Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 180113 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 913 PM EDT Mon Jul 17 2017 .SYNOPSIS... A weak front across southern sections of the region will linger near the Blue Ridge overnight into Tuesday before dissipating. A residual upper trough should also remain in place east of the mountains later Tuesday into Wednesday. High pressure will then return warmer and more humid weather by late in the week. && .NEAR TERM /THROUGH TUESDAY/...
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As of 855 PM EDT Monday... Widely scattered pulse convection has faded for the most part but still have areas espcly near the Blue Ridge and foothills that have not been worked over per lingering instability. In addition decent vort/wave aloft to cross the region overnight as a broad upper trough drifts east over top the residual surface boundary across southern sections. This combined with less cap aloft and a wetter boundary layer could lead to a few added shra/tsra through about midnight before loss of heating wins out. Latest short term guidance quite inconsistent with where or if any lingering convection will take shape, with most convection very slow moving and outflow driven given chaotic light winds aloft off evening soundings. Therefore will leave in some isolated pop mention mainly south-east over the new few hours with more clouds near the front over the southeast per latest satellite and lingering strato-cu northwest. Expect with some overnight clearing to again see fog development espcly where earlier heavy rainfall occurred and across the western mountain valleys. Left temps similar to previous with overall lows in the 60s. Previous discussion as of 326 PM EDT Monday... Regional radar mosaic as of 3:00 pm this afternoon shows scattered, slow-moving strong thunderstorms from just west of Galax southwestward into the NC mountains. More isolated, garden-variety storms were also developing in the central VA Piedmont. One storm has pulsed to radar-indicated severe limits in Grayson County this afternoon. Convection has formed in a moderately unstable environment along moisture convergence zone and aided by upper support from vorticity lobe currently spiraling near Knoxville. Lapse rates in mid-levels being on the steep side and freezing levels being around 12,000 ft AGL...though deep- layer wind shear values only support pulse convection. Strongest pulse storms would be capable of gusty winds and small hail, with pea- size hail having recently been reported near Boone. Still at least isolated storms possible along the northern Blue Ridge up into the southern Shenandoah Valley, but more nebulous forcing may keep coverage more isolated. These trends generally reflected in recent runs of the HRRR and 3-km NAM, which show decreasing convective coverage and strength toward 9 pm. Aforementioned mid-level vort/trough digs into the central NC Piedmont, with daytime cu clouds diminishing. Winds tonight become extremely light through the entire column tonight which may lead to a patchy fog/stratus development later in the overnight. Patchy fog also possible in the western river valleys and in areas where it has rained. Overall similar lows to last night in the upper 50s to upper 60s areawide, coolest west and milder/muggier east and south. For Tuesday...any overnight fog should burn off with partly sunny skies. Skies should fill in with cumulus readily associated with cold pocket of air aloft (-12C at 500 mb). Related mid-level trough should be slowly moving eastward into coastal NC through the daylight hours, leaving the forecast area on the northern side of the trough. Thus it appears forcing likely to be being even more limited than today. Though the 12z/17th GFS and ECMWF solutions try to paint much of the forecast area with some light QPF, it`s not clear if the coverage will be that widespread. Differential heating along the terrain may be the only substantive source of lift. Started with the global model PoPs but also blended some of the available mesoscale models to show the spatial coverage variation. Namely: isolated/slight chance PoPs for the VA/upper NC Piedmont and in southeastern WV, scattered/chance PoPs along the Blue Ridge. As the case today, storms should be slow moving in a weak shear environment, but weaker lapse rates should keep thunderstorms more general/garden-variety. Highs in the upper 70s/mid 80s mtns with low 90s confined to the Piedmont.
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&& .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY NIGHT/... As of 326 PM EDT Monday... You can expect an increase in temperatures this period as the axis of the upper trough moves from the piedmont of VA/NC east to the Atlantic coast. 5h heights will stay mainly in the 591 DM range, while 8h temps rise from +18C to +20-22C. At the sfc, high pressure is going to be in place over the southern Appalachians, with lee trough in the piedmont. Weak convergence along the Blue Ridge and differential heating may lead to isolate to widely scattered storms Wednesday afternoon/evening. Appears subsidence behind the trough may limit coverage. More subsidence/drying aloft will keep any convection at bay Thursday so expect a mostly sunny/hot day, with highs in the mid 80s to near 90 mountains, to lower to mid 90s foothills/piedmont. Heat indices should not be an issue as dewpoints will be in the lower to mid 60s. && .LONG TERM /FRIDAY THROUGH MONDAY/... As of 325 PM EDT Sunday... Keeping the theme of hot and humid with daily threat of storms. Will be on the northern periphery of upper high, so any convection on the southern belt of the westerlies may scrape our area in the Fri-Sun time frame. Models diverge Monday with new ECMWF showing a trough digging through the Ohio Valley which is more favorable for convection. The GFS showing less amplification with better ridge over the southeast. With this being 7 days out will keep token low chance pops in mainly west. Saturday-Sunday appear to be hottest days with 8h temps around +24C. Looking at upper 80s to around 90 west, to mid to upper 90s east. Convection and any upstream blowoff of high clouds may inhibit this though. && .AVIATION /01Z TUESDAY THROUGH SATURDAY/... As of 700 PM EDT Monday... Convection remains isolated to widely scattered across southern/eastern sections early this evening and will likely linger mainly around KLYH another hour or two before fading. May also see added isolated shra/tsra pop up due to outflow until shortly after sunset when instability should finally diminish. Thus keeping in a VCSH or VCTS mention where convection continues to linger while basically leaving out elsewhere for now into the evening. As an upper level trough swings into central NC tonight, light winds and clearing skies should allow for development of IFR or lower fog/stratus in the river valleys including KLWB/KBCB, with another area of development possible in the piedmont where have seen pockets of heavier rainfall this afternoon/evening. For Tuesday, anticipate any fog from the overnight to burn off with sunshine. Should see deepening VFR cumulus, but potential for showers or thunderstorms mainly after 18z/2pm Tuesday. However isolated coverage and short lived extent of any convection makes things too uncertain in include any mention in any of the terminals. Winds light and variable. Aviation Extended Discussion... Increasingly diurnal nature to any convection as we progress into the middle and later part of the week should make for mainly VFR excluding any late night sub-VFR fog/stratus. An upper ridge across the Midwest will slowly build east- southeast through the period. No organized convection expected through this time frame, perhaps toward the weekend in the form of an MCS from the upper Midwest/Ohio Valley. Confidence is this occurring is low to medium at this point. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...AL/JH SHORT TERM...WP LONG TERM...WP AVIATION...AL/JH/RAB

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