Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 271414 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1014 AM EDT Thu Jul 27 2017 .SYNOPSIS... A cold front will approach from the northwest today into Friday sliding across our area Friday afternoon into Friday night. Upper system should keep it cooler with a few showers and storms around Saturday before it dries out Sunday into early next week. && .NEAR TERM /THROUGH TONIGHT/...
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As of 950 AM EDT Thursday... Showers and isolated thunderstorms are slowly approaching the western slopes of SW VA/SE WV this morning. With the slower arrival time, we have cut back on PoPs along and east of the Blue Ridge through early this afternoon. Clouds are keeping instabilities at bay but should see an uptick when clouds break this afternoon. Weak flow and weak deep shear will also limit severe activity this afternoon. Can not rule out an isolated strong storm producing sporadic wind damage and/or wind gusts. Previous discussion... As of 250 AM EDT Thursday... Stratus not as prevalent as last night but is out there across the NC mountains into portions of southwest Virginia. Mostly have high clouds streaming in from the northeast in association with convection moving across Illinois into Indiana. Latest Hi-Res models are painting a smattering of showers this morning mainly east of the Blue Ridge. However, area radars show little development and will limit any isolated shower threat toward the NC mountains where better low level convergence resides. For today, will be watching upstream convection and how it evolves to the east/southeast along with outflow interactions to generate storms over our area. Models are in agreement that the mid-Atlantic into the central Appalachians will see a decent coverage of storms, but where it lines up for likely to categorical pops vs low chance pops is iffy. Confidence is high based on upstream shortwave and model initialization of it over Indiana, that best lift and dynamics should push into WV by 18z, then move east into the Virginia piedmont by early evening. Leaning pops this morning toward a blend of the Hi-Res ARW, GFS and ECM, then blend of the GFS/ECM thereafter which paints the mountains with high chance to likely pops, with 40 pops later in the east, but would not be surprised if this gets bumped up with later model runs/forecasts. The issue today will be severe wx threat from said storms. SPC keeps marginal risk over the entire mid-Atlantic and central/southern Appalachians. Per model solutions, the better upper forcing and shear will move along and north of our northern forecast area (Lewisburg-Buckingham), while the southern area gets more heating and steeper low level lapse rates. Given we are already getting some high clouds, issue remains cloud cover and destabilization. We have the higher dewpoints to juice the low levels. At the moment it looks like a day where storms could fire ahead of the main shortwave along outflow with more organized convection moving into the Shenandoah Valley to northern WV area this afternoon. Should be a lull in convection this evening behind the main cluster of storms, while more forms upstream across the Ohio Valley. There is a potential per GFS for some clustering of storms in the evening moving out of southeast KY into the NC mountains/Mountain Empire, but will not try to get too specific beyond today. After midnight, expect 1-2 areas of concentration, mainly NC south and southeast of our forecast area, and the other moving from southern Ohio into northern WV/VA/MD. Will have to see how the mid-upper flow evolves as more complex area of storms can create density and shift southward toward better instability. Needless to say will maintain high chance to likely pops especially after midnight-toward morning. Severe threat overnight should be limited with loss of heating, though cannot rule out some rogue strong to severe thunderstorm clusters. Another potential issue will be hydro related, but given dry antecedent conditions will have low confidence at the moment for flooding, as will have to see how storms line up. The hydro issue may come more into Friday if the front slows down. With more clouds around today, highs will be close to normal with upper 70s to lower 80s mountains, to mid to upper 80s foothills/piedmont. You will be feeling the humidity though with dewpoints in the upper 60s to lower 70s. Lows tonight stay elevated due to storms/clouds and high dewpoints, ranging from the mid to upper 60s west, to lower 70s east.
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&& .SHORT TERM /FRIDAY THROUGH SATURDAY/... As of 330 AM EDT Thursday... A significant closed low will develop over the mid Atlantic region and linger through the weekend fueled by vigorous short wave energy moving out of the Great Lakes. This will drive development of a surface low over the mid Mississippi valley that will slide along a stalled frontal boundary draped across the region before reaching the coast as a well developed storm system. Indications are that the energetic dynamic environment will combine with ample instability to drive deep convection, and may be sufficient with marginal shear profiles to produce some strong to severe storms especially heading further south and east. However, there is much uncertainty in exact timing/placement of features so the severe threat for Friday remains a bit ambiguous. So while confidence is decent for high POPs, will have to wait for later model runs to converge on a solution before the nature of our severe threat can be better defined. Antecedent dry conditions will help mitigate potential hydro issues from heavy rainfall but will fine tune this thinking based on extent of activity Thursday/Thursday night. With the stacked low spinning over the mid Atlantic, wrap around moisture will keep a chance of showers/possibly some thunder in the forecast into Saturday night before the system starts to lose influence over our region by Sunday. Northwesterly flow on the backside of the low this weekend will make for breezy and cool conditions with temperatures 5 to 10 degrees below normal. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... As of 330 AM EDT Thursday... An upper level trough will sit over a dry surface high through the middle of next week with no rain is in the forecast except for a slight chance of a shower/thunderstorm from the Grayson Highlands through the mountains of North Carolina. Expect a good amount of diurnally driven cumulus clouds forming in the afternoon, then fading in the evening. Temperatures will moderate back to normal by midweek. Dew points will remain low (mid 50s to lower 60s) through Tuesday, then edge up towards being muggy again on Wednesday. && .AVIATION /14Z THURSDAY THROUGH MONDAY/... As of 730 AM EDT Thursday... Sub-VFR cigs/vsbys are scattered from the piedmont into the mountains this morning, with MVFR cigs at DAN into the NC foothills, while MVFR to IFR vsbys exists at LWB/BCB into the Mountain Empire and NC mountains. Expect these cigs/vsbys to improve to VFR between 12-15z. This afternoon will see scattered to numerous showers and thunderstorms move in and/or develop starting in the mountains by 18z, moving into the DAN/LYH area by 21z. Confidence is high enough to have predominant showers with VCTS with tempo +TSRA at all sites. Still will have to amend as needed based on radar indications. Storms could be robust with gusty winds possibly exceeding 45-50kts, so aviation interests should monitor wx conditions. In thunderstorms sub-VFR down to IFR possible. Should see somewhat of lull in activity this evening followed by another of showers/storms after midnight, with at least MVFR vsbys with moderate showers. Aviation Extended Discussion... Showers/storms continue into Friday along and ahead of a front, so terminals will be affected off/on with sub-VFR cigs/vsbys with heavy rain and gusty winds into Friday evening. Saturday starts to dry out some but upper low will enhance northwest flow over the area keeping sub-VFR cigs stuck in BLF/LWB similar to winter scenarios with isolated showers around. Expect widespread VFR to return Sunday/Monday under high pressure following the front. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH/WP NEAR TERM...RCS/WP SHORT TERM...MBS LONG TERM...MBS/RCS AVIATION...AL/JH/WP

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