Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 201149 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 749 AM EDT Sat May 20 2017 .SYNOPSIS... Unsettled weather is anticipated through early Monday as an upper level trough of low pressure over the central U.S. moves east, erroding the warm subtropical ridge over the eastern United States. && .NEAR TERM /THROUGH TONIGHT/... As of 430 AM EDT Saturday... Today will be the last day of the unseasonable warmth, before cooler air arrives tonight. Busy weather regime with mid-CONUS trough continuing to pound the center of the country with thunderstorms. Farther east, subtropical ridge of high pressure has been acting as a road block to the upper trough, but still providing the heat and humidity needed to support scattered showers and storms per either daytime surface generated instability or nocturnal mid level instability. Either way the coverage has not been that extensive, mainly confined to the higher terrain, although we are now beginning to see an eastward drift into the piedmont from any activity that develops per increasing westerlies aloft. Today will be no different, showers/storms developing during the peak heating of the day. Only caveat today will be a better focusing mechanism, a surface front that will enter the CWA from the northeast. A surface front extends from northern VA/WV west across the OH valley and into an area of low pressure over Kansas. The low over Kansas is associated with the parent upper level trough. Cool high pressure was moving east across eastern Canada, this High poised to build over New England and wedge down the east side of the northern Appalachians. This will give the surface front over northern VA/WV a nudge to the south today and tonight with this feature entering our northeast CWA this afternoon and evening, then southwest (backdoor style) down the east side of the central Appalachians. This front will be converging with the warm southwesterly flow aloft which will persist per the subtropical ridge just off the southeast Atlantic coast holding its ground. Net result will be an area of convergence along the front which will provide the focus for showers and thunderstorms this afternoon and evening. Attm it appears the axis of greatest instability will bisect the CWA from northwest to southeast just ahead of the front and right through the center of the CWA vicinity of an LWB-ROA-DAN sort of corridor during the peak heating part of today. As such, expect the greatest CAPE to pool within this region providing the fuel for strong to potentially severe storms. Similar to yesterday, the main severe threat would be from wind (40-60 mph sort of max gusts), in addition to some hail. Another concern would be potential for isolated flash flooding per orientation of the front which would support backbuilding along the boundary. On the average, anticipating between 0.50 to 1.00 inch of rain, but the potential will exist for locally higher amounts, with some of the hires models suggesting isolated cores of 2 to 3 inches associated with storms which track over the same area. Temperatures today will once again head for the 80s, but with more cloud cover around, these numbers will be a few degrees lower than Friday. Areas northeast of Lynchburg will also begin to encounter a cooler northeast wind as the surface front enters the forecast area from the northeast, the cool advection negating some of the daytime insolation. For tonight, the surface front will continue to make inroads to the CWA, and is expected to push cooler air into all areas by daybreak Sunday. Showers will continue along the frontal boundary which will favor the southern and western sections of the CWA with time. Overrunning of this airmass by southwest wind flow aloft will maintain mid-level instability, so not eager to pull pops completely, even after midnight. Temperatures tonight will retreat into the upper 50s-lower 60s...the coolest readings northeast of the Roanoke valley into northern VA. && .SHORT TERM /SUNDAY THROUGH TUESDAY/... As of 400 AM EDT Saturday... It continues to be unsettled for Sunday and into the first half of Monday. 00z global model guidance continues to display reasonably good agreement on the idea that the mid-level subtropical ridge axis builds eastward, being replaced by increasing southwest flow aloft. A surface cold front trailing from the eastern Ohio Valley southward to the western Alleghanies then south-southwest into the Tennessee Valley early Sunday morning is apt to make slow eastward progress. With a continued feed of moisture ahead of the front from the western Atlantic and the eastern Gulf, there may be multiple rounds of showers and thunderstorms through the Sunday late morning into Monday afternoon timeframe. Abundant cloud cover appears likely to limit potential for stronger storms through this period. Flow aloft is sufficiently strong enough to keep individual showers and storms moving along. However given we have had several periods of showers and thunderstorms in recent days, rainfall from this frontal system projected to total 1 to 1.75" based heavily on latest WPC QPF (locally higher in thunderstorms) opens the door for potential localized flooding. Rainfall appears to be the steadiest/heaviest during the Sunday evening into Monday morning timeframe. Will continue to mention this period as one to bear close watch for at least minor hydro issues in the HWO. Front slips far enough southward Monday evening into the central Carolinas that an weak high pressure builds into the northern two- thirds of the CWA. Cooler 850 mb temperatures to around +7 to +10C and at least some radiational cooling makes Monday evening the coolest of the period. Didn`t include in the grids but potential for patchy fog if radiational cooling is strong enough and winds light enough as well. Into Tuesday, dominant feature in mid-levels is a sprawling trough over the northern tier of states. Unfortunately what this means for us is that with spokes of energy rotating around the trough and interacting with remnant baroclinic zone over the Gulf states, another shot of wet weather emanating from the Deep South appears to be in the cards for later into Tuesday. Compared to climatology, high temperatures trend near to slightly below normal through Tuesday, coolest earlier in the period. Lows on the other hand trend near to slightly above normal given several periods of cloudiness, coolest Monday evening. && .LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/... As of 400 AM EDT Saturday... Cold front crosses through on Wednesday leading in a colder air mass over the area for Wednesday and Thursday. ECMWF shows the potential for some gusty winds behind Wednesday`s front. Expecting showers and thunderstorms along each of the front with the potential for heavy rain. Overall troffing and periods of rain will result in below normal temperatures for much of the week. Have trended toward cooler daytime temperatures for Monday through Friday. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... As of 800 AM EDT Saturday... Mainly VFR expected the first half of the day. Debris cloudiness from last nights showers will dissipate later this morning, followed by afternoon cumulus buildups and a repeat cycle of afternoon and evening showers and thunderstorms. Aside from local sub-vfr conditions associated with the deep convection, will be watching a backdoor cold front slip southward along the east side of the Appalachians later today and tonight. Models suggests a low stratus deck will develop behind this front for tonight, with potential for IFR CIGS. Extended Aviation Discussion... Unsettled weather will continue Sunday and into early Monday with an opportunity for showers and thunderstorms, especially during the peak heating hours of the day over the western mountains by late Sunday, and again Monday associated with a cold frontal passage from the west. Potential for sub-VFR will exist until the passage of the front Monday. Improvement is expected behind Monday`s front with dry weather returning Monday night into Tuesday. Another front may then bring the next round of showers and subsequent MVFR by Wednesday. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...PM NEAR TERM...PM SHORT TERM...AL LONG TERM...AL AVIATION...PM

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