Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 181409 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1009 AM EDT Thu May 18 2017 .SYNOPSIS...
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High pressure east of the Carolinas will continue to produce a warm and humid southwest flow of air across the region through this afternoon. The increasing moisture will result in a threat for widely scattered afternoon and evening showers and storms, especially across the higher terrain. A weak cold front will approach from the northwest Friday, providing a better focus for showers and thunderstorms.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 955 AM EDT Thursday... Main concern with convective coverage espcly over the mountains this afternoon as increasing instability and moisture act to break through a weakening cap per slowly lowering heights aloft. However latest morning soundings still show quite a bit of dry/warm air lingering aloft, which current models seem too fast to erode as suggested by a quick jump in PWATS to over an inch this afternoon. Latest HRRR has backed off a bit on widespread coverage west with it seeming to focus on a residual weak surface trough to the west that will cross into 1500-2000+ J/KG forecast Cape values along the Blue Ridge later on. Appears this weak feature combo with some differential heating per early clouds, and convergence within the low level southwest flow the main kickers for development Blue Ridge. However appears a little slower for deeper convection outside of the NC mountains where the earlier Cam ensemble and current 12z Nam have the better focus. Thus outside of isolated lighter showers far west early, running with higher chance coverage mountains from mid to late afternoon, where any deeper storms could tap into the dry air aloft, as well as the inverted V type profile to produce some stronger pulse storms. Otherwise kept temps a bit cooler than yesterday due to more clouds early and at least spotty convection later, but overall only small changes to the previous forecast highs for now. Previous discussion as of 500 AM EDT Thursday... Ridge of High pressure covers much of the southeastern CONUS. Clockwise flow around the high will continue to provide for a very warm airmass in addition to increasing dewpoints and instability. For the last two days we have been capped with no precipitation. Today will start off much the same, however, cooling aloft from approaching mid-CONUS trof will result in a weakening of the cap this afternoon with potential for widely scattered showers/storms to develop across our western CWA. Greatest threat for hearing a rumble of thunder today will be along and west of the Blue Ridge. Cap is forecast to remain strong enough across the piedmont for little or no coverage there. Temperatures today will be very similar to yesterday, maybe a degree or two cooler per more cloud cover. Diurnally driven showers are expected to dissipate after sunset, lingering moisture and higher dewpoints then leading to patchy fog/stratus for the overnight and early Friday. Mid Conus trough and associated surface front will begin to impact the area by Friday, with potential for debris cloudiness and showers from anything that develops over the TN valley to reach our mountain counties toward daybreak Friday. Can`t totally rule out an isolated severe storm late this afternoon, but the threat will pale in comparison to the High Risk that has been posted for parts of KS/OK. Our storm mode will be Pulse Storms with focus on microburst potential per forecast DCAPE over 1000.
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&& .SHORT TERM /FRIDAY THROUGH SUNDAY/... As of 400 AM EDT Thursday... In mid-levels, a deamplifying shortwave trough responsible for the rash of severe weather in the central Plains and upper Midwest will progress across the Northeast CONUS and into the northwest Atlantic early on Friday. The implication that this will have is a (temporary) weakness in the subtropical ridge contributing to summerlike temperatures across our area Friday into Friday night. The ridge becomes re-established/amplifies into the upcoming weekend in response to downstream amplified troughing over the central Rockies and western/central Plains states. With the weakness in the ridge, if there is a period of greater potential for thunderstorms it would be Friday. The capping inversion is progged to be weaker than prior days. Combined with daytime insolation and mid-level lapse rates of 6.5-7 C/km, moderate instability values are likely (~1500-2000 J/kg MUCAPE per 00z GFS, overdone NAM is a bit higher due to higher sfc dewpoints than realistic). Questions regarding a triggering mechanism Friday continue into this recent model cycle; 00z GFS and ECMWF showed a weak vorticity max embedded in moderate SSW flow that would fit the bill, but they differ on timing with the ECMWF arriving its vort max after peak heating. So in that sense, it will probably come down to what evolves upstream on Thursday. Absence of stronger winds aloft (< 35 kts) keeps deep-layer shear at/below 30 kts; if storms do form they likely would be of the pulse variety and/or be disorganized. Coverage would be greatest from the Blue Ridge west, likely weakening as they progress eastward into Friday evening. SPC`s 07z Day2 Outlook calls for 5% severe probs/Marginal Risk for much of the forecast area. There may be one or two stronger storms but based on the forecast shear/instability parameter space and lift described above, I wouldn`t think much more than that. Friday projects to be the last warm day for a while with highs in the 80s coolest west; a potential spot 90 degree reading may still be in the cards for the VA/NC Piedmont areas. Later Friday evening, we do get into more of an onshore easterly flow from a backdoor cold front as ridge to the north. NAM/GFS RH values are rather high with likely increased cloudiness for the evening hours along and east of the Blue Ridge. Forecast lows in the lower to mid 60s for Friday night, with some decrease in dewpoints/humidity levels as well into the overnight. For Saturday, majority of guidance depicts some residual light QPF early in the day associated with the backdoor front as it continues its southward migration toward the VA/NC border. I think that seems reasonable; however, the coverage of PoPs/QPF for the afternoon appears overdone for a few reasons: (1) lower instability due combined effects of cooler high temperatures in the mid 70s due to cloudiness and (2) background mid-level subsidence. I`ve therefore opted toward a dry forecast for many areas except slight chance for southeast WV, far southwest VA and the NC mountains. I`ve biased temperatures toward some of the cooler raw model guidance for highs in the 70s; admittedly if clouds hold tough through the day and they might, these may still be a few degrees too warm. Lows should be in the upper 50s/lower 60s with continued cloudiness. While the ridge still remains strong early into Sunday, it does begin to weaken later Sunday as the mid-level energy along the Rockies/associated sfc cold front progresses east into the OH/TN valleys and acts to weaken it. Should see increasing chances for PoPs with western extent for showers and thunderstorms toward the late afternoon/early evening, but feel greater coverage of precip expected toward evening. Instability is on the low side and not thinking any stronger storms though precipitable water values increase to values near 1.5" late in the day. Highs should be a couple degrees warmer than Saturday in the mid/upper 70s, though low 70s along the Blue Ridge. && .LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/... As of 400 AM EDT Thursday... A cold front will bring a better chance for showers and thunderstorms to the area Sunday evening. This front may become disconnected from its parent low over the Great Lakes on Monday, keeping the chance for storms in the area. The ECM has a surface low developing along the stalled front Monday afternoon. This low will eventually push the front to the coast Monday night. Weak high pressure will give the region a brief break from the storms Tuesday before the next round comes in with frontal passage on Wednesday. With a good chance for showers and thunderstorms this week, temperatures will remain close to normal. && .AVIATION /14Z THURSDAY THROUGH MONDAY/... As of 400 AM EDT Thursday... VFR conditions expected to continue through the TAF valid period. The only exceptions will be the development of widely scattered shra/tsra this afternoon between 18z/2PM and 00Z/8PM, and some late night - early morning fog Friday morning. VCNTY TSRA has been added to all TAF sites along and west of the Blue Ridge for a few hours late this afternoon/evening. Period of fog/mist will be added to the TAFs for later tonight and a few hours Friday morning. SCT-BKN layer of stratus is also possible early Friday morning. Could result in local IFR. High confidence in VFR through 18Z today. Medium confidence for the remainder of the TAF period per thunderstorm uncertainties and fog potential late tonight. Extended Aviation Discussion... Better threat for greater coverage of afternoon showers/thunderstorms Friday with periodic rounds of sub-VFR possible Friday afternoon/evening as a frontal boundary moves southeast toward the central Appalachians. The front is forecast to stall across the region into this weekend, with best shower/storm chances over the mountains Saturday afternoon/evening. Coverage should then trend more isolated until late Sunday as this residual boundary lifts out to the northeast ahead of another cold front approaching from the west. This front may then bring another round of deeper convection to the mountains by Sunday evening and over much of the region into Sunday night. && .CLIMATE... As of 800 PM EDT Wednesday... Record high temperatures were tied at Blacksburg and Bluefield yesterday with 85 and 84 degrees, respectively. With increasing cloud cover today, record high temperatures are unlikely, but here they are just in case. List of record highs for Thursday 5/18 for our five climate sites. Thursday 05/18/2017 Site MaxT Year Bluefield 87 1996 Danville 93 1974 Lynchburg 93 1911 Roanoke 94 1962 Blacksburg 87 1962 && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH/PM NEAR TERM...JH/PM SHORT TERM...AL LONG TERM...AL/RCS AVIATION...PM CLIMATE...DS/WP

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