Area Forecast Discussion
Issued by NWS Pittsburgh, PA
Issued by NWS Pittsburgh, PA
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795 FXUS61 KPBZ 270119 AAB AFDPBZ Area Forecast Discussion...UPDATED National Weather Service Pittsburgh PA 919 PM EDT Wed Jun 26 2024 .SYNOPSIS... Thunderstorms capable of producing damaging wind and localized flooding are expected this afternoon and evening ahead of a cold front. Dry and seasonable weather will return Thursday, with rising temperature expected Friday. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... KEY MESSAGES: - Flash Flood threat is trending as highest probability focused north of Interstate 80. - A Severe Thunderstorm Watch remains in effect through 10pm EDT for northwest PA with damaging wind and hail as the mentioned concerns. - Probability of impact via either wind, hail and flooding lowering for locations outside of southwestern Pennsylvania ------------------------------------------------------------------- Active severe wx is waning as storms shift ewd/sewd out of the region, with strongest storms located in eastern Fayette and Preston Counties. Residual showers in the wake of the storms will wane quickly this evening, likely by 27/0200-0300 UTC. In the wake of storms, a moist, cool airmass remains. Light wind and temps approaching saturation will support rapid fog development tonight, potentially limited mainly by low stratus. Whether stratus or fog develops tonight, all should erode by mid-morning, with mainly upper-level clouds lingering thru the day Thu. Previous discussion follows... 315pm Update: The area stretching from northeast Ohio to northwest PA continues to be the focus for flash flood potential as analysis shows an overlapping maxima of 700-400mb convergence, surface convergence, rising PWAT values and slow storm motion. Rainfall rates of 1-2"/hr or higher remain possible, through the axis likely stays north of I-80 (and potentially even just north of northern forecast zones). While a Severe Thunderstorm Watch for northwest PA remains in effect til 10pm, the overall threat environment for wind and hail seems to be decreasing and forcing/lift sits north of the region and vort advection from the southwest continues to spread more stable air (highlighted by -100 CIN values via MLCAPE mesoanalysis). Latest hi-res modeling suggests the focus over the first 1-3 hours will be flash flooding along the frontogenetic banding of heavy rain thunderstorms in northwest PA, which should gradually progress east through that time. While showers and low probability thunderstorms remain possible south, the most likely occurrence is dry weather until the approach of the surface cold front. A quick surge of warm, moist air at the surface ahead of the boundary should help destabilize the environment for thunderstorm development; timing of entering eastern OH is 5-7pm EDT and reaching western PA between 6-8pm with storms exiting the entire region generally by 10pm EDT. There will be a narrow window then for potential damaging wind/hail and localized flash flooding for areas south of I-80 but the shortened window for instability maximization makes probability of occurrence low. Shower and thunderstorm activity is likely to weaken late tonight as surface instability wanes along with the passage of the cold front. Much of the severe threat should subside by 10pm EDT while lingering post-frontal showers and convective stratiform rain regions exit the higher terrain of WV by early Thursday morning. Dry air and residual mixing of a NW post- frontal wind should mitigate morning fog risk, but residual boundary layer moisture in river valleys (favoring locations SE of Pittsburgh) could have patchy fog development. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY NIGHT/... KEY MESSAGES: - Dry weather returns. - Temperature rises above normal again Friday. ------------------------------------------------------------------- Cool, dry northwest flow will develop Thursday as high pressure builds in from the northwest. Residual moisture and delay in subsidence is likely to allow for lingering stratocu Thursday morning that turns into scattered to occasionally broken afternoon cumulus. Cold advection and cloud cover will result in more seasonable afternoon temperature while post-frontal mixing may support NW wind gusts around 20 mph. The surface high will shift over New England by Friday as heights aloft rise ahead of the next upper level shortwave trough. Positioning of the high will support weak warm, moist advection amid mostly clear skies to raise area temperature about 5 degrees above the daily average. Broad ascent may increase mid to high level clouds and even low probability showers Friday night as weak perturbations ahead of the main upper level trough axis. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... KEY MESSAGES: - Shower and thunderstorm chances return Saturday into Sunday with the next low pressure system. - Low probabilities exist for a damaging wind and flooding threat with these thunderstorms. ------------------------------------------------------------------- Broad ascent within warm, moist southwesterly flow ahead of the main upper level trough axis will support increasing shower and thunderstorm probabilities starting early Saturday morning. Arrival of a more notable embedded shortwave and the surface cold front Saturday afternoon and evening is likely to help concentrate convective activity (currently favoring NW PA) and spreading it southeast through Sunday morning. Variation in storm timing and environment remains large due to differences in upper trough depth/timing as well as convective evolution prior to its (and cold fronts) arrival. Latest analysis suggests a low probability risk for damaging wind (mean values currently show 30-40kts of 0-6km shear) and localized flooding (tall-skinny CAPE values around 1500 J/kg with PWATs approaching daily maxima). Will mention threat in HWO and monitor evolving forecasts for this period. Dry weather and seasonable temperature is likely by late Sunday into Monday as the upper trough axis moves east and high pressure builds in from the western Great Lakes. && .AVIATION /00Z THURSDAY THROUGH MONDAY/... Storms continue along a line from near IDI to MGW, advancing ewd. In the wake of these storms, residual showers will last an hour or two before quickly eroding. The cool, moist airmass in the wake of storms is likely (70-80%) to support fog development overnight and possibly low cigs, with potential for LIFR vsby early Thu morning. Rapid improvement to VFR is expected by mid-morning after any morning restrictions. .Outlook... VFR is expected through Fri amid high pressure. Restrictions are possible Fri night with a crossing warm front. Restrictions are likely Sat and Sat night with showers and thunderstorms ahead of and along a crossing cold front. VFR returns Sun under high pressure. && .PBZ WATCHES/WARNINGS/ADVISORIES... PA...None. OH...None. WV...None. && $$ SYNOPSIS...Frazier NEAR TERM...Kramar/Frazier SHORT TERM...Frazier LONG TERM...Frazier AVIATION...Kramar/WM/Hefferan