Area Forecast Discussion
Issued by NWS Charleston, WV

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354 FXUS61 KRLX 271023 AFDRLX Area Forecast Discussion National Weather Service Charleston WV 623 AM EDT Sat May 27 2017 .SYNOPSIS... Rounds of convection are expected this holiday weekend, until a cold front crosses first thing Monday morning. Additional cold fronts cross Tuesday and Wednesday. && .NEAR TERM /THROUGH TONIGHT/... As of 515 AM Saturday... Decaying MCS will continue to move east across far northern zones through early this morning. Focus then shifts to increasing chances for severe weather as the day progresses. Surface boundary, at 4 AM stalled out across central IN/Ohio, will sag south into the region today as low pressure wave moves east. Convection will fire later this afternoon in vicinity of boundary, while a separate shortwave will move east across our south later this evening, creating additional convection. This looks to be a favorable severe weather set up today, with a very unstable airmass progged to be in place. Model soundings indicating high CAPE, on the order of 3000 J/KG on average MU cape, and strong shear, on the order of 30-40 kts, with unidirectional profiles. With such high cape, particularly in the hail growth zone, and shear, expecting severe hail, along with the threat for damaging winds. In addition, the threat for tornadoes cannot be ruled out either. Models indicating low lcl`s and stp of around 1 across the south. SPC has highlighted the entire CWA in a slight risk for today with a 2 percent tornado risk. However, greatest threat is strong/damaging winds and hail. In addition to the overall severe threat, there of course remains the threat for flash flooding. PW values progged to rise to around 1.4 to 1.6 inches today as high moisture content air feeds into the region. Areas that receive repetitive convection will be most vulnerable. FFG values are higher in Ohio and Kentucky, and the threat for repetitive storms today should mainly be across the southern half of the CWA, providing boundary sets up this afternoon where models are predicting. The watch was issued to account for additional convection on Sunday and Sunday night, and may with time, need to be expanded in area. Will allow future shifts to monitor development of convection today, and determine if expansion in future forecast updates is necessary. && .SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/... As of 515 AM Saturday... Additional convection Sunday and Sunday night will be dependent upon previous convection leaving behind a worked over environment. Several boundaries are likely to be in the area Sunday, including at least one outflow boundary somewhere across the south, and a more synoptically driven boundary to the north. There may also be a north to south oriented boundary crossing as a surface reflection of an upper level short wave trough. Model differences in moisture and instability on Sunday reflect the possible effects of a cool pool from previous convection. Models are in better agreement on most of the CAPE being elevated and thickest in the hail growth layer. This somewhat tempers the diurnal dependency of convection. Deep layer shear is forecast to be in the 40 to 50 kt range. All of this suggests showers and thunderstorms around from time to time Sunday through Sunday night, with repetitive downpours possible. Also, afternoon recovery would add surface based instability to the equation, resulting in very strong, heavy thunderstorms. Finally, to go along with these parameters, sigtor values are forecast to climb to 1 to 1.5 in an axis ahead of a cold front approaching from the west Sunday night. The flood watch goes until 12Z Monday when the cold front goes through, and the HWO also highlights the convective threat. Yet another cold front crosses late Tuesday. While weaker and with less available moisture than the weekend system, it will still be able to garner enough moisture and shear for thunderstorms. No deviations from central guidance temperatures but values will fluctuate with the convective complexes. && .LONG TERM /WEDNESDAY THROUGH FRIDAY/... As of 515 AM Saturday... This period features a large upper level low over eastern Canada with long wave trough over the eastern United States. This spells a cool, more tranquil pattern once a reinforcing cold front crosses on Wednesday. PoPs increase again next weekend, as that trough lifts out, and allows another system to approach from the west or southwest. Central guidance temperatures reflect the slightly below normal pattern. && .AVIATION /10Z SATURDAY THROUGH WEDNESDAY/... As of 623 AM Saturday... Isolated showers have developed this morning in unstable airmass. Brief MVFR conditions are possible. Otherwise, VFR conditions initially, although convection will develop once again, particularly after 18Z, with MVFR and IFR conditions, heavy rain, and strong-damaging winds in vicinity of storms. Bulk of convection will diminish after 04Z-06Z Sunday, however, widespread MVFR and IFR conditions are possible in post rain stratus or fog. Additional showers and thunderstorms expected towards the end of the TAF period and beyond. FORECAST CONFIDENCE AND ALTERNATE SCENARIOS THROUGH 12Z SUNDAY... FORECAST CONFIDENCE: Medium AFTER 12Z SUNDAY... IFR likely in storms Sunday and Monday. && .RLX WATCHES/WARNINGS/ADVISORIES... WV...Flash Flood Watch from 2 PM EDT this afternoon through Monday morning for WVZ005>008-013>016-024>027-033-034-515>520. OH...None. KY...Flash Flood Watch from 2 PM EDT this afternoon through Monday morning for KYZ102-103-105. VA...Flash Flood Watch from 2 PM EDT this afternoon through Monday morning for VAZ003-004. && $$ SYNOPSIS...TRM/SL NEAR TERM...SL SHORT TERM...TRM LONG TERM...TRM AVIATION...SL

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