Area Forecast Discussion
Issued by NWS Blacksburg, VA

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853 FXUS61 KRNK 300507 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 107 AM EDT Fri Sep 30 2016 .SYNOPSIS... A strong upper level area of low pressure will remain just west of the mountains today before slowly lifting to the north later Friday night into Saturday. This system will combine with a residual stationary front across the region to produce periods of showers and thunderstorms into Friday night. The low will move northeast, away from the region, resulting in drier weather this weekend. && .NEAR TERM /THROUGH TODAY/... As of 1146 PM EDT Thursday... Keeping the flash flood watch over the north though we have had some narrow bands of torrential rain further south. The saving grace so far is that these bands have been moving enough to prevent flash flooding, but still causing some minor street flooding or low lying flooding. Two areas of upper level divergence associated with jet over KY. With still good deep moisture convergence per southeast low lvl flow, will see bands of showers/few thunderstorms continue to pivot north across most of the forecast area, with best focus lying from along the Blue Ridge near the VA/NC border north into the Alleghany Highlands. Previous evening discussion... Decided to hoist a flash flood watch for our Alleghany Highlands to Amherst County til early Friday. High-res models and radar trends are focusing deeper convection setting up across this area through overnight. Severe threat will start to wane overnight with loss of heating but still some decent shear could lead to isolated wind damage or hail. Previous discussion from late afternoon... The Flash Flood Watch for the piedmont of Virginia into Rockbridge County will be allowed to expire at 6pm. Deep convection has been lacking this afternoon here, and short term models are indicating main focus through early evening will be in the west. However, the slow moving band from MTV-ROA-HSP at 2150z will keep shifting north and northeast. Will have to monitor the Alleghanys and Shenandoah Valley of our forecast area for possible watch if deep convection maintains its intensity into early evening. Forecast grids updated to account for latest radar and high-res models trends favoring likely to categorical pops in the west to little to low chance pops in the NC piedmont through 8-9pm. Previous afternoon discussion... Upper low was along the Ohio River as seen in all satellite loops. A well defined area of mid level drying over the central and southern Appalachians into the eastern Great Lakes. The upper low is forecast to drift southeast into Kentucky then loop back into Indiana. Good consensus in the guidance with the track of the low. As a result there will be little change in the weather pattern tonight or Friday. Surface through low level winds will keep dew points in the 60s in all but the southwest tip of Virginia and extreme western North Carolina. This will fill low clouds and fog in over the foothills and to the east. The western edge of multiple bands of showers and thunderstorms will gradually move northeast tonight and Friday. Have taken a blend of bias corrected MET and MAV guidance for lows tonight. Areas closer to the upper low, west of Bluefield and Wilkesboro will have the coolest temperatures overnight. Highs on Friday will very dependent on cloud cover, similar to today. Locations that have more sunshine will be close to 80 degrees. Spots that stay in the low clouds longer may only reach the lower 70s. && .SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/... As of 330 PM EDT Thursday... The closed low over the Ohio valley that has been dominating the upper level pattern will slowly pull up to the Great Lakes region as it starts to open up, dragging a well occluded surface low with it. This will position our region essentially in the dry slot under a neutral thermal advection regime with slight warming aloft. This should yield a weekend forecast devoid of significant precipitation though an isolated shower may still be possible. By sunday night, high pressure pushing in form the upper midwest will nudge a cold front to our western doorstep but the best forcing and instability will be shearing off to our northwest so the chances for any frontal precipitation encroaching Sunday night are low. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 330 PM EDT Thursday... The upper low will give way to ridging in the eastern US through the early part of next week. This stall the approaching front and allow a large area of high pressure to settle over New England and wedge down the east side of the Appalachians. The big variable lies with the potential track of TS Matthew as it is expected to turn north and move up the Atlantic coast. The exact track of Matthew will determine how much, if any, precipitation it can push into the region from the east and how much overrunning precipitation will develop as it interacts with the wedge. Right now, the most likely scenario is for some light overrunning precipitation to develop by Tuesday and increase a bit into Wednesday especially in the east, with any significant effects form Matthew remaining to our east. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... As of 1250 AM EDT Friday... Variable but overall sub-VFR conditions will prevail overnight with cigs fluctuating between MVFR and low end IFR at times. Best coverage of showers resulting in vsby restrictions look to occur along the KLYH-KHSP corridor for the rest of the night. However another narrow band of showers and storms may affect areas from KBLF to KBCB/KROA into the early morning hours. Thus will include either a vicinity or tempo mention with perhaps a brief prevailing group of lower cigs/vsbys. Should finally see convection wane before daybreak but still enough to keep thunder in a few western sites over the next few hours. Uncertainty continues with the degree of added convective coverage on Friday with the upper low starting to retrograde while the occluded boundary to the west shifts east and starts to cutoff deeper moisture late. Models continue to focus showers and storms across the northwest corridor Friday morning, then along and north of a KLWB- KDAN line. Should see any IFR conditions slowly improve to MVFR/VFR by midday/early afternoon though think places like ROA/LYH/LWB will keep MVFR to the end of this taf period. Will include more showers and storms given decent probability during the afternoon in most locations with tapering of showers by the evening as the main corridor of deeper lift slides north. Extended aviation discussion... A deep upper low will finally shift farther to the north by Saturday and away from the region on Sunday. This should allow for a gradual improvement in cigs after early sub-VFR Saturday with widespread VFR Sunday as weak high pressure builds in. Looks like overall VFR to then prevail into early next week outside of some upslope low clouds Monday and perhaps along the Blue Ridge Tuesday as another weak wedge develops under high pressure to the north. && .HYDROLOGY... As of 750 PM EDT Thursday... Banding of heavier showers and storms look to form up across the Bath County to Amherst County area through overnight. Mainly looking at potential training here which could bring small streams out of their banks if deeper convection can maintain itself. Expect some weakening with loss of heating. Effects on main stem rivers will still keep them well below flood stage, so main concern in smaller creeks, as well as urban areas like Lexington and Covington. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Flash Flood Watch until 8 AM EDT this morning for VAZ019-020- 023-024-035. NC...None. WV...None. && $$ SYNOPSIS...JH/AMS NEAR TERM...AMS/WP SHORT TERM...MBS LONG TERM...MBS AVIATION...AMS/JH/WP HYDROLOGY...WP

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