Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 200140 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 940 PM EDT Tue Jul 19 2016 .SYNOPSIS... A weak cold front will drift southeast across the region this evening before passing into the Carolinas overnight. Weak high pressure follows the front with drier weather for much of the remainder of the week. && .NEAR TERM /THROUGH WEDNESDAY/... As of 930 PM EDT Tuesday... Very difficult to tell exactly where the weak front lies as convective outflow boundaries have contaminated the low-level wind field. Soundings and wind fields suggest it is near the Blue Ridge. Earlier round of strong to severe convection has subsided, but as HRRR suggested, we may not be completely done yet. CAPES are still 1000-2000 j/kg across the Piedmont and DCAPES are still well over 1000 j/kg in this same region. There is quite a gradient in PWATS across the region this evening with less than an inch on the 00Z RNK sounding to well over an inch east and south of the Blue Ridge. Have left some low end slight chance pops across much of the CWA through 06Z, then keep mainly east of the Blue Ridge there after, dropping to below 15% before daybreak. Will be monitoring trends as additional changes may be needed. No significant changes needed to T/Td grids at this time. As of 339 PM EDT Tuesday... Previous updated for higher pops/stronger storms to reflect issuance of severe thunderstorm watch 398 for counties east of the Blue Ridge through early this evening. Surface front currently drifting through the extreme northern portion of the area should finally spill south and through the region after sunset. Ahead of the boundary still seeing the potential for at least scattered deeper convection but so far limited via weak northwest downslope flow, and earlier mid deck that slowed the onset of instability over the west. Latest HRRR continues to suggest that the current convergence area heading off east/southeast of the Blue Ridge to push southeast in the next few hours with only widely scattered coverage through about midnight and mainly north-east. This looks basically on track given overall weak instability and lack of much cloud development across the west where drier air aloft continues to invade on better nw flow aloft. Think mostly scattered pops for the most part which may be too much over the northwest sections before only having isolated coverage southeast overnight ahead of the dewpoint boundary. Likely still moist enough in the valleys for fog late, otherwise clearing overnight with lows low/mid 60s west to upper 60s east. Weak high pressure builds to the north on Wednesday with good drying seen aloft in forecast soundings across the region. Added dry entrainment should also help dry out the low levels enough to avoid convection except perhaps the higher ridges down in the mountains of northwest NC. Thus left in a token shower pop there given decent instability and weak late day convergence between easterly flow in the lee of the ridges, and weak northwest/variable trajectories over the west. Otherwise should be mainly sunny under weak subsidence and slightly less humid with dewpoints falling a bit espcly north and west. Highs still around 90 Blue Ridge east and low/mid 80s rest of the mountains, as heating of dry air should offset any weak/brief cooling seen aloft behind the front. && .SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY NIGHT/... As of 230 PM EDT Tuesday... The northern trof over the northeastern part of the country will dip a bit further south as the broad upper ridge over the central part of the country begins to flatten out. This will bring us seasonably warm temperatures for Thursday as high pressure moves into the middle Atlantic region, though there will still be a chance for some scattered orographically forced convection mainly along the Blue Ridge. Then as the high moves off to the east we will get back into southerly flow as a frontal boundary approaches from the northwest. This will allow the heat and humidity to build throughout the weekend with air temperatures reading the middle/upper 90s and heat index values approaching 110 for locations in the piedmont, a bit cooler in the elevations along and west of the Blue Ridge. Also worth noting is that temperatures at night will remain warm with lower/middle 70s east of the Ridge so there will be little opportunity for folks to recover from the heat of the day. Definitely important to take breaks from the heat and remain well hydrated in the days ahead. Our chances for thunderstorms will also be increasing in the warm and unstable environment, but dynamic forcing is weak so any severe threat seems limited to pulse activity and very heavy rain as precipitable water values start creeping above 1.5 inches as we head into the first part weekend. && .LONG TERM /SATURDAY THROUGH TUESDAY/... As of 230 PM EDT Tuesday... By Sunday the upper pattern will have transitioned to a quasi zonal regime with the jet stream riding well to our north. This will keep us in the heat and humidity through the first part of next week. The zonal flow will result in little push to get fronts trough the region and as a result, a washed out frontal boundary combined with a lee/thermal trof will keep an elevated chance for thunderstorms across the region on Sunday, followed by another weak front moving in from the northwest and continuing our chances for thunderstorms as it washes out over the area Tuesday. Air temperatures and heat index values will remain at hazardous levels on Sunday especially across the piedmont, with only a slight tempering of the heat through the first part of next week. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/... As of 800 PM EDT Tuesday... Weak cool front drifting southward through the CWA, but very diffuse and difficult to find given several residual convective outflow boundaries across the region. Best analysis is that it is along or near the Blue Ridge. Isolated strong to severe convection has all now moved south of the CWA. HRRR tries to bring convection back into the eastern part of the CWA from the Shenandoah Valley and central VA area, but this is questionable. Too much uncertainty to indicate in any TAF sites at this point. For Wed...upper ridge from the central U.S. will continue to build eastward resulting in increasing subsidence. With the front south of the area this should finally yield a mostly convection free day. The only area to watch would be far SW VA and perhaps the NW NC mountains, but not at any of the TAF sites. Otherwise, the only real concern will be potential for fog overnight/early Wed. Confidence in this occurring is fairly low given lack of rainfall in/near TAF sites late today and drier air advecting southward. Have left fog in TAFs as earlier advertised...but confidence in less than 50% at this point. Winds will be light northwest through the period, mostly less than 5kts. Extended aviation discussion... Strong high pressure aloft will bridge east across the region Thursday into next weekend. Cant totally rule out an isolated storm each afternoon/evening through the period but appears overall VFR outside of the typical late night valley fog in spots. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...JH/RAB SHORT TERM...MBS LONG TERM...MBS AVIATION...JH/RAB is the U.S. government's official web portal to all federal, state and local government web resources and services.