Area Forecast Discussion
Issued by NWS Blacksburg, VA

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS61 KRNK 271402 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1002 AM 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/...
-- Changed Discussion --
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.
-- End Changed Discussion --
&& .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 /14Z SATURDAY THROUGH WEDNESDAY/... As of 715 AM EDT Saturday... Expecting scattered to broken ceilings at 5 to 7 Kft today. Isolated light showers with no visibility restriction are possible at KLWB before noon. Scattered thunderstorms will develop this afternoon. Medium confidence on timing. Based on SPC-HRRR and when short wave out of the Mississippi arrives in the central and southern Appalachians, the best chance of thunderstorms will be from 21Z/5PM through 02Z/10PM. Any heavier rainfall will result in MVFR visibilities. As winds become west to northwest overnight, low level moisture will pool along the west slopes of the Appalachians. MVFR to possible IFR ceilings are expected at KLWB and KBLF after midnight with medium confidence on timing and height of the ceilings. Patchy MVFR fog will develop late tonight once storms and clouds dissipate, especially in areas that get rain today. Extended Aviation Discussion... 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 with the passage of a cold front. Thursday is expected to be VFR and dry. Confidence concerning the general weather pattern during this part of the aviation forecast period is moderate to high, but confidence on specific timing of any sub-VFR condition is low. && .HYDROLOGY... As of 450 AM EDT Saturday... Minor river flooding continued on the Dan River at South Boston and on the Roanoke River at Randolph this morning. For today localized flash flooding is possible since any of the stronger thunderstorms this afternoon and evening will produce high rainfall rates and Flash Flood Guidance was generally 1.5 to 2.5 inches in 3 hours. Storm motion is east around 35 mph so locations that have repeated storms will have a higher threat for flooding. Depending on where any heavy rain falls today, a similar scenario is expected on Sunday. Locations will high rainfall rates or training thunderstorms will have an increased threat of flash flooding and rises on smaller creeks and streams. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AMS NEAR TERM...AMS/DS SHORT TERM...RAB LONG TERM...AL/RAB AVIATION...AMS HYDROLOGY...AMS is the U.S. government's official web portal to all federal, state and local government web resources and services.