Area Forecast Discussion
Issued by NWS Pittsburgh, PA

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391 FXUS61 KPBZ 251725 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 125 PM EDT Sat May 25 2024 .SYNOPSIS... Dry weather is expected through the morning. The chance for showers and strong to severe thunderstorms will increase this afternoon and evening. Sunday is expected to be mostly dry under high pressure, but thunderstorms will returning Sunday night into Memorial Day. As we flip the calendars to June, temperatures fall back to just below normal. && .NEAR TERM /THROUGH TONIGHT/... KEY MESSAGES: - Dry through most of the morning hours - Increasing chances for strong to severe storms this afternoon/evening with a crossing shortwave - Damaging wind, hail, and excessive rainfall will be the main threats ------------------------------------------------------------------- A brief period of weak ridging that will keep the area dry through the morning hours will break down and shunt to the east. A nearly stalled boundary that has been situated across the area is now lifting north as a warm front and southerly flow has already advected low to mid 60s dew points into the area with temperatures forecast to rise to the low to mid 80s by this afternoon under continued warm, moist advection. The 12z PIT sounding doesn`t exhibit much of an impressive wind field currently, but the upstream ILN sounding and current analysis shows that low to mid level flow will increase later this afternoon in tandem with a passing shortwave and just ahead of an approaching cold front. These will be the initiating mechanisms for today by early afternoon (most likely after 1-2pm, but possibly as early as noon) based on latest guidance. There`s also a hint of some redevelopment along the boundary itself later tonight, but confidence is low with disagreement amongst the latest CAMs on the coverage. Hi res ensemble probability indicates that, initially, the best deep shear will be north and west of Pittsburgh (80% chance of >30 kt) coincident with better proximity to the passing wave, but the best instability will be displaced further south (60% chance of >2000 J/kg) in the area with more residence time in the mostly clear open warm sector. Nonetheless, the shear increases further south and the best overlap of parameters looks to be just north and west of Pittsburgh by late afternoon with a 50-70% chance of both CAPE >1000 J/kg and deep layer shear >30 kt. Primary threats will be downburst wind potential and low end severe hail; thetaE minimum (updraft core height for potential downburst concerns) is around 10kft but the amount of DCAPE available may be a limiting factor. Will also be looking for solid cores to reach above 27kft for severe hail concerns. Weak low level shear should preclude a tornado threat. SPC has kept the region in a Marginal Risk (1/5) for severe weather. Will also have to monitor a low end flood threat as well as we are in a Marginal Risk (1/5) for excessive rainfall, especially for areas that got hit hard yesterday (northern WV). Despite PWAT values topping 1.2", storm motion should help alleviate flooding concerns, and hi res ensemble probability for >0.5"/hr rates peak on the low end north of Pittsburgh. Training of storms would likely be see flash flood threats. Lastly, areas that receive rain will likely see fog overnight with ensemble probability of >70% to drop to less than 5 mile visibility. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... KEY MESSAGES: - High pressure returns dry, warm weather for Sunday - Another round of potentially strong to severe thunderstorms Sunday night into early Monday morning, then Monday afternoon ------------------------------------------------------------------- Flat upper ridging builds for Sunday as surface high pressure arrives in the wake of the departing cold front. This will be the best day of the holiday weekend with little noticeable difference in the airmass behind the front as NBM probability for >85F is 40-60%. A warm front approaches late in the day on Sunday as low pressure ejects out of the Plains. Increasing moisture in developing southwest flow aloft will overspread mid/upper level clouds by evening. Latest guidance has continued the trend noted in the last update with slightly slower progression and onset not until late evening/overnight. While this timing is not typically favorable for severe weather in our area, a strengthening 30-40kt low level jet across the area late Sunday night may help maintain several stronger storms with damaging wind and/or large hail into eastern Ohio. That said, a general weakening trend is expected as these storms cross into our region, and thus just the far western half of the area remains outlined in a Marginal Risk (1/5). Ensembles indicate only slight deviations from the mean in regard to the depth of an upper trough and additional shortwave energy traversing an upper low spinning across the Great Lakes through Monday. Aided by daytime heating and slow passage of a cold front, yet another round of strong to severe thunderstorms is possible with our area again outlined in a Marginal Risk. Machine learning comes a bit more aggressive with an extension of severe probabilities back into our area from the east, but one potential limiting factor may be morning cloud cover as NBM spreads pin even 60-70% coverage on the low end of the distribution. Highs Monday will be thus be cooler than those over the weekend with mid 70s most likely. && .LONG TERM /TUESDAY THROUGH SATURDAY/... KEY MESSAGES: - Periodic rain chances continue through midweek. - Dry weather favored to close out the week. - Temperatures fall back toward seasonal average. ------------------------------------------------------------------- Upper level troughing plagues the area into mid week and rain chances continue into next week, although coverage should be scattered and mostly diurnally driven aside from a secondary cold front crossing the region Tuesday. By Wednesday, ensemble disagreement increases with the axis and strength of the upper trough, but any flavor of solutions doesn`t kick it out of the eastern CONUS until toward next weekend. Broad surface high pressure should finally turn the area dry by late week while temperatures fall back towards seasonal average with lows in the 40s not out of the question come mid to late week as the cooler airmass settles in. && .AVIATION /17Z SATURDAY THROUGH WEDNESDAY/...
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High confidence forecast (95%) save for the afternoon and early morning window around daybreak. Confidence falls back into the 40-50% range during the afternoon and evening given uncertainty with thunderstorm placement. The pre dawn fog from 9Z-12Z could produce IFR vis for any airport that sees a thunderstorm during the day. A shortwave trough passes this afternoon. This will kick off showers and thunderstorms. Brief IFR vis are possible with heavier downpours, but it won`t last long /less than 30 minutes/, while CIGS could drop into the MVFR category. There will be a break in the activity this afternoon as we are in a lull behind the shortwave and waiting for the forcing associated with the front to approach later this afternoon - early evening. VFR quickly returns after 2Z area wide then a wait and see where fog develops. NBM probs of less than 1SM are below 25% so not super gung ho on IFR weather. Any IFR weather quickly burns off by 12-13Z to VFR the balance of the forecast. .Outlook... Thunderstorm probabilities have decreased overnight Sunday into Memorial Day as another low pressure system moves through the Great Lakes. A pattern shift thereafter will introduce periodic precipitation chances and potential for more prolonged cig restrictions.
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&& .PBZ WATCHES/WARNINGS/ADVISORIES... PA...None. OH...None. WV...None. && $$ SYNOPSIS...Rackley/MLB NEAR TERM...MLB SHORT TERM...MLB LONG TERM...Rackley/MLB AVIATION...Shallenberger/McMullen