Area Forecast Discussion
Issued by NWS Charleston, WV

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000 FXUS61 KRLX 221611 AFDRLX Area Forecast Discussion National Weather Service Charleston WV 1211 PM EDT Sat Jul 22 2017 .SYNOPSIS... Strong to severe thunderstorms are possible this weekend with risk for heavy rainfall and potential flooding. Fronts cross late Sunday and again late Monday. High pressure crosses midweek. Next front late week brings more relief next weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 825 AM Saturday... Quick update to tighten up pops a bit across the north,and decrease across the south this morning per recent radar trends and high res models. Showers and thunderstorms producing heavy rain and gusty winds across the north. Will need to monitor the situation, and possibly issue a ffa for the potential of additional convection later this afternoon and overnight. As of 600 AM Saturday... Another batch of convection has developed north of CRW into s-central OH. Therefore made some grid adjustments to account for this activity early this morning. There remains some uncertainty regarding timing and evolution of convection later today. However, the latest HRRR/HRRRx are in general agreement with other convective-allowing model (CAM) guidance in depicting a re-intensification of remnant MCS activity spreading east-southeast from OH across at least the northern 1/2 of WV by 18z. Any outflow to the south of the more organized activity moving into PA could trigger additional diurnally driven storms from the Tri-State area eastward into the central Appalachians. Deep moisture will remain in place along with very unstable conditions which should promote more storms into tonight. SPC has maintained a SLGT risk across all but the far southern portion of RLX CWA. High PWs along with increasing westerly flow aloft suggests damaging winds are the primary severe threat. Other concern is for rounds of heavy rain and risk for flash flooding. Confidence in QPF details is lower with time but potential is there for some training cells with local rain rates exceeding 2 in/hr. A flash flood watch was considered in collaboration with ILN but thinking this would be a more short- fused watch situation with higher confidence in convective rainfall trends. && .SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/... As of 420 AM Saturday... Convectively active, wavy, roughly west to east oriented front, right over the area first thing Sunday morning, sinks southward through the area during daylight the morning hours. However, with the cold front still west of the Ohio River, models insist the atmosphere easily recovers, with CAPE values climbing to 4 kj/kg over the middle Ohio Valley Sunday afternoon, to go along with modest deep shear into the 30s. Models do show hints of the atmosphere being worked over, with holes in the 2 inch PW air, where values are progged as low as 1.5. With a flat pressure gradient per disorganized low pressure along and south of the cold front, mesoscale processes will be dominant. All things considered, the Sunday forecast is complicated, but likely to be active, especially with forcing being provided by the front, and a digging upper level short wave trough approaching from the northwest. This is reflected in SPCs slight risk for damaging wind, and perhaps even large hail, given thick CAPE through the hail growth level and the modest deep layer bulk shear, although the discussion suggests that the forecast area may be between complexes to the southeast and northwest. The marginal risk for excessive rainfall reflects the variable PW distribution, and the weak low level inflow, but still copious amounts of moisture in place. Flood watch issuance would likely follow a shorter summer fuse similar to convective watches, as the convective complexes evolve and track. With loss of heating, and the ring of fire shifting south as the upper level trough digs in, convection should gradually wane Sunday night. The upper level short wave trough pushes a second cold front through on Monday into Monday night, but moisture will become a bit more limited by then, and storm coverage should be limited and the storms on the move. This should limit any further water concerns, but daytime heating driving CAPE values to perhaps 3 kj/kg in said narrow band could still lead to strong storms in modest deep layer bulk shear. Canadian high pressure brings cooler and less humid air in for Tuesday and Tuesday night. Convection spells high bust potential on highs Sunday or Monday, but lows and dew points will be typical of the summer muggies. Central guidance Monday night through Tuesday night reflect values returning to near normal in the fresher canadian air. && .LONG TERM /WEDNESDAY THROUGH FRIDAY/... As of 420 AM Saturday... High pressure exits Wednesday, and afternoon thunderstorms are possible in the mountains on return southerly flow. A cold front approaches Thursday, and moves through Thursday night, with a round or two of showers and thunderstorms likely. Model divergence suggests it may take until sometime Friday to get the front, and its associated precipitation, entirely south of the area. A large canadian high pressure system will bring fresh, cooler and less humid air by next weekend, as the pattern amplifies with a deepening trough in the eastern U.S., and a building ridge in the west. Central guidance reflects warm weather Wednesday through Thursday, followed by near, or even slightly below normal temperatures in the drier air for next weekend. && .AVIATION /16Z SATURDAY THROUGH WEDNESDAY/... As of 600 AM Saturday... Big change with the 22/09z TAF update was to improve visibility at most terminals based on observational trends. More isolated convection has developed over the northern 1/2 of the airspace with KPKB most likely to be briefly impacted. The other significant change with the intermediate update was to adjust timing of thunderstorms expected to spread east-southeast from OH and re-intensify later this morning into the early afternoon. The northern 1/2 of the airspace is the area most likely to experience thunderstorm impacts between 14-18z and added 2hr IFR vis TEMPO groups to KPKB, KCKB and KEKN. Convection may be a bit more isolated over the southern airspace and will probably stick with VCTS at KCRW, KHTS and KBKW. Not quite sure how to play the later periods in the wake of the early-day storms with thunderstorm risk lasting through tonight. There may also be some areas of fog but later issuances can address this better going forward. FORECAST CONFIDENCE AND ALTERNATE SCENARIOS THROUGH 18Z SUNDAY... FORECAST CONFIDENCE: Medium. ALTERNATE SCENARIOS: Limited impacts from upstream thunderstorms early today and no fog formation tonight into Sunday morning. EXPERIMENTAL TABLE OF FLIGHT CATEGORY OBJECTIVELY SHOWS CONSISTENCY OF WFO FORECAST TO AVAILABLE MODEL INFORMATION: H = HIGH: TAF CONSISTENT WITH ALL MODELS OR ALL BUT ONE MODEL. M = MEDIUM: TAF HAS VARYING LEVEL OF CONSISTENCY WITH MODELS. L = LOW: TAF INCONSISTENT WITH ALL MODELS OR ALL BUT ONE MODEL. UTC 1HRLY 15 16 17 18 19 20 21 22 23 00 01 02 EDT 1HRLY 11 12 13 14 15 16 17 18 19 20 21 22 CRW CONSISTENCY H H H H H H H H H H H M HTS CONSISTENCY H H H H H H H H H H H M BKW CONSISTENCY H H H H H H H H H H H M EKN CONSISTENCY H H H H H H H H H H H M PKB CONSISTENCY M H H H H H H H H H H M CKB CONSISTENCY M M H H H H H H H H H M AFTER 18Z SUNDAY... IFR possible Saturday night and Sunday in heavy showers and thunderstorms. && .RLX WATCHES/WARNINGS/ADVISORIES... WV...Flash Flood Watch through Sunday morning for WVZ007>011- 014>020-027>032-039-040-517>526. OH...Flash Flood Watch through Sunday morning for OHZ066-067-075- 076-083>086. KY...None. VA...None. && $$ SYNOPSIS...TRM/99 NEAR TERM...SL/99 SHORT TERM...TRM LONG TERM...TRM AVIATION...99 is the U.S. government's official web portal to all federal, state and local government web resources and services.