Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 261954 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 354 PM EDT Fri May 26 2017 .SYNOPSIS... High pressure will remain over the area this evening. A cold front will approach the region later tonight, and stall over the area during the day Saturday. This front will act as the focus for yet another disturbance that will pass overhead Sunday into Sunday night. && .NEAR TERM /THROUGH SATURDAY/... As of 315 PM EDT Friday... An upper level shortwave ridge of high pressure was centered over the region this afternoon. An upper level low was centered over south central Canada. A little farther south in the Wabash Valley of IL/IN an upper level disturbance was progressing eastward. A cold front extended southwest from this feature into central Oklahoma. As we progress through the overnight, the IL/IN feature will continue heading eastward. It will have the impact of flatting the shortwave ridge overhead, and place the region in a steering flow that will be almost due west-east from the mid-Mississippi Valley into our region. This will open the door for mid-West disturbances that form along the associated cold front to make headway toward and into our area late tonight into Saturday. The bulk of the energy associated with the IL/IN feature will remain to our north overnight, skirting the far northwestern sections after midnight. By sunrise Saturday, convection is progged to have fired along the approaching cold front and will be moving eastward through the Ohio Valley and Tennessee Valleys. SPC Day 2 Convective Outlook still has a Slight risk of severe weather across the region tomorrow into tomorrow night in anticipation of convection forming and racing ahead of the approaching cold front. Around noontime onward into the evening hours looks to be the greatest potential for the forecast area to be impacted by this activity. Large hail and damaging winds are expected to be the primary hazard. After today`s brief dry period, humidity levels will be on the increase again on Saturday, and temperatures will also inch up a few more degrees. Low temperatures tonight will be a mix of low to mid 50s. High temperatures on Saturday will range from the upper 70s to lower 80s across the mountains with mid to upper 80s across the Piedmont. && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/... As of 230 PM EDT Friday... Potential continues to exist for strong to severe thunderstorms and an accompanying potential for heavy rainfall/isolated flash flooding for the Memorial Day weekend. Identifying intensity, timing and geographic areas in a more specific risk are very difficult to pin down, with guidance varying widely on each of these aspects. This appears to be a convectively active pattern across a broad area from the mid-Mississippi, OH/TN Valleys eastward to the mid- Atlantic/Carolinas. These details probably will not avail themselves any sooner than each day, so the forecast really is not all that specific. With the stated uncertainty, and as many likely have plans outdoors, we encourage those to keep a close ear to the latest forecast for the holiday weekend. On Saturday night, likely capped atmosphere will keep a lid on what is expected to be warm, increasingly humid and conditionally unstable air mass conditions. While most areas stay dry, latest convective-allowing guidance shows isolated to widely scattered storms developing early evening across northern and eastern sections of the forecast area, perhaps along any old convective outflow that sags southward from northern VA. This area roughly bounded from Greenbrier and northern Monroe Counties in WV eastward across the southern Shenandoah Valley in VA and into the northern foothills of the Blue Ridge and the VA Piedmont. Any storm that were to form may become strong given MLCAPEs on the order of 1500 J/kg and LI values of -6, but may prove isolated. It`s not until you look westward into the faster westerly flow across the Mississippi and OH/TN valleys where guidance develops more organized MCS clusters, some of which may need to be watched for the overnight period as they near our western border. Dependent on how Saturday night evolves upstream, Sunday probably features at least early-day convective debris cloudiness and potential showers/embedded thunder. Strengthening mid-level height gradient supports a greater increase in mid-level winds and unidirectional, deep-layer shear values supporting organized storms. Questions for Sunday include degree of instability/breaks in cloud cover, any potential outflow boundaries left behind from overnight/morning and timing any weak shortwave/vort max that may help trigger storm development. SPC presently has a 5% severe/Marginal Risk for Sunday for much of the forecast area. Pending more clarity on these details, not out of the realm of possibility that this may change. Region becomes dominated by broadly cyclonic mid-level flow on the southern end of a large upper low over the Great Lakes. A cold front should be working its way eastward across the forecast area, though it may spark showers and thunderstorms into Memorial Day. Think this would be mainly east of the Blue Ridge according to the 12z NAM and GFS guidance. As deep layer shear is still on the high side, could still have a few stronger storms in that corridor during the afternoon if frontal timing is favorable. Should see clearing take place with cold/dry advection taking place behind the front Monday evening, putting an end to warm and humid conditions. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... As of 230 PM EDT Friday... A brief reprieve from the unsettled conditions appears to be in the offing for Tuesday as region becomes more influenced by broad cyclonic flow in mid-levels. 850 mb temperatures about 3-5 degrees cooler than the Memorial Day Weekend. Appears that Wednesday into Wednesday afternoon may offer another period of showers/storms associated with a weak shortwave trough rounding the broader longwave feature, though the better chance appears to be along and north of I-64 at this point. We remain in the stronger belt of zonal westerlies into the late week and the early weekend, separating warm and humid air to the south and cooler cP air to our north. Kept a generic slight/Chance PoP for the weekend. && .AVIATION /20Z FRIDAY THROUGH WEDNESDAY/... As of 150 PM EDT Friday... A ridge of high pressure over the region will hold fast over the region through tonight. Precipitation in advance of an approaching cold front will start to enter the western section of the area late Saturday morning. The result will be period of VFR conditions through roughly 15Z/11AM Saturday. After that time, MVFR ceilings will start to increase across the area from northwest to southeast, reaching at KMKJ-KLWB line by 18Z/2PM. Scattered showers with mainly VFR visibilities will accompany this cloud cover. Winds will remain gusty this afternoon from the west to northwest with 15 to 25 kt gusts common until just before sunset. Overnight light winds are expected, and the direction will start to slowly back. On friday, winds will increase from the southwest in advance of the approaching system. Confidence levels high on all parameters for this near term part of the aviation discussion. Extended Aviation Discussion... Saturday afternoon, shower and thunderstorm chances increase across more of the area, with a greater abundance of the western sections having sub-VFR conditions with the arriving convection. Activity will continue into the evening hours. Additional convection and sub-VFR conditions probable Sunday into Sunday night as another disturbance crosses the area. Convection becomes less organized Monday into Tuesday with hit and miss sub-VFR conditions. Perhaps late night and early morning sub-VFR river and mountain valley fog. Better organization of convection Wednesday into Thursday with the passage of a cold front. Confidence concerning the general weather pattern during this portion of the aviation forecast period is moderate to high, but confidence on specific timing of any sub-VFR condition is low. && .HYDROLOGY... As of 220 PM EDT Friday... We continue to have river flood warnings in effect for the Dan River at South Boston and on the Roanoke River at Randolph. Flood wave from upstream gages has moved into the lower reaches of these river basins, with crests expected at each gage Friday afternoon but will remain above flood stage until Saturday See specific point flood warnings in effect for these gages. Elsewhere, hydrographs at many small creek and larger main stem rivers continue to show a general receding trend. While the receding trend is expected to continue, a reminder that rivers still are elevated and there are still fast flows that can be dangerous. Looking ahead into the Memorial Day weekend...there is the potential for multiple rounds of showers and thunderstorms. Heavy rainfall is possible from any thunderstorms, especially in storms that can be slow moving, affect urban catchments and/or anchor to the terrain. Flash flood guidance values are low (1.25-2.5" in 3 hours lowest in the mountains and highest in the southern Piedmont), and would precondition the region for flash flooding and rises on smaller creeks if rainfall amounts and rates prove high enough. Confidence is low on expected rain amounts and specific periods of time of greater risk. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...DS NEAR TERM...DS SHORT TERM...AL LONG TERM...AL AVIATION...DS HYDROLOGY...AL

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