Area Forecast Discussion
Issued by NWS Blacksburg, VA

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878 FXUS61 KRNK 272327 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 727 PM EDT Thu Jul 27 2017 .SYNOPSIS... Low pressure was over the Ohio Valley with a cold front extending into the the Southern Plains. This low will move southeast tonight through Friday night, which will bring a cold front across the Mid Atlantic region on Friday and Friday night. High pressure follow behind the front covering from the Great Lakes to Virginia and North Carolina by Tuesday. && .NEAR TERM /THROUGH FRIDAY/... As of 625 PM EDT Thursday...Sent an early update to increase QPF and to add heavy rain wording through midnight mainly for locations along and southwest of a line from Bluefield to Blacksburg to Hillsville and North Wilkesboro. Regional radar shows bands of embedded heavy rain producing thunderstorms over northern and eastern Kentucky that are progressing east- southeast. The area I`ve included for heavy rain wording looks to be on the eastern extent of favorably high moisture flux convergence from developing 30-kt southwesterly low-level jet advecting in PWAT values nearing 2.0" per SPC mesoanalysis. WPC`s mesoscale precip discussion issued around 2 pm does a nice job describing the larger-scale meteorological elements associated with heavy rain in this area as well. HRRR and the 18z 3-km NAM progress this ongoing area across the Mtn Empire/Grayson Highlands and into the NC High Country through midnight. As profiles have generally stabilized from earlier showers and thunderstorms, think convective threat is over outside of some intra-cloud lightning flashes. In the next couple of hours, will also need to monitor ongoing convection over the northern NC Piedmont for localized heavier rainfall issues. Overall message for tonight is such that the already- limited severe threat has ended, but in turn is turning toward localized hydro concerns. As for the after-midnight period, it`s a little unclear on the spatial extent of rainfall per 18z guidance suite. The NAM and the CAM guidance generally keeps us dry with a lot of cloud cover around; while the GFS hints at re-developing at least showers roughly along/north of I-64 associated with broad diffluent mid-level flow ahead of potent shortwave now over the upper Midwest. Started to reduce PoPs for areas south of Rt 460 after midnight, but keeping them relatively high across the northern third of counties. Further adjustments may be needed later. All in all, expect a cloudy and rather muggy evening with lows in the mid/upper 60s to mid 70s. Previous discussion issued at 205 PM follows... A weakening line of convection will fall apart over the New River and Roanoke Valleys this afternoon. Following this line of convection is an area of light rain with embed cloud-to-cloud lightning. This area of rain will stay over a county for two to three hours before moving east. There is another line of convection coming across eastern Kentucky with another line behind that one. These two lines will move over the Mountain Empire and North Carolina Highlands later this afternoon. Convection east of the Blue Ridge is likely however, severe potential is limited. Morning soundings at RNK/GSO shows plenty of moisture in the low levels (below 6kft), but abundant dry air above. Both soundings are also showing a very strong cap between 6500-8000 ft. It was this capping inversion that kept the first line of convection weak coming over the mountains this morning. The difference this afternoon is that diurnal heating, especially across Southside has allow instabilities to creep up to moderate levels. A few storms may find a weakness in the cap and break through to become severe. These lines of convection and areas of rain will help saturate the column and erode capping inversion by this evening. An ill-defined surface low is expected to track along a cold front tonight into Friday. Models are developing two areas of convection with this front. One area of convection is expected to occur across Tennessee early Friday morning and track southeast into Alabama and Georgia. This line of convection is expected to fade through the morning, but will send an outflow boundary across the southern Appalachian and into North Carolina in the afternoon. The second area of convection will occur across the southern Ohio Valley tonight then track east into the Mid Atlantic region Friday. This area of convection will have the stronger dynamics as a closed low drops out of the Great Lakes. Being in between these two convective systems, our chances are very good to see showers through the day Friday. Convection coming across the area today will take out some of the instabilities but will saturate the environment. PWATs will increase to nearly 2 inches Friday. With our recent dry spell, we are not expecting any widespread flood concerns today. However, depending on how much rain falls today and does rain continue overnight, may prime the area for Flash Flooding tomorrow. For now, we will hold steady and only highlight in the Hazardous Weather Outlook (HWO). && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/... As of 125 PM EDT Thursday... Closed low moves southeast from the Ohio Valley into Virginia by Saturday. Upper low moves offshore and trough axis will be along the coast on Sunday. This brings a vorticity maximum over the region on Saturday. This synoptic lift combined with good lapse rates due to colder temperatures aloft will result in scattered afternoon showers. More questionable is the presence of enough instability and lift for thunderstorms. BUFKIT forecast soundings showed little, if any CAPE, Saturday afternoon. Models, especially the 12Z NAM, suggest 850MB temperatures by Sunday morning as low as +8. NAM. SREF and ECMWF were not that cool. Clouds and wind will limit cooling potential on Saturday night. will be leaning toward blend on MOS/MAV guidance for lows Saturday night. Deeper moisture wraps around the low pressure system with the dry slot over southwest Virginia Friday night and moisture back in the area on Saturday and Sunday. A surface low off the southeast Virginia coast may enhance the north to northeast surface winds across the region on Saturday night and Sunday. Will be increasing wind speeds at higher elevations this time frame. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... As of 1245 PM EDT Thursday... Upper low opens up and lifts northeast back into the northern jet stream Sunday night through Tuesday with 500MB heights rising Tuesday and Wednesday over the Mid Atlantic region. Still under long wave troughing over the eastern United States through the end of the week with models showing some signs of through amplifying by Friday. 850MB temperatures gradually warm Monday through Wednesday back close to +18 to +20 for Wednesday and Thursday. May take until Monday for low pressure system to move far enough away that wrap-around clouds exit foothills and piedmont of Virginia and North Carolina. Will maintain dry forecast Sunday night through Thursday. Once surface high moves offshore on Friday a slight chance of precipitation will return to the mountains. && .AVIATION /23Z THURSDAY THROUGH TUESDAY/... As of 726 PM EDT Thursday... Light, unrestricted isolated/scattered rain showers producing generally VFR conditions through about 03z. Next wave of moisture/heavier showers now over eastern KY then advances southeast to areas mainly along and southwest of I-77, potentially bringing MVFR-visibility showers to Bluefield and the southern Blue Ridge and Mtn Empire, perhaps as far northeast as Blacksburg. Threat mainly from 03-05z. Wouldn`t discount rumble or two of thunder and embedded intracloud lightning but likely to be sporadic. Ceilings should begin to fall to a BKN/OVC VFR/MVFR range after 06z. Kept VCSH going after 06z across the TAFs, though this may be overdone and potential for adjustment possible in 06z TAF issuance. Winds mainly light southwest, variable to calm at times. Confidence in aviation ceiling/visibility and timing elements medium this period, though is medium to high on winds. Low-confidence TAF forecast for Friday. Appears that VFR to possibly MVFR ceilings may linger with early-day showers a possibility especially further northwest. Potential will exist for re-development of showers and thunderstorms as early as noon. Timing, strength and extent of thunderstorms highly contingent on sufficient cloud breaks. Any thunderstorm may be capable of torrential downpours and associated IFR to LIFR visibility and lightning. A few stronger storms may be possible, again contingent on cloud breaks. Current TAF calls for extensive, though VFR cloudiness with VCTS indicated for the afternoon hours (starting 16-18z). Winds turn south/southwest 4-8 kts. Aviation Extended Discussion... Saturday starts to dry out some but upper low will enhance northwest flow over the area keeping sub-VFR cigs stuck in BLF/LWB similar to winter scenarios with isolated showers around. Expect widespread VFR to return Sunday/Monday under high pressure following the front. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...RCS NEAR TERM...AL/RCS SHORT TERM...AMS LONG TERM...AMS AVIATION...AL/RCS

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