Area Forecast Discussion
Issued by NWS Blacksburg, VA

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006 FXUS61 KRNK 210220 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1020 PM EDT Thu Jul 20 2017 .SYNOPSIS... Warm and humid conditions will persist over the area into Saturday under weak high pressure. Mainly afternoon and evening showers and thunderstorms are possible Friday and Saturday, becoming more numerous in coverage with the approach of a cold front Sunday into Monday. Slightly cooler and less humid conditions will overspread the region behind the front by midweek. && .NEAR TERM /THROUGH FRIDAY NIGHT/... As of 1015 PM EDT Thursday... No stray showers hanging around tonight given warm cap aloft and diminishing instability. Mainly thin high clouds but do not appear to be having much influence on temps dropping this evening but nevertheless a mild evening expected once again. A look at temps last night compared to this time tonight and expected generally clearing skies for most locations later tonight, dropped lows just a couple degrees in parts of the Piedmont where it already has fallen to a few degrees of previous forecast mins. Still not especially confident in any upstream convection making it into far NW forecast area toward dawn since so far the late evening development across the Midwest is behind schedule according to most near term models. Fewer clouds into Greenbrier Co could help valley fog formation once again. Decided to hold on to a small area of slight chc far NW edge of forecast area just in case, but have my doubts if anything will get in there. Made some minor adjustments to Friday PoPs as well based on latest blend of near term models and idea that at least widely scattered convection may reach a little farther south into the Piedmont late but generally think coverage will be minimal unless a more obvious outflow can be generated from any upstream convection. If no real boundary or wave, then may really be limited in coverage until max heating can get the Blue Ridge and a few other higher ridges going due to slightly better lapse rates. Biggest story for Friday is continued heat with most locations in the Piedmont likely to see another couple degrees added on to what they had today. With dew pts probably mixing a bit during the and decent possibility of scattered storms forming by mid afternoon, are current heat index values fall just a tad shy of heat advisory criteria so no plans yet to issue a headline unless we see something in new guidance overnight to indicate even higher temps or dew points. Previous discussion as of 315 PM EDT Thursday... Synoptic-scale pattern remains well-specified compared to prior days, that being a large heat ridge of 594 dm centered over the center of the U.S.. North of this feature, a belt of moderately strong west-northwest mid-level winds exists across the lower Ohio Valley and northern mid-Atlantic region. Periods of elevated convective instability - essentially a degraded portion of a elevated mixed layer (EML) over the northern Plains - along with the fast belt of westerlies fostered an overnight MCS last evening across the upper Midwest. Remnant energy associated with that feature has re-developed north of Cincinnati, OH, with another area of developing thunderstorms in western NY. Corfidi vectors which tend to steer organized complexes such as these directed east-southeast, so it`s anticipated that these features should begin to progress southeastward into the mid- Atlantic/southern New England area overnight. The eventual evolution of these features tonight will ultimately play a significant influence on what are the two major highlights in the period: heat/humidity and the potential for thunderstorms. For Tonight: Cumulus clouds with not much vertical development given dry mid-levels and capped atmosphere should dissipate with sunset. Shown resulting clearing skies through midnight. After midnight though, did show an increase in cloudiness in our northern and northwestern counties. This should take the form of MCS blowoff clouds. NAM-based guidance does indicate some showers developing toward early morning in central and eastern WV, so I`ve added some slight chance/isolated PoPs for showers or thunderstorms toward early morning hours. With light west to southwest flow tapping into greater dewpoints, along with 850 mb temperatures in the +20 to +22C range, it should be a warm and muggy night areawide. Forecast lows are in the mid 60s in the Blue Ridge and lower valleys to the lower/mid 70s in the Piedmont and Southside areas. For Friday: Very warm and humid conditions are expected. 850 mb temperatures being pretty similar to tonight at least yield highs in the mid 80s to mid 90s. Along with muggy dewpoints in the 60s to lower 70s, 100-105 degree heat indices in the Piedmont, Southside, and potentially the Roanoke/Lynchburg metro areas are values that are near heat advisory level. Overall though, confidence wasn`t high enough to consider issuing a heat advisory for those areas. Some level of uncertainty exists on the mesoscale during Friday afternoon thunderstorm development and related cloudiness that may keep high temperatures closer to official forecast values. Will continue to mention heat in the HWO. On the thunderstorm threat for Friday, note SPC`s Day-2 Convective Outlook shifts the 5% severe probability/Marginal Risk area a bit further southeastward from a Bluefield to Appomattox line northward. Today`s 12z NWP solutions have shown enough of a weakness in the western extent of the heat ridge, associated with the southern extent of the westerlies advancing at least as far south as the Marginal risk area, to allow for weaker capping than earlier indicated. More so in the NAM-based solutions and in the 00z NCAR ensemble, they depict re- development of showers and thunderstorms by early to mid afternoon along what likely would be an old outflow from tonight`s forecast MCS well north of the area. This may move in a corridor generally depicted in the Marginal Risk area. Given light southwest flow in lower-levels with mid- level southwest flow, another area of potential re-development is back across the KY/western WV/OH area. Deep-layer shear is forecast to be low enough for generally pulse storms that may become strong to locally severe. Gusty winds would be a potential hazard in stronger thunderstorms, and in a pattern that may promote cell training may have some localized hydro issues. PoPs were shaped such that Chance/scattered PoPs were drawn Summers and Mercer counties in WV eastward toward Appomattox County northward. Best chance for the aforementioned hazards described would be in the Chance-PoP area. Lower/slight chance range PoPs south and east of these areas, with storm threat here conditional on new cell growth in these areas from outflow produced from storms that may develop Friday. This is no better than medium confidence at this point and, as mentioned, depend significantly on developments tonight and overnight. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... As of 315 PM EDT Thursday... Battle will be in place between tenacious upper ridge extending from the central plains east into the mid-Appalachian region and how successfully short wave trofs and/or mesoscale convective circulations/vorticity scooting east through the Upper Midwest and Ohio Valley and Great Lake region on the northern periphery of the ridge can push baroclinic zone and surface cool front into and/or through the Blacksburg forecast area. First wave expected to push east-southeast out of the Ohio Valley and through or just north of the Blacksburg forecast area later Friday into Friday night. At least northern portions of the forecast area may be close enough to the edge of the upper ridge to allow sufficient erosion of capping inversion in area where better moisture and dynamic forcing will exist. Therefore, any associated upstream convective activity that fires up on Friday afternoon may linger well into Friday evening, at least near and north of I-64 until passage of wave later Friday night. A second wave is expected to race east-southeast out of the upper Great Lakes into the lower Great Lakes/southern New England region Saturday afternoon. Forecast soundings in advance of this wave forecast to yield 2000-3000 j/kg CAPE and LI values of around -5C to -7C with increasing mainly straight-line hodographs supportive of an even more expansive convective complex on the northern periphery of the upper ridge...possibly folding southeast into portions of the Blacksburg forecast area by late Saturday afternoon or Saturday evening. This scenario will have to be watched carefully, since hot temperatures and a relatively dry lower troposphere, yet with impressive low and mid level lapse rates should yield sufficient inverted-V soundings and DCAPE to support generation of extensive and self-sustaining deep convective cool pool/outflow that could yield a heightened threat for broadscale strong to damaging straight- line winds. Not yet convinced that indexes and dynamics/thermodynamics support Derecho-scale event, but threat for at least a more limited event in time/space does indeed exist, and obviously has sufficient attention by the Storm Prediction Center to warrant at least a broad region of Slight Risk which, at least right now, falls just north of our area of concern. If sufficient outflow boundaries from any Friday afternoon/evening convection can force surface boundary a bit further south, then Slight Risk area may also have to be adjusted south to encompass the Blacksburg forecast area - so will bear close monitoring over the next day or so. As is often the case, the extreme southern tail of any mesoscale complex convective outflow and associated strong winds will be penetrating into an increasingly drier and more capped environment closer to upper ridge axis such that winds on southern flank may actually intrude into areas that receive little to no rain. Obviously, details in time and space will be inserted into the grids as the potential event nears, but for now, will hold highest threat for precipitation across northwestern third of the forecast area for later Saturday afternoon into Saturday evening, with lower threat further south and east. Weather after Saturday night will be largely governed by extent of post-convective outflow, and whether atmosphere can sufficiently recover prior to arrival of final upstream short wave Sunday night into Monday that will support amplification of developing upper trof over the lower Great Lakes - driving cool front south through the forecast area early in the long-term period (see discussion below). As would be expected, convection and associated cool outflow boundaries have the potential to significantly influence temperatures during the period, but mean environment prior to frontal passage will be for hot days and warm nights - yet not high enough to trip heat- related advisories or threaten any thermal records. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 315 PM EDT Thursday... At least some ongoing threat for additional shower/thunderstorm activity, dependent on earlier convective influences, will continue until cool frontal passage later Monday or Monday evening. Thereafter, deepening upper trof over New England will support good push of cooler and drier air back into the mid-Appalachian and mid- Appalachian region Tuesday through Thursday as surface ridge builds southeast into the area. Little to no post-frontal precipitation threat will exist during this period - along with a return to near normal temperatures. Only exception may be across the extreme southern flanks of the forecast area into midweek - where an ongoing threat of mainly diurnally- driven showers/storms may persist if less robust of a penetration of drier/cooler air occurs behind the front. && .AVIATION /02Z FRIDAY THROUGH TUESDAY/... As of 805 PM EDT Thursday... Overnight there is fairly high confidence VFR conditions should prevail with the exception of KLWB where valley fog formation depends on any mid to high clouds sneaking in late tonight from weakening convection to the northwest. Here have added in some MVFR and possibility of brief IFR visibilities due to lack of confidence in any clouds arriving in time to influence the climatologically expected river valley formation. Elsewhere, with dry ground due to lack of rain last couple of days, fog is not expected to create sub VFR conditions. Skies will be mainly clear with exception of some thin high clouds. High confidence in light west or calm winds as well. For Friday, confidence is no better than medium on timing and evolution of potential thunderstorms. Atmosphere will be a little more unstable compared to last couple of days under hot and humid conditions but the triggering mechanism is in question, with most near term guidance relying on an old outflow boundary from stronger convection over Ohio Valley tonight, which has yet to develop. This boundary may produce some isolated showers in the mountains morning to noon time, and then may be in the foothills and Piedmont by afternoon where stronger storms could develop along it. Will certainly not be widespread but scattered about enough that airports from KROA to KLYH to KDAN have a decent chance of having some impact. Chances seem better farther north, but not especially confident, and if boundary is weak or non-existent, then coverage of an storms will be much more isolated and could also threaten areas farther west. Updates by early Friday morning should have noticeably higher confidence. So while VFR should predominate, included VCTS with CB for KLWB, KROA, KLYH and KBCB. Less confident at KBLF and KDAN. Potential exists for storms to become strong with gusty, erratic winds with localized turbulence, lightning and brief IFR or lower visibility in downpours. Outside of thunderstorms, winds should be light southwesterly. Aviation Extended Discussion... Confidence is growing in a potential more active extended aviation forecast period for the weekend into Monday with multiple opportunities for thunderstorms. Most of the period will be VFR but with better chances for storms in more locations, mainly afternoon and evening, and with better coverage of rainfall the chances for IFR fog in the early morning hours increases again especially for KLWB and KBCB. A weak cold front is likely to push some of this moisture off to the southeast by Tues or Wed for at least a couple of relatively dry days. Overall, 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...AL NEAR TERM...AL/SK SHORT TERM...WERT LONG TERM...WERT AVIATION...AL/SK is the U.S. government's official web portal to all federal, state and local government web resources and services.