Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 210726 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 326 AM EDT Fri Jul 21 2017 .SYNOPSIS... Very warm to hot and humid conditions will persist over the area into this weekend under high pressure. A frontal boundary across the mid-Atlantic to midwest will stay situated in this area into Saturday, and will finally see a cold front tracking across our area by early next week. && .NEAR TERM /THROUGH TONIGHT/... As of 240 AM EDT Friday... Belt of westerlies will continue to stay north of us today, though some hint of a southern shift appears in the models and will have to see how upstream mesoscale convective systems over Iowa/Illinois this morning shift east and allow for cooling of upper levels and lowering heights later today. Have backed off somewhat on temperatures as models are all favoring some cloud cover arriving thanks to cu development and possibly some convective blowoff by early afternoon. Still hot and humid, but not enough to send heat indices to 105F. A few locations around Buckingham/Keysville/South Boston/Danville could touch 100F for a heat index, but no advisories planned. SPC has us in a general thunderstorm outlook. We have plenty of instability today and model soundings showing lack of a cap as mid/upper ridge stay and warmer temps aloft stay south and southwest of us. Appears best forcing and jet dynamics will favor stronger storms across the Ohio Valley. Still any storms that fire up could become strong to severe but isolated threat. Following a blend of the HiRes-ARW/GFS/NAM should see widely scattered showers and storms either arrive or develop over the Alleghanys of WV/VA late this morning with potential outflow and weak upper vort allowing for convection to fire downstream over the Blue Ridge by 1-2pm. Models are favoring better coverage north of Lewisburg WV to Lynchburg, VA then isolated/widely scattered south along and either side of the Blue Ridge, with lesser threat further southeast in the NC Piedmont. Should be enough sunshine to get temperatures into the mid 80s to around 90 mountains, with lower to mid 90s east, possibly upper 90s if clouds are fewer. Tonight, should see a decreasing trend of coverage of storms with potential small area moving into the piedmont after sunset. Will have to watch upstream once again as models are favoring convection/MCS over the midwest, with upper vorts moving along the northern periphery of the 5h high. Potential for another area of showers/storms toward dawn Saturday moving over the WV mountains toward the Alleghanys. Clearing late Friday evening followed by increasing clouds in the west overnight. Low temperatures running from the upper 60s mountains to lower to mid 70s foothills/piedmont. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... As of 315 AM EDT Friday... The heat is on for the weekend with a tenacious upper ridge fighting shortwave trofs and a frontal zone pushing southward. On Saturday, a shortwave will travel eastward out of the Great Lakes Region to southern New England region Saturday afternoon into Saturday evening. Capes of 2000-3000 j/kg CAPE and LIs around -5C to -7C are expected with this wave. Convective complexes on the northern periphery of the upper ridge could fold southeast into portions of the forecast area. Impressive low and mid level lapse rates should yield sufficient inverted-V soundings and DCAPE to support potential for damaging winds. The SPC Day 2 convective outlook highlighted a marginal potential for severe thunderstorms Saturday for our forecast area, with slight risk to our north. There is a potential for damaging winds and hail with stronger storms or storm clusters moving quickly east-southeast across the region. High temperatures on Saturday will range from around 80 degrees in the mountains to the mid 90s in the Piedmont. The combination of heat and humidity will produce heat indices around the century mark in the Piedmont. These values are not high enough to trip heat- related advisories of 105 degrees for several hours or threaten any thermal records. Elected to keep the highest pops in the northwest for Saturday night, with lower threat further south and east. Low temperatures Saturday night will remain mild with readings from the mid 60s in the west to the mid 70s in the east. Waves of low pressure will roll east along the frontal boundary Sunday and try to move it south into our region. Scattered showers and thunderstorms will develop on Sunday with the potential for strong to severe storms. High temperatures Sunday will range from the upper 70s in the northwest mountains to the mid 90s in the piedmont. Heat indices will be once again top 100 in portions of the piedmont. A healthy shortwave Sunday night into Monday will support amplification of developing upper trof over the lower Great Lakes. Low temperatures Sunday night into Monday morning will generally be from the mid 60s in the mountains to the mid 70s in the piedmont. && .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 /08Z FRIDAY THROUGH TUESDAY/... As of 110 AM EDT Friday... Have maintained a generally VFR forecast this period with high pressure aloft entrenched from the TN valley into the southern Appalachians. Since cloud cover will be lagging, have moved down a category for fog at LWB with IFR tempo LIFR in the 10-12z time frame. Some mid clouds may slide in toward 12z to cut off fog sooner. Elsewhere fog will be limited if at all, so none added to other taf sites, with mainly cirrus overhead in a thin veil. 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. Latest models runs are favoring upstream outflow to induce showers/storms in the mountains by late morning, then across the Blue Ridge after midday. Will have VCTS at LWB/BCB/ROA/LYH although the coverage is questionable but looks more concentrated north of a BKW-LWB-LYH line. 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 westerly. 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/WP NEAR TERM...WP SHORT TERM...KK LONG TERM...WERT AVIATION...SK/WP

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