricky as would only take some breaks to punch readings well into the 80s east, while the far west, where clouds remain more prevalent, may struggle to reach the mid 70s. Previous discussion as of 330 AM EDT Tuesday... Scattered to numerous showers early this morning from WV/SW VA east to the piedmont. This is occurring along sfc boundary with wave of low pressure over KY/TN, interacting with pwats over 2", as well as a couple of vorts from TN to western NC. Will see greatest concentration of showers over the piedmont of VA and from the Mountain Empire north toward Greenbrier this morning. Overall high-res solutions painting a wet morning for most of the forecast area. Once main axis of upper level energy shifts east of the mountains this afternoon, should see decreasing threat over the mountains, then this evening in the east, though some of the high- res models pushing batch of showers east of the WV mountains/far SW VA by 12z this morning. Given radar trends, should be close. Nonetheless 50 pops to 80 pops warranted with highest along/east of the Blue Ridge. Again, hedging on whether we get sun this afternoon or not, and high-res models showing some breaks, so instability should increase with help of upper vort to allow thunderstorms to be isolated to scattered, or embedded in the main area of showers. Should see skies clearing tonight from the northwest as deeper moisture shifts to the east/southeast, though how far given Hurricane Gert further east slowing thing movement of the front down. Gert should be pushing back out toward the northeast late tonight allowing this to happen. Another shortwave skims across the TN/NC tonight which may keep showers isolated along/south of the NC/VA border. With clearing skies, light winds, and moist ground look for more fog late tonight into Wed morning. Highs today a few degrees warmer than Monday since wedge erodes, but still a few degrees below normal, with mid 70s to around 80 mountains, to lower to mid 80s east. PWATS are expected to drop off some late tonight, and should see lows still held up some, with lower to mid 60s mountains, to upper 60s to around 70 east of the Blue Ridge.
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&& .SHORT TERM /WEDNESDAY THROUGH FRIDAY/... As of 245 AM EDT Tuesday... Another surface high will drift over the northern Mid Atlantic Wednesday and wedge south into the Carolinas Wednesday night. Upslope flow across North Carolina High Country may be enough to spark some storms late in the afternoon. Weak steering flow will have any storms drifting east over the foothills in the evening, then fading by sunset. Storms are also possible along the SW VA Blue Ridge in the afternoon. However, mid level cap is lower, therefore storms will have a tough time developing or be short-lived. The wedge boundary should not advance into the area until the evening, therefore temperatures Wednesday will warm to or a few degrees above normal. Shallow wedge comes sliding across the region Wednesday night. Easterly flow will bring some low deck clouds into the region, but models look to dry and stable for any precipitation overnight. A cold front is expected to move from the Midwest Thursday to the Appalachains Friday. Pressure falls Thursday will erode the wedge by the afternoon. Prefrontal short waves will then bring the first round of storms into the region Thursday afternoon and evening. The next round will be with the passing of the cold front Friday. Thursday looks to be the better of the two day for strong to severe storms to pass over the area. If Thursday`s activity is lack luster, then Friday may be busy. Temperatures Thursday will warm above normal with mid to upper 80s west of the Blue Ridge to the lower 90s east. Similar temperatures expect Friday but could be cooler depending on timing of the frontal passage. && .LONG TERM /FRIDAY NIGHT 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 /18Z TUESDAY THROUGH SUNDAY/... As of 720 AM EDT Tuesday... Poor flying conditions til midday though some spots of VFR out there, but overall most of the time will be IFR with either low cigs or vsbys, with period of showers, especially BLF-LWB east to ROA/LYH. This afternoon will see another round of showers/few tsra but overall keeping VCTS out except DAN. Should be mainly low end VFR to high end MVFR this afternoon outside any rain. Tafs will be amended as necessary for approaching convection. Models showing it drying out from the northwest this evening and should see skies become skc/sct. Seems with wet ground and light winds, fog will be a good bet, and expect most to see at least MVFR vsby with LIFR vsbys possible. Will keep most MVFR but take LWB/BLF/BCB to IFR or lower after 05z. Extended Aviation Discussion... Fog should burn off Wednesday after 14z. High pressure will keep the skies sct/bkn at VFR. Scattered showers/storms possible Thursday but mainly VFR. 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. && .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/WP NEAR TERM...DS/JH/WP SHORT TERM...RCS LONG TERM...AL AVIATION...AMS/DS/WP EQUIPMENT...RAB/WERT

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