Area Forecast Discussion
Issued by NWS Pittsburgh, PA
Issued by NWS Pittsburgh, PA
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314 FXUS61 KPBZ 290137 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 937 PM EDT Fri Jun 28 2024 .SYNOPSIS... Quiet weather can be expected this evening under high pressure. A warm front brings showers and thunderstorms to the region early Saturday. The probability of severe weather increases Saturday afternoon and evening ahead of a cold front. Torrential rain, tornadoes and damaging wind all will be possible. && .NEAR TERM /UNTIL MIDNIGHT TONIGHT/... KEY MESSAGES: - Dry weather and light wind expected through this evening ------------------------------------------------------------------ Cumulus field has broadened in response to upstream ascent spreading ewd across the upper Midwest, with cirrus from showers in sern MI and far-nrn OH spreading across the area north of Pittsburgh. These showers may graze the region north of I-80 this evening on their current trajectory, but confidence remains low in accumulating precipitation reaching the ground with abundant dry air to overcome between H8 and H5. Some modest updates to the sky and PoP/Wx grids have been made to reflect the near-term evolution. Previous discussion follows... Southerly flow has gradually allowed rich moisture to move back across the Ohio River with a decent cumulus field and dew point of 69F at KHTS this moisture is now poised to return rapidly later tonight. && .SHORT TERM /MIDNIGHT TONIGHT THROUGH SATURDAY NIGHT/... KEY MESSAGES: - Showers and thunderstorms expected Saturday and Saturday night as a cold front approaches the area. Some storms could become severe. - The primary threats will be damaging winds and possibly a few tornadoes. - Flash flooding will also be possible due to torrential downpours in the heavier thunderstorms, mainly from the Pittsburgh metro on north. ------------------------------------------------------------------- Southerly flow late tonight will prompt a warm, moist airmass to return to the region ahead of an approaching low pressure system. Overnight low temperatures will likely range in the upper-60s to low-70s; 10 degrees above the climatological average. The warm front lifting north and increasing flow just above the nocturnal inversion should quickly push 850 mb theta-e values into the 335-340K range after midnight and this will result in development of elevated showers and thunderstorms especially toward daybreak in the northern Ohio counties of our northwest forecast area. Saturday is shaping up to be another potential severe weather day across the region. Low pressure advancing eastward from the upper Great Lakes into southern Ontario will result in southerly flow developing across the Ohio Valley and lower Great Lakes tonight, bringing moisture and showers back into the area. Ongoing convection is anticipated Saturday morning, most likely over eastern Ohio and northwest PA where a weak warm frontal zone will reside along with deeper moisture. Despite the negative impacts this morning round may have on diurnal heating, convective-allowing models continue to suggest moderate instability developing by mid to late afternoon, with HREF probabilities for 1000+ J/kg SBCAPE exceeding 60% areawide. Meanwhile, deep-layer bulk shear profiles will be supportive of organized storm modes (sfc-6km around 40kts), including bowing segments and supercells. What catches my eye when interrogating model-derived soundings for Saturday afternoon is how they compare to Wednesday`s setup. There are striking similarities between the two, most notably the fact that instability and wind shear are packed in the lowest ~4km of the atmosphere, the former being driven by steep 850-700mb lapse rates and the latter by jets at those two respective levels overspreading the region ahead of the advancing cold front. These two factors combined could support strong low-level updrafts where your best streamwise vorticity will also reside, leading to low-level mesocyclones that can rapidly spin up and locally enhance tornado potential. We saw this manifest in the QLCS that moved through southwest PA on Wednesday, which produced 5 confirmed tornadoes. This will be something to watch closely as this system evolves. The Storm Prediction Center has highlighted this local enhanced tornado threat with a 5% tornado probability contour extending across portions of western PA and eastern OH (the rest of the region remains in a 2% probability area). The final threat to discuss for Saturday`s convection is flash flooding. The setup may be such that there is training, or at least multiple rounds, of thunderstorms in some areas (particularly Along the I-70 corridor on north). Eventually, the cold front will sweep through and storm motion will become more progressive before ending altogether behind the front. However, ahead of the front, corfidi vectors decrease to 5-10 knots in a moisture- rich environment characterized by PWATS exceeding 2 inches, nearly an inch above normal levels for this time of year. 850 mb theta-e values rise into 345-350K range and there is decent prolonged deep layer moisture transport into the area. These factors, along with abundant buoyancy and warm cloud depths >13kft supporting warm rain processes, are an indicator for backbuilding and training thunderstorms capable of producing torrential downpours. The Weather Prediction Center has highlighted much of the area in a Slight Risk (Level 2 of 5) for excessive rainfall leading to flooding. Issued the Flash Flood Watch for roughly northern half of forecast area, where widespread 1 to 2 inch rainfall is expected and locally 3-4 inches could fall. && .LONG TERM /SUNDAY THROUGH FRIDAY/... KEY MESSAGES: - Dry weather expected Sunday through mid week under building high pressure. - Showers and thunderstorms return Wednesday and Thursday with another low pressure system. ------------------------------------------------------------------- A cold front sweeps through the area early Sunday morning, bringing an end to showers and thunderstorms and ushering in a drier airmass. This will bring dry weather and a slight cool down Sunday and Monday with temperatures returning to normal or just below normal levels for this time of year. Dry weather persists through Wednesday under high pressure. The upper ridge then pushes off to the east Wednesday night and low pressure moves back into the Great Lakes region, bringing showers and thunderstorms back to the area Wednesday and Thursday. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... VFR will prevail through much of the overnight. Warm/moist advection in advance of an approaching disturbance will increase mid-level clouds overnight and eventually increase rain chances before 12z Saturday. Thunder is possible with morning showers, but probabilities remains low for now, so have opted to exclude TS mention. MVFR to IFR ceilings are expected north of PIT after around 14z until mid-afternoon. Some clearing is anticipated late morning/early afternoon before more robust convection is expected ahead of and along a crossing cold front. Strong to severe thunderstorms are possible generally after 18z, with a most likely timeframe in the 20-22z range at PIT. Brief clearing is expected Saturday evening by around 00z Sunday, though additional rain is expected along the cold front by early Sunday morning. .Outlook... MVFR conditions may continue north of PIT with lingering low-lvl moisture. Widespread VFR will likely return Sun afternoon under high pressure and is likely to persist Mon and Tue. && .PBZ WATCHES/WARNINGS/ADVISORIES... PA...Flood Watch from Saturday morning through late Saturday night for PAZ007>009-013>016-020>022-029-073-074-077-078. OH...Flood Watch from Saturday morning through late Saturday night for OHZ039>041-048>050. WV...Flood Watch from Saturday morning through late Saturday night for WVZ001-002. && $$ SYNOPSIS...Craven NEAR TERM...Craven/Kramar SHORT TERM...Craven LONG TERM...Craven AVIATION...Hefferan/Rackley