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Area Forecast Discussion
Issued by NWS Pittsburgh, PA
Issued by NWS Pittsburgh, PA
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298 FXUS61 KPBZ 291558 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 1158 AM EDT Sat Jun 29 2024 .SYNOPSIS... The probability of severe weather increases after 12pm today. Torrential rain, tornadoes and damaging wind all will be possible. && .NEAR TERM /THROUGH TONIGHT/... KEY MESSAGES: - Favorable environment for severe weather today. Damaging wind and tornadoes will be the main threat. - Flash flooding will also be possible with heavy rainfall mainly from Pittsburgh metro and north. ------------------------------------------------------------------- Low pressure currently moving across the upper Great Lakes has resulted in broad and deep warm, very moist southerly flow throughout the Ohio Valley and lower Great Lakes regions. A shield of showers and thunderstorms to our west out ahead of a cold front and with an attendant shortwave will arrive in our neck of the woods after noontime today. Our severe weather threat will likely be tied to this feature later this afternoon and evening. The 12z PIT sounding supports the expected threats of damaging wind, a few tornadoes, and heavy rainfall leading to flash flooding. Notable elements on the sounding in regard to the flash flood threat include a precipitable water value of 1.98" (above the daily climatological max), warm cloud depths to around 14kft, and, above the lowest 3 km or so, unidirectional west-southwest flow. Corfidi propagation vectors of 10 knots or less across the area this morning suggest slow moving and backbuilding potential with the convection today. This, coupled with high PWATs even pushing 2.2" north of I-80, signals a flash flood concern for roughly Pittsburgh and north especially with any training showers and thunderstorms. The latest hi res ensemble data supports a 60% chance of 1 hour rainfall rates >0.5". Localized areas of 1-2" of rain aren`t out of the question. For the severe threat, despite morning cloud cover and showers, temperatures are already in the mid 70s/low 80s, so destabilization isn`t expected to be all that limited. Latest ensemble joint probability is 60-90% for >1000 J/kg SBCAPE and >30 kt of shear. Similar to the event this past week, the bulk of the shear is packed into the lowest levels in the 25-35 knot range which could support a tornado threat. Latest convective models continue to suggest some organization into one or multiple line segments that push across the area through the afternoon and evening. The environment ahead of these storms will support thunderstorm intensification to severe levels, with damaging straight line wind and a few tornadoes being the primary threat. The greatest tornado threat will reside generally north of the I-70 corridor where 850mb and 700mb flow will be stronger resulting in more bulk shear in the sfc-3km layer and elongation of the low level hodographs. If any discrete cells can fire ahead of the more organized segments, deviation to a right moving motion vector of east/northeast will be favorable for streamwise vorticity ingestation and increased tornado potential. In addition, the bulk of the shear packed into the lowest levels could also pose a QLCS tornado threat with any bowing segments. Farther south, weaker winds and a more unidirectional wind profile in the low levels appear to reduce the tornado threat somewhat, though steep low level lapse rates will still promote strong low level updrafts in thunderstorms which can overcome the more marginal profiles as we`ve seen so far this spring and summer, so the tornado threat certainly is not zero in those areas. The Storm Prediction Center has maintained a Slight Risk (Level 2 out of 5) for severe thunderstorms across the entire area. In terms of timing, the primary wave of thunderstorms carrying the greatest severe threat pushes through between roughly 3-9pm, though there is some indication of weak backbuilding occuring towards the latter half of that window with storms lingering in areas south and east of Pittsburgh until closer to midnight. Interestingly, some 12z CAMs have backed off some on the threat today, though not entirely buying this as those that do are struggling with resolution of the upstream MCS and painting a much drier boundary layer with dew points mixing out this afternoon which seems very low probability given the moist southwest flow and morning rain. Additionally, there may be redevelopment overnight into early Sunday morning along the cold front itself as it pushes through the area. Hi-res guidance suggests a low-end severe threat carries into this overnight wave, though limiting factors will be a stabilizing surface layer and available instability. && .SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/... KEY MESSAGES: - A cold front sweeps through the area Sunday morning/afternoon. - A few storms along the front could become strong, primarily east and south of the Pittsburgh metro. Damaging straight line winds will be the primary threat. - Cooler and drier weather expected in the wake of the front Sunday night as high pressure builds into the region. ------------------------------------------------------------------- A cold front enters the area from the northwest around or shortly after sunrise on Sunday. Showers and thunderstorms could be ongoing along the front itself as it pushes through, leading to lingering rain chances across much of the area through the morning hours. The front is not expected to exit to the southeast until early to mid afternoon, which may allow enough destabilization to occur ahead of it to support a few strong thunderstorms that will carry a low-end damaging wind threat, primarily east and south of the Pittsburgh metro. As such, the Storm Prediction Center has highlighted the higher terrain of southwest PA and much of northern WV in a Marginal Risk (Level 1 out of 5) for severe weather. Drier air moves into the area in the wake of the front Sunday evening and Sunday night, as well as cooler temperatures with overnight lows dipping into the 50s areawide by early Monday morning. && .LONG TERM /MONDAY 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. ------------------------------------------------------------------- Dry weather persists through Tuesday night under high pressure. The upper ridge then pushes off to the east and low pressure moves back into the Great Lakes region, bringing showers and thunderstorms back to the area Wednesday through the end of the week. Temperatures will be below seasonal levels Monday, followed by a warm up back to near or slightly above levels Tuesday through the end of the week. && .AVIATION /16Z SATURDAY THROUGH WEDNESDAY/... Much of the forecast remains on track for the early morning period with VFR expected with increasing mid-level cloud coverage as a system approaches from the west, ushering in warm/moist advection. VFR will prevail with moderate to high probability for all airports south of FKL/DUJ for the TAF period well into the warm sector of the passing low. At DUJ/FKL, deeper saturation maintains chances of MVFR for most of the day. Beyond daybreak, shower chances slowly increase from northwest to southeast as pre-frontal convergence is strewn across the area. With increased daytime instability, shower chances transition to storm chances in the afternoon/evening for all ports. Strong to severe thunderstorms are anticipated as the front crosses the area, mainly after 18Z. The first ports to be affect will likely be ZZV, BVI, and FKL. Storms are most likely to finish at MGW around 02Z. The most likely timing for Pittsburgh area ports is between 20Z and 22Z. Low clouds and patchy fog are possible tomorrow night with cold advection and radiative cooling forcing saturation, given recent rainfall. IFR restrictions remain possible, most likely for eastern ports. .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 through late tonight for PAZ007>009-013>016- 020>022-029-073-074-077-078. OH...Flood Watch through late tonight for OHZ039>041-048>050. WV...Flood Watch through late tonight for WVZ001-002. && $$ SYNOPSIS...Hefferan NEAR TERM...Cermak/Hefferan/MLB SHORT TERM...Cermak LONG TERM...Cermak AVIATION...Milcarek