Area Forecast Discussion
Issued by NWS Pittsburgh, PA
Issued by NWS Pittsburgh, PA
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270 FXUS61 KPBZ 292350 AAE AFDPBZ Area Forecast Discussion...UPDATED National Weather Service Pittsburgh PA 750 PM EDT Sat Jun 29 2024 .SYNOPSIS... A lull in showers and storms is expected this evening before renewed showers and storms develop late tonight. A cold front moves across the region Sunday morning, with high pressure building in its wake to start the week. && .NEAR TERM /THROUGH SUNDAY/... 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. ------------------------------------------------------------------- PoPs have been updated to reflect latest expectations. Storms across nrn WV will continue for an hour or two as they translate ewd and across the mountains. Blowoff from storms near KILN also may eventually graze Muskingum County in OH. An upswing in storm coverage in the far srn zones (nrn WV to near the Mason-Dixon Line is increasingly probably late tonight/pre-dawn tomorrow, and may expand in coverage with approach and passage of the cold front Sun morning. Previous discussion follows... Despite modest CAPE recovery within filtered sunshine and increasing dewpoints, a lack of focusing mechanism for strong updrafts appears to be lacking as ascent spreads across the region evidenced by an increase in showers south and west of Pittsburgh. These showers are insufficiently deep to pose a flash flood threat, so the Flash Flood Watch has been cancelled, and the tornado threat is waning, so the Tornado Watch will be trimmed further. Previous discussion follows... 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 NIGHT THROUGH MONDAY NIGHT/... KEY MESSAGES: - Cold front moves through Sunday morning. - A few storms may be strong along it south and east of Pittsburgh. - Drier air builds in with cooler temperatures. ------------------------------------------------------------------- The cold front itself then moves through Sunday morning and carries an additional lower confidence severe threat. Unfavorable mid to late morning timing with at least some degree of cloud cover around as even the lower end of the latest ensemble guidance indicates 40- 50% sky coverage through the morning hours lends less of a severe threat, though that said, can`t rule out some stronger storms capable of damaging wind southeast of Pittsburgh where a Marginal Risk (1/5) remains. We then finally get a push of drier air behind the front on Sunday afternoon and evening as high pressure builds across the Great Lakes and reinforces northerly flow in the wake of the boundary. Still a bit of discrepancy in the 12z guidance on the timing of the front; highs will be dependent on said timing as a quicker boundary could keep PIT in the upper 70s while a slower push could result in mid 80s. Either way, clouds will scatter into Sunday evening, though a passing upper wave looks to reinforce cloud coverage toward morning on Monday. Still, overnight lows will be refreshing dipping into the 50s for most, though some 40s are possible (50-70% chance) in the ridges and north of I-80. Dry weather continues with high confidence through Monday night as heights begin to rise and high pressure moves further east. Temperatures remain below average on Monday with ensemble probability <30% of highs topping 80 degrees. && .LONG TERM /TUESDAY THROUGH SATURDAY/... KEY MESSAGES: - Dry weather continues Tuesday with increasing temperatures. - Showers and thunderstorms return Wednesday and Thursday with another low pressure system. ------------------------------------------------------------------- Good agreement amongst the various ensemble clusters supports dry weather on Tuesday as ECONUS ridging builds. Temperatures begin to rebound as flow becomes more south of east and pushes highs back toward or a couple degrees above normal. By mid-week, the pattern becomes more unsettled as ensembles break the ridge down and low pressure moves by to our north. Deep southwest flow on Wednesday surges temperatures back to 5-10 degrees above normal, though timing of a cold front passage may throw a wrinkle in that. Ensembles show increasing deviation from the mean come Wednesday night through the remainder of the week, so confidence is lower, but a cold front looks to pass through sometime late Wednesday into Thursday returning shower and thunderstorm chances. Machine learning becomes more interested in severe chances by mid-week as the front pushes into a warm, moist airmass, so this will bare watching through the coming days. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/... Scattered showers, with isolated thunderstorms south of PIT, will continue this evening as a shortwave trough crosses the Upper Ohio Valley region. The threat for thunderstorms elsewhere continues to diminish, with weak forcing for ascent. VFR is expected for much of the evening outside of any thunderstorms. Expect MVFR/local IFR cigs late tonight with low level moisture in place. In addition, scattered showers and possible thunderstorms are expected late tonight/early Sunday morning as a cold front crosses the region. MVFR will continue until mixing and dry advection lift cigs back to VFR by Sunday afternoon. Wind gusts should also increase after FROPA, with gusts to 20kt Sunday afternoon. .Outlook... Outside of early morning river valley fog, VFR is expected until a slow moving front sets up across the region Wednesday and Thursday. && .PBZ WATCHES/WARNINGS/ADVISORIES... PA...None. OH...None. WV...None. && $$ SYNOPSIS...MLB NEAR TERM...Cermak/Kramar/Hefferan/MLB SHORT TERM...MLB LONG TERM...MLB AVIATION...WM