Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 191344 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 944 AM EDT Wed Jul 19 2017 .SYNOPSIS... A weak upper low over southeast Virginia will move south along the North Carolina and South Carolina coast line through Friday. High pressure aloft will build east from the Midwest toward the southern Appalachians and Southeast U.S. through the weekend. A cold front will move southeast into the region from the Great Lakes late in the weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 940 AM EDT Wednesday... Fog over parts of the New and Greenbrier River Valleys is starting to erode, and that trend is expected to continue. Anticipate little if any fog remaining across this region by 1100 AM EDT. Have reflected the forecast accordingly. Am making no significant changes at this time to the remainder of the forecast for today. Have made minor tweaks to hourly temperature and dew points based upon the latest observations and expected trends into the early afternoon. As of 330 AM EDT Wednesday... Showers and thunderstorms, associated with a weak upper low now located over southeast Virginia, dissipated quickly after sunset with no convective activity left in or near the CWA at this hour. The upper low will drift further south along the North Carolina coast today and tonight. The associated cold pocket aloft will dissipate with 500mb temps warming from -10C yesterday to around -6C by the end of the day today. Lapse rates will be markedly weaker and overall the atmosphere much more stable. All this will be the result of the upper low moving away and heights building eastward from the Midwest where a strong 597dm upper ridge is located. Confidence in convection today is low. Situation is entirely different than yesterday and none of the models really indicating much occurring at all. HRRR and NSSL WRF very quiet with little to no convection indicated through the end of its cycle. Nonetheless, not comfortable with temperatures warming into the 90s and dewpoints well into the 60s of completely unmentionable pops, so have included a 15% pop for all areas east of the Alleghany front. Feel that areas along and west of the Alleghany front will be closer to the upper high and too stable for convection. The Piedmont and/or along the Blue Ridge should be the most favored areas for convection today. Then again, I could also see a scenario with no convection at all. It is rare to have two back-to-back days like yesterday and again it is worth noting that the synoptic situation is far from favorable for such today compared to yesterday. Temperatures will be on an upward trend the next couple of days as the upper ridge builds east. 850mb temps have been hovering near or just below 20C. These will be creeping up toward +22C to +24C over the next couple of days. Not enough rise today to really be reflected in the max temps, so have only raised them a degree or two in most areas. This will yield low 90s Piedmont to mid 80s west, except upper 70s higher elevations of the mountains. Overnight lows Thursday morning should be considerably warmer than this mornings temps as a consequence of the lack of convection to lead off with a rather cool evening. Lows Thursday should range from the lower 70s Piedmont to the lower and mid 60s across parts of the Greenbrier and New River Valley as well as mountain valleys throughout the west. Finally, do expect fairly widespread areas of dense fog this morning from late day rain Wednesday and calm winds, clear skies, and effective radiational cooling this morning. Fog will likely be less widespread Thursday morning with the lack of late day rainfall and warmer overall minimum temps, hence greater T/Td spreads. && .SHORT TERM /THURSDAY THROUGH SATURDAY NIGHT/... As of 230 AM EDT Wednesday... Quieter yet hotter weather in store Thursday, as dome of high pressure slides east from the southern Plains and Mid-Mississippi Valley. Still enough low level convergence where an isolate storm could pop up along the higher terrain in the afternoon but most will be dry. Temperatures should peak out in the mid to upper 80s across the mountains, to the lower to mid 90s east. Dewpoints in the 60s should keep heat indices mostly in the 90s east. Friday, the upper center of the high remains over the TN valley, while southern belt of westerlies stays situated from the northern Plains, east to the mid-Atlantic. Will have to watch for convection firing upstream and density of an MCS being able to propagate southeast into our area. Most of the 0z models are favoring some convection sliding toward the northern CWA in the afternoon, roughly from the Greenbrier valley of WV eastward toward Lynchburg. Not too confident given the flow aloft and the warmer temps aloft with the ridge in place, so will have mainly slight chance pops in the mountains, to low chance over the Alleghanys, while keeping most of the piedmont/foothills south of Roanoke/Lynchburg dry. Friday`s highs should be a few degrees warmer if we have no convective blowoff, with upper 80s to lower 90s in the mountains, to mid to upper 90s in the piedmont/foothills/Roanoke. Dewpoints creeping up will make this the day where heat indices could exceed 105 degrees in some of the piedmont area. Heading into Saturday, 5h heights fall slightly with southern belt of westerlies dropping toward the south into Virginia. This combined with an frontal boundary across the mid-Atlantic and upstream convection should allow for at least outflow driven convection to fire during the afternoon. Highs again will be dependent on clouds, but should be a degree or two cooler than Friday but hot nonetheless, with mid 80s to near 90 west, to mid to upper 90s east. Again with dewpoints near 70, heat indices could threaten 105 degrees along/east of Highway 29 Lynchburg/Danville south into Yanceyville. Any convection Friday-Saturday should weaken/dissipate during the nighttime hours, while lows stay elevated in the upper 60s to mid 70s. && .LONG TERM /SUNDAY THROUGH TUESDAY/... As of 315 PM EDT Tuesday... Very warm/hot weather expected to persist through much of the long-term forecast period. Weak upper closed low expected to drift west into AL over the weekend, then shear slowly northeast back toward the southern and central Appalachians by early next week. Meanwhile, a series of short wave trofs in the northern stream are forecast to scoot east through the Great Lakes, with the strongest on Sunday into Monday likely to push cool front/baroclinic zone south toward or through the Blacksburg forecast area later Monday or Monday night. Whether the surface front can progress this far south while pushing against a strong Bermuda high pressure system still remains somewhat in question, but the frontal proximity, combined with at least some moisture streaming back northeast in advance of southern closed/shearing low should support a slowly increasing threat for mainly diurnally-driven showers/thunderstorms into portions of the Blacksburg forecast area, especially in the mountains (Saturday) and into most areas Sunday into early next week as the front nears or moves southeast through the region. Little overall change in airmass expected until the possible frontal passage - with a continuation of relatively hot days with warm nighttime lows, but with only a very limited threat for any record breaking readings. Cooler air may arrive by Tuesday, driving temperatures back down 5-7F and closer to seasonal levels. && .AVIATION /14Z WEDNESDAY THROUGH SUNDAY/... As of 745 AM EDT Wednesday... Patchy dense fog is the immediate problem this morning with some sites, such as LWB and BCB reporting 0SM. This will continue for another hour or so as a result of rainfall late in the day Tuesday leaving the ground wet/moist at sunset. With calm winds and mostly clear skies now, conditions are ideal for radiational cooling and fog. Most of the dense fog is confined to the New and Greenbrier River Valleys. Again, this should only be an issue for another hour or so. There is some concern that patchy dense fog and associated low ceilings will reoccur Thursday morning, but with less rainfall to perhaps no rainfall late today, the fog may tend to be less in duration, intensity, and areal coverage. After sunrise, conditions will improve quickly to VFR and should remain that way through the remainder of the TAF valid period. Not expecting a repeat of the widespread showers and thunderstorms that were observed Tuesday under the weak upper low as the upper low drifts further east-southeast and upper ridging moves eastward from the Plains. 500mb temps will warming from -8C to -10C to around -5C during the next 24 hours, which will leave a much more stable atmosphere in place. Showers and thunderstorms will be isolated today and confined mainly to areas east of the Blue Ridge closer to the upper low. Not enough confidence to include in the TAFs at this time. Winds will be light and variable through the TAF valid period and mostly calm at night. Some tendency for winds to be southeast across the Piedmont, but even there, speeds will be 5kts or less. Medium confidence in visibilities and ceilings through 13Z, then high confidence in VFR conditions. Medium to high confidence in wind speed and direction through the TAF valid period. Aviation Extended Discussion... The pesky weak upper low will drift into the southeast states toward the weekend and then eventually retrograde westward along the Gulf Coast. Eventually it could figure into our weather once again early next week. However during the time frame of Thursday through Sunday, generally VFR conditions are expected as ridging aloft spreads eastward from the Midwest toward the Southeast U.S. A frontal system could approach the area late in the weekend with an increase in convection at that time. Late night and early morning fog can be expected at the usual sites (e.g., LWB, BCB, LYH) most likely each day through the period with IFR-LIFR conditions possible in the 08Z-13Z time frame each day. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...RAB NEAR TERM...DS/RAB SHORT TERM...WP LONG TERM...WERT AVIATION...RAB is the U.S. government's official web portal to all federal, state and local government web resources and services.