Area Forecast Discussion
Issued by NWS Blacksburg, VA

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923 FXUS61 KRNK 271757 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 157 PM EDT Sat May 27 2017 .SYNOPSIS... Several low pressure systems will track across the central and eastern United States today through Monday with a series of fronts over the Mid Atlantic region. Tuesday a stronger low will develop over the Northeast pushing a final front through the area. High pressure builds in for the end of next week. && .NEAR TERM /THROUGH TONIGHT/...
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As of 115 PM EDT Saturday... There will be no major adjustments to the ongoing forecast for this afternoon. Have tweaked hourly temperatures, dew points, and sky cover to reflect the latest observations and expected trends through the afternoon. Also, have delayed by an hour or two the onset of showers and storms this afternoon. However, the same general pattern is expected. Activity will form upstream in WV/OH and work its way into the area. Also, development is expected across eastern parts of the area this afternoon where sunshine has been more abundant. Once developed, this convection will head eastward. As of 945 AM EDT Saturday... Latest guidance still offering solutions that fire showers and storms across our western flank early this afternoon, and then these become outflow based and translate southeastward across the region. Limiting factors currently are a weakening capping inversion near the surface and warm nose aloft around 660mb. Likewise, cloud cover heading eastward will limit the surface warming to overcome these caps. However, any convection that is able to push through the 660 mb warm nose, will have plenty of instability aloft to become strong/severe quickly once that threshold is surpassed. Our office will be conducting an 18Z/2PM upper air weather balloon launch to allow for better assessment of how the thermal and wind profiles of the atmosphere have been changing since this morning. As of 400 AM EDT Saturday... Watching two potential areas of precipitation today. The first is the cluster of showers and thunderstorms in Ohio and northern West Virginia. Showers from this decaying mesoscale convective system may cross southeast West Virginia and the Alleghany Highlands, mainly north of a Lewisburg to Lexington line this morning. The outflow boundary from these showers will be on of the locations that thunderstorms develop this afternoon. Models showed differing solutions on the location of storm development. Next, the short wave in northern Arkansas and southern Missouri will move through the westerly flow aloft, reaching the Mid Atlantic region late this afternoon into this evening. This feature will also trigger scattered thunderstorms. Expecting enough heating today, even with the residual cloud cover from this morning, to have an unstable airmass this afternoon. Enough instability and bulk shear that isolated severe storms are possible. At the surface, low pressure will track from the Ohio Valley southeast to the Virginia coast by Sunday morning. No real change in airmass today or tonight but moisture is increasing,with precipitable water getting above one inch and surface dew points rising into the upper 50s to mid 60s. Only minor changes to highs today and lows tonight.
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&& .SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/... As of 400 AM EDT Saturday... The second consecutive day of severe convective weather potential should evolve on Sunday as a strong short wave, embedded within the base of the broad upper trough centered in south central Canada, moves through the area during peak heating. Associated with this short-wave feature is a 100+kt jet at 250mb. Strong to extreme instability is indicated by several models, with the GFS likely overdone with 5000-6000 J/kg of CAPE and lifted indices of -8C. However, all of the models suggest good dynamics and instability will be in place as the short wave moves into the area during peak heating. For now, the region is largely in a marginal risk for severe, but would not be at all surprised to see this upgraded one or even two categories before all is said and done. The NAM NEST model shows the development of discrete supercells developing during the mid to late afternoon Sunday then quickly evolving into an intense squall line that races across the Piedmont. It appears large hail and damaging winds would be the main threat, but tornadoes cannot be ruled out given the strong helicity/shear in place with the jet streak over the area at the time. Will need to watch closely with later model runs. Individual cells should be moving fairly quickly, reducing the threat of flooding. However, we could see high precipitation supercells at the outset resulting in localized flooding, which won`t take much given the very low FFG values and saturated soil from all of the recent rainfall. Of course the caveat to all of this occurring is the potential for an early morning much weaker squall line or thunderstorm complex to leave behind a more stable/bubble high air mass that reduces the potential for strong afternoon convection. Unfortunately, this does not appear to be the favored mode, so we will certainly need to keep a close eye on the severe potential Sunday afternoon. Given the speed of movement of the Sunday evening convection, would expect any lingering activity to quickly race off toward the coastline during the mid to late evening, leaving a weak outflow boundary or front draped from northwest NC into southeast VA. A broad upper low will remain across the Great Lakes region with upstream energy/short waves still in place. However, at this point, the most moist and unstable air mass will be east and south of the area. Afternoon convection will likely develop once again near the boundary, mainly along and east-south of the Blue Ridge. SPC has indicated a marginal risk for severe with this activity as well, but it certainly does not seem to be as concerning as the Sunday situation. Tuesday, the area remains just north of the baroclinic zone with upstream energy from the broad upper trough still in place. These features will likely continue to interact with each other bringing a daily risk of showers and thunderstorms to the area. The best instability and moisture will remain just south of the area, so the overall severe threat seems low, as well as the flash flood threat. Temperatures during the period will be mostly just above seasonal normals. Sunday will be the hottest day as 850mb temps tease the +20C level. This will yield max temps in the 70s mountains and 80s elsewhere with lows in the 50s and 60s. For the remainder of the period, look for temperatures to run just above normal with lows in the 55-60 range and highs in the 75-80 range. && .LONG TERM /WEDNESDAY THROUGH FRIDAY/... As of 430 AM EDT Saturday... Broad low pressure aloft will remain anchored across the south central Canadian provinces, shifting slowly east through the later half of the week. This will keep a less moist and more stable air mass across the region. As the trough axis moves through the area Wed, look for a good chance of showers and a few thunderstorms, but the severe threat with this activity appears low. Thu-Fri should be dry before the next embedded short wave moves through the region from the northwest. Temperatures will average near normal through the period with lows mainly in the 50s and highs in the 70s west to lower 80s east. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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As of 145 PM EDT Saturday... Scattered showers and thunderstorms are expected to develop this afternoon and progress east-southeast through the area. Some of these will originate upstream in WV/OH, others will develop across central to eastern parts of the are where more sunshine has helped allow conditions to be a bit more unstable. Most of the area will experience VFR conditions through the evening. However, some MVFR cloud cover is expected across the mountains this afternoon, and any heavier showers and storms will produce some temporary sub-VFR conditions. Some late night and early morning MVFR fog is expected. Any sub-VFR conditions should improve to VFR by early morning. Additional showers will arrive in western section of the area late Sunday morning. Extended Aviation Discussion... Additional convection and sub-VFR conditions probable Sunday afternoon 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 with the passage of a cold front. Thursday is expected to be VFR and dry.
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&& .HYDROLOGY...
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As of 115 PM EDT Saturday... 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.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AMS NEAR TERM...AMS/DS SHORT TERM...RAB LONG TERM...AL/RAB AVIATION...DS HYDROLOGY...AL

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