Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 200513 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 113 AM EDT Thu Jul 20 2017 .SYNOPSIS... Low pressure off the coast of North Carolina will head south to near the coast of South Carolina and Georgia by Thursday morning. Starting Thursday, high pressure will gradually build into our region through the week. A cold front will move into the area Sunday into Monday. && .NEAR TERM /THROUGH TODAY/... As of 957 PM EDT Wednesday... Weak convergence near Blue Ridge and a bit of an instability gradient are keeping an isolated shower or two going late this evening, mainly across Wilkes Co NC. Kept a mention of an isolated shower in a few spots going until 11pm but with instability weakening more quickly now am still expecting even these very isolated showers to diminish. otherwise forecast of mainly clear and calm conditions overnight looks on track with patchy valley fog developing again in favored locations, perhaps a little less dense and slightly smaller area compared to previous night. no updates to overnight lows, with low to mid 60s in the mountains to upper 60s to around 70 piedmont. Rather muggy across the southern piedmont especially where dew points are remaining in the 70s this evening. Previous discussion as of 300 PM EDT Wednesday... A few isolated showers, and even fewer thunderstorms, will dot the countryside the remainder of this afternoon. Any that survive past 6pm will quickly dissipate around or shortly after sunset. Through the overnight hours, a closed upper low off the coast of North Carolina will head south to along the coast of SC/GA. While this is taking place, an west-east oriented upper ridge will start nosing into the area. We are expecting mostly clear skies to start the night, with the potential for some mountain/river valley fog and associated low stratus to develop towards daybreak Thursday. Our airmass will remain on the humid side with surface dew points a few ticks higher as compared to last night. Expect low temperatures to range from the low to mid 60s across the mountains with mid to upper 60s across the Piedmont. On Thursday, building heights, rising temperatures aloft, and increasing subsidence will allow for even less potential for any isolated afternoon convection as compared to today. At best, an isolated shower or storm may develop along or near the crest of the Blue Ridge, or across western Greenbrier County, WV. Forecast 850mb temperatures are forecast to be close to the +22C mark tomorrow afternoon compared to today`s readings right around +21. Will have forecast highs a degree or two higher than those forecast for today. Will have the mid to upper 80s across the mountains. However, the highest elevations will be a little cooler with highs around 80. Across the Piedmont, expect highs of the low to mid 90s. && .SHORT TERM /TONIGHT 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 /05Z 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...DS NEAR TERM...DS/SK SHORT TERM...AL LONG TERM...AL AVIATION...DS/SK/WP

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