Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 200759 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 359 AM EDT Thu Jul 20 2017 .SYNOPSIS... High pressure building eastward across the area will provide hotter temperatures today through Saturday. At the surface, a frontal boundary will slowly track from the lower Great Lakes today, to the Northern Virginia area by Friday evening, before stalling. A cold front will move into the area early next week. && .NEAR TERM /THROUGH TONIGHT/... As of 310 AM EDT Thursday... Upper ridge over the southern Plains and Mid-Mississippi Valley this morning heads extends eastward today to the mountains, while the upper trough along the coast of the Carolinas, weakens and actually starts to slide back southwest toward the Florida Panhandle tonight. 8h temps reach +22C today and with a lack of cloud cover somewhat of a westerly downslope component, though not strong, temperatures should heat up into the mid 90s over the piedmont this afternoon as well as urban areas further west, like Roanoke and Lexington. Subsidence behind the upper trough and westerly flow should keep dewpoints somewhat lower to offset heat indices, but some areas could exceed 100 degrees for an hour in the Buckingham to South Boston area around 3-4pm this afternoon. Threat of convection looks very low, so have removed isolated threat along the Blue Ridge. May see some storms nearby over the Alleghanys west and north of Hot Springs after 4pm, but appears better coverage will be further north over northern WV/PA. Tonight, 8h flow turns more northwest and a couple of models have a piece of vort energy/possible convective vort moving from Indiana/Ohio around midnight, to the central Appalachians by 8am Friday. Some hint in the previous forecast for this possibility so keeping slight chance of thunderstorms late tonight northwest Lewisburg/Hot Springs. Otherwise, muggy with lows from the mid to upper 60s mountains, to lower to mid 70s east. && .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... As of 315 PM EDT Wednesday... Relatively busy period on the whole, with the following highlights (listed in order of greatest forecast confidence): (1) Very warm/hot and humid through the period, with little nighttime relief. (2) Progressively greater coverage of thunderstorms, some at times strong. A well-advertised 594 dm heat ridge will become established over the mid-Mississippi Valley, a portion of which will spread over the forecast area Friday into Saturday. On the north and northeastern periphery of this synoptic feature will be a belt of moderately strong mid-level NWly flow, initially from the northern Great Lakes/northern mid-Atlantic which will progressively build south or southeast with time. The upper Midwest and the Ohio Valley stand to be convectively active, as surges of mid-level conditional instability advect southeast on the southern periphery of the stronger belt of mid-level NWlys. For Thursday Night: Mainly quiet weather, but will be turning quite warm as 850 mb temperatures warm to values around +19 to +21C. May see some MCS blowoff mid-level cloudiness at least, indicated more bullishly by the 12z NAM. Kept forecast dry with muggy lows in the upper 60s to mid 70s given the warmest thermal profiles. For Friday/Friday Night: Hot and humid. Amid at least partly sunny skies and as 850 mb temperatures warm to values near +24C off the 12z GFS, high temperatures should reach the upper 80s to low 90s, with highs in the Piedmont reaching the mid/upper 90s. Blacksburg has yet to reach 90 degrees this year, but Friday may offer the best potential. Forecast max heat index values from Roanoke into the VA/NC Piedmont project to reach to values around 101-106F with afternoon mixed dewpoints in the upper 60s to mid 70s. Could see some potential for heat headlines in the Southside. As far as potential for convection goes, it appears limited as rising 850 mb temperatures induce a strong cap that appears unlikely to be broken with sources of lift limited to wind shifts on the mesoscale in the mtns. PoPs were kept on the isolated/slight chance range along the mountains; though did increase to scattered/chance range (~30%) from the Greenbrier Valley southeastward into the Shenandoah Valley and central VA Piedmont near/northeast of Lynchburg given some uncertainty if subtle disturbances in the mid-level flow that are impossible to time could spark somewhat greater convective coverage. As mid-level lapse rates are on the steep side (between 6.5-7 C/km), any storm that overcomes the cap could become strong fairly quickly, but also be of the pulse variety. Maintained slight to lower Chance range PoPs (greatest along and north of I-64) into the evening given the uncertainty on what may evolve upstream to our northwest in the Ohio Valley. Little relief from daytime heat with muggy lows in the upper 60s to mid 70s. For Saturday/Saturday Night: There is some level of uncertainty here, in that potential for early-day cloudiness/afternoon convection would be substantially influenced by the pattern Friday. Southern extent of moderate mid-level westerlies should be near the far north of the forecast area. BUFKIT soundings showing weaker capping and enough of a light southerly 2-m AGL convergent wind against the Blue Ridge to allow for an increase in PoPs into the Chance range, as well as northern tier of counties. Lower/slight chance PoPs along the foothills and Southside areas. High temperatures should be similar to Friday, but kept heat indices in the 99-104 range in the Southside and VA/NC Piedmont areas. Could see some heat headlines in this period as well, but confidence is a little lower as storm coverage/cloudiness could be somewhat greater. Lows should also be rather similar as well, in the upper 60s to mid 70s. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... As of 315 PM EDT Wednesday... Heat ridge weakens Sunday with mid-level westerlies steadily sagging southward (i.e. getting stronger) each day in the long-term period. The net effect this should have is for more subtle shortwave disturbances that may trigger more widespread convective coverage, particularly for Sunday and Monday associated with a surface cold front. Cooler temperatures should also result as 850 mb temperatures fall back to more seasonal levels, but highs in the mid 90s still are possible in the Piedmont on Sunday. && .AVIATION /08Z THURSDAY THROUGH MONDAY/... As of 110 AM EDT Thursday... Fairly quiet aviation wise this period with high pressure aloft building in from the west. The exceptions will be a period of sub-VFR vsbys and some cigs with fog early this morning, especially LWB/BCB, but also LYH/DAN. LWB should be favorable for 1/2sm after 08-09z. Most of the sub-VFR conditions will improve to VFR by 14Z/10AM Thursday. it is unlikely any showers or thunderstorms will develop Thursday under a warm ridge, but if they do they will be extremely isolated and likely confined to near the higher ridges. Winds through the forecast period will mainly be light westerly or calm. Confidence levels are high regarding the wind through the 24 hour TAF period. Confidence levels regarding ceiling and visibilities are high during the daylight and evening hours. Confidence is moderate into early Thursday morning. Aviation Extended Discussion... Mainly VFR conditions are expected through Sunday as upper level ridging allows for at best isolated to scattered afternoon and evening showers and storms, most likely in the mountains and less likely to impact farther east such as KLYH and KDAN. Brief, sub- VFR conditions will be possible with the heavier showers and storms. Late night/early morning sub- VFR fog will be possible, mainly in the mountain and river valleys. A cold front will move into the region Sunday night into Monday. Look for an increase in convection, and better chances of sub- VFR conditions in association with convection along the front. Overnight fog will be more likely Sunday night into Monday morning. Confidence of all weather parameters is moderate during the extended portion of the forecast. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...WP NEAR TERM...WP SHORT TERM...AL LONG TERM...AL AVIATION...DS/SK/WP is the U.S. government's official web portal to all federal, state and local government web resources and services.