Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 220000 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 800 PM EDT Fri Jul 21 2017 .SYNOPSIS... 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. && .NEAR TERM /THROUGH SATURDAY/... 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. && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/... 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. && .LONG TERM /TUESDAY THROUGH FRIDAY/... 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. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
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As of 750 PM EDT Friday... High confidence in diurnal trends this TAF period with some late night fog/stratus and afternoon convection. Believe fog will be most prevalent at sites that experienced rain today, especially KLYH and KDAN, as well as the climatologically favored site of KLWB. There was precipitation near KBLF and a light upslope west wind may loft some fog/stratus onto the airfield late but confidence in this is not as high as for other sites. Meso guidance indicates the potential for some scattered showers to be stewing around overnight feeding of residual instability and weak short wave energy moving through aloft, but do not expect activity to be very widespread so will go with a dry forecast through daybreak and amend if showers become imminent at an airfield. Tomorrow will be similar to today as upstream convective remnants/outflow moves into the region and helps trigger another round of storms. Will include VCTS all sites for the latter portion of the period. Expect a well mixed boundary layer Saturday will allow for some low end gusts at TAF sites but nothing overly problematic for aviation interests. 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...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. && $$ SYNOPSIS...JH/WP NEAR TERM...JH SHORT TERM...PW LONG TERM...PW AVIATION...MBS/JH

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