Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 211954 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 354 PM EDT Fri Jul 21 2017 .SYNOPSIS...
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High pressure will remain over the region tonight into weekend keeping a hot and humid airmass across the area. A frontal boundary across the mid- Atlantic will remain just north of the region as well into Saturday night. A second stronger cold front will then arrive from the northwest and affect the area later Sunday into Sunday night.
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&& .NEAR TERM /THROUGH SATURDAY/...
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As of 255 PM EDT Friday... Broken band of convection crossing the mountains, preceded by a few storm clusters, will continue to slide east across the region through late afternoon before exiting. This could again result in a few stronger cells mainly across the north per better support/instability. Once this outflow driven band exits expect may see added isolated storms redevelop mainly east per latest HRRR, so will hold onto some low pops into early this evening. Will be in between this lead feature and the next upstream MCS that will be nearing the Ohio Valley by early Saturday. However weak shortwave in between may result in enough lift and weak westerly convergence to support isolated showers far west late where keeping in a 20/30 pop western mountains late. Otherwise another very warm and humid overnight with some patchy fog around and lows mostly mid 60s to low or even mid 70s east. Upper heights will continue to slowly fall on Saturday as a surface wave tracks east along the residual front to the north by late in the day. This looks to keep the track of the next upstream MCS heading more east than south per latest model Corfidi vectors with less lift to the south. However will be quite unstable to the south of this complex with only a weak cap above a rather well mixed westerly trajectory. This may allow storms to unzip over the mountains per weak progged convergence and eventually out east late along the lee trough once mixing relaxes. Thus running with overall chance pops spreading/developing east mainly during the afternoon. Otherwise heat will be the main issue with Saturday perhaps the hottest day so far out east given even warmer 850 mb temps and weak downslope to start. However this likely wont be enough to mix out dewpoints enough to preclude possible heat advisory levels in parts of the piedmont as seen by latest Mos guidance. Therefore bumped up highs some with even 90 degrees possible in spots across the mountains and perhaps near 100 far east with indices around 105 degrees for a couple of hours. This along with it being the weekend supports going ahead with a heat advisory for a few eastern counties Saturday afternoon.
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&& .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/...
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As of 330 PM EDT Friday... Several mesoscale convective systems will track across to the north during this period. The focus for these systems will be a stalled frontal boundary over West Virginia and Maryland. It should be trapped between an upper level trough passing eastward over the Great Lakes during Saturday night and Sunday and a broad upper level ridge over the Southeast. This ridge will keep the heat and humidity over the Mid Atlantic on Sunday. Expect heat indices to reach over 100 degrees in the Piedmont. This heat and humidity should also fuel the aforementioned systems. Models are showing the potential for severe weather within these clusters of thunderstorms as indicated by the slight risk from SPC for Sunday. It is possible that some of these systems could dive southward toward our CWA, and model soundings show CAPE approaching 2,000 J/kg by the afternoon. Depending on how well the convection can push the frontal boundary southward over our CWA will shape up the forecast for Monday. Model spread increases considerably at this point while trying to refine where the frontal boundary will reside. So, it requires keeping the chance of showers and thunderstorms into Monday afternoon. The highest chance appears to be along the southern Blue Ridge and through the northwest North Carolina mountains. Temperatures will likely be a couple degrees cooler for Monday as compared to Sunday. More relief from the heat appears by Monday night as high pressure approaches from the north.
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&& .LONG TERM /TUESDAY THROUGH FRIDAY/...
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As of 330 PM EDT Friday... High pressure should build over the Northeast by Tuesday. The frontal boundary will be shoved southward southward toward the North Carolina coast by this time. Lingering moisture may still spark a few diurnally-driven showers and thunderstorms during each afternoon through this period. The best chances appear confined to the VA/NC border on Tuesday, but more potential exists for Wednesday with easterly to southeasterly flow from high pressure offshore. There could be some wedging possible east of the Blue Ridge, so high temperatures were pushed downward for Wednesday to allow for the potential of upslope flow along the Blue Ridge and more cloud cover. Temperatures should bounce back upward by Thursday as high pressure heads further out into the Atlantic Ocean. Long range models hint at another frontal boundary approaching our CWA by Friday.
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&& .AVIATION /20Z FRIDAY THROUGH WEDNESDAY/... 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. && .RNK WATCHES/WARNINGS/ADVISORIES...
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VA...Heat Advisory from 11 AM to 7 PM EDT Saturday for VAZ044>047- 058-059. NC...Heat Advisory from 11 AM to 7 PM EDT Saturday for NCZ005-006. WV...None.
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&& $$ SYNOPSIS...JH/WP NEAR TERM...JH SHORT TERM...PW LONG TERM...PW AVIATION...JH/SK/WP

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