Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 211643 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1243 PM EDT Fri Jul 21 2017 .SYNOPSIS...
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High pressure will maintain hot and humid conditions this afternoon into Saturday. A frontal boundary across the mid- Atlantic will remain just north of the region into Saturday night. A second stronger cold front will then arrive from the northwest and affect the area Sunday into Sunday night.
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&& .NEAR TERM /THROUGH TONIGHT/... As of 945 AM EDT Friday... Isolated convection continues to push across the far northwest in association with residual outflow from earlier upstream showers. Expect this coverage to fade upon crossing the ridges ahead of another upstream MCS complex from overnight. Eastward extent of this system remains iffy given residual ridging across the region but lots of moisture within an overall uncapped environment and high afternoon instability espcly western half. Latest short term guidance basically weakens this system upon approach before redeveloping at least a band of convection along the leading outflow west of the Blue Ridge. This espcly in line with the latest overnight Cam solutions with the HRRR fading coverage pushing deeper into the ridging out east. Looking at current upstream trends in progression of convection, will go ahead and up pops to higher chances mountains espcly far west this afternoon and include at least a widely scattered mention out east. Still enough dry air aloft to support decent DCAPES so a stronger storm or two possible. Otherwise heat remains a concern out east where a little more of a southwest trajectory should act to hold dewpoints up more this afternoon, making for slightly higher heat indices but still below advisory criteria overall. Expect more clouds to limit heat a bit more across the mountains but still well into the 80s west to a few mid 90s east. Previous update as of 710 AM EDT Friday... Few showers arriving into Greenbrier County this morning off old outflow from upstream convection and along theta-e ridge gradient. High-res models are slower than radar indicates, so upped pops a little but still slight to low chance across the Alleghanys prior to 10-11am. Previous discussion from early morning... 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 /16Z FRIDAY THROUGH TUESDAY/...
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As of 1240 PM EDT Friday... Additional bands/clusters of convection continue to approach from the northwest ahead of the next residual upstream MCS. Also expect to see some added development take shape over the mountains as upstream forcing/outflow works into the weak instability gradient over the far west. Thus will follow closer to the current short term HRRR and include a bit more convection at KBLF/KLWB through the afternoon, as well as a mention elsewhere from mid afternoon onward as perhaps a broken band makes it across the Blue Ridge and into eastern sections. With any of the showers/storms, vsbys/cigs could drop to MVFR/IFR, with gusty winds around. Otherwise, winds will be light from the west to southwest. Depending on where it rains and before next wave of clouds from upstream convection arrives early Saturday morning, fog is possible and for now have it advertised at KLWB after 06z in the IFR range, with also potential for cigs around 2 Kft. Also included brief MVFR at KBCB and KLYH. However left out mention at KBLF for now given uncertainty but given showers around this afternoon, and overnight westerly flow cant rule out sub-VFR in fog/stratus late. Any low clouds/fog should quickly fade Saturday as another MCS slides by to the north. Expect westerly flow to limit added convective development to mainly northern/western sections early on with afternoon potential farther south later along a lingering lee trough. Otherwise appears a period of VFR within a developing cumulus field without any mention of shra/tsra for now since threat appears mainly beyond the valid taf period later Saturday. Aviation Extended Discussion... Confidence is growing in a potential more active extended aviation forecast period for Sunday into Sunday night with multiple opportunities for thunderstorms. May see coverage shift into southern sections by Monday afternoon as residual outflow from Sunday jumps into North Carolina so appears more in the way of overall VFR possible Monday based on latest guidance. Otherwise most of the period will be VFR but with better chances for storms in more locations, mainly afternoon and evening late in the weekend, 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.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH/AL/WP NEAR TERM...JH/WP SHORT TERM...KK LONG TERM...WERT AVIATION...JH/SK/WP

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