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Area Forecast Discussion
Issued by NWS Pittsburgh, PA
Issued by NWS Pittsburgh, PA
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998 FXUS61 KPBZ 162225 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 625 PM EDT Fri Aug 16 2024 .SYNOPSIS... A slow moving upper level system will promote rounds of showers and thunderstorms through Monday, with convection peaking Sunday afternoon. There is a low probability for severe weather Saturday afternoon and evening. && .NEAR TERM /THROUGH SATURDAY/... KEY MESSAGES: - Trends suggest a lowering threat for severe weather this afternoon. - Showers and thunderstorms will remain fairly scattered while tapering off into Saturday morning. ------------------------------------------------------------------- The evening update feature adjusting pops and lower thunder probs as lack of instability this afternoon and quickly waning daytime heating is keeping convection for getting deeper. In fact, showers are having a difficult time breaking the cap. Have lowered pops and thunder chances through 00Z. Previous discussion...Shortwave movement southeast of a well developed upper level low over NW Wisconsin continues to promote areas of showers and thunderstorms primarily along and south of I-70 early this afternoon. Splitting flow aloft is aiding to the development of a more northern stream area of lift (well N and W of region) and a southern stream area of lift (favoring locations south of I-70). The expectation is for the southern stream wave to move ENE through this evening and offering some NE expansion of convective activity ahead of the mid-level 700mb warm front. Little to no instability within this environment means damaging wind and high rainfall rates are not expected. Hi-res CAMS suggest potential for new convective development along a weak surface trough near central OH late this afternoon, and moving said convection east. Confidence is lowering in this occurrence, however, due to excessive cloud cover in the area continuing to limit destabilization (with waning upper support and lack of notable cooling aloft). As a result, the probability of isolated damaging wind threats developing appears to be lowering at this time. If greater destabilization does occur (resulting in SBCAPE values around 1000 J/kg), then the highlighted marginal severe threat by SPC is closer to reality. Subsidence behind the shortwave movement and loss of diurnal heating will lead to a downward trend of precipitation chances after 03z (11pm) tonight. Temperature will be mild with varying cloud decks, averaging 5 to 10 degrees above the daily average. && .SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/... KEY MESSAGES: - Low probability for strong to severe thunderstorms exist Saturday, favoring a wind and isolated flash flood threat. - Widespread convection is favored Sunday, but poses a more limited hazard risk. ------------------------------------------------------------------- The organized upper level low will creep ESE towards the eastern Great Lakes Saturday into Sunday, with varying shortwave movement through its base crossing the Upper Ohio River Valley region. This will promote periods of showers and thunderstorms favoring the afternoon and evening hours and tapering off with the loss of diurnal heating. For Saturday, there may be lingering light rain showers east of Pittsburgh from prior shortwave movement during the morning. Otherwise, generally dry conditions will exist until the afternoon and evening with the approach of a pre-frontal trough and the likely thunderstorm development along and ahead of this boundary (in the form a broken convective line). There is a marginal risk for damaging wind with these thunderstorms due to potential SBCAPE reaching 2000-2500 J/kg and notable mid-level dry air overcoming weaker lapse rates and more limited deep layer shear. Confidence in the overall threat coverage is only on the medium side as probabilistic guidance suggests the potential for lingering cloud cover to reduce the degree of instability while area forcing proves too weak for greater storm organization amid the likely weak shear/lapse rates. Progression of the trough eastward is expected through 06z, with dry air and loss of heating contributing to a downward trend in precipitation chances. For Sunday, influence from the nearly overhead cold core trough center will lower convective temps and allow for more widespread thunderstorm development after 15z. There is some uncertainty in favored locations due to slight variations in trough positioning that may create subsidence holes, but the overall pattern/environment should support on and off storms through the evening. The lack of strong instability despite 90th percentile PWATS and some forward motion of storms should preclude a flood threat. && .LONG TERM /MONDAY THROUGH FRIDAY/... KEY MESSAGES: - Residual shortwave movement favor lower probability showers Monday along with abnormally cool temperature. - Long range trends favor dry weather and near normal temperature. ------------------------------------------------------------------- Ensembles indicate fairly minor variations in trough progression Monday into Tuesday, lending to the likelihood of the trough axis shifting east of the region and NW flow remaining in place. Lingering, diurnally driven showers will taper off Monday evening while area temperature ends well below the daily average. Tuesday will then be dry with the residual cold air advection leading to temperature around 10 degrees below the daily average. Long range analysis favors a continuation of the fairly amplified pattern of ridging over the Rockies and troughing along the east coast. This should position the region within the drier NW flow pattern where precipitation chances will be heavily reliant on strong shortwave movement over the ridge. Latest trends don`t favor this and thus suggest a prolonged period of dry weather and seasonable temperature Wednesday through the end of the week. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... Meager instability will build across the region this afternoon, limited by cloud coverage across most of the area save eastern OH where some breaks are more likely. A shortwave will cross the Upper Ohio Valley region this afternoon and early evening. This should result in scattered showers and thunderstorms developing, but confidence in coverage low given limited instability and lack of surface forcing mechanism. Continued to carry only VCTS in the TAFs for the most likely time of occurrence. Instability will diminish this evening with a few showers possible until the shortwave completes its passage. Ceilings will then lower overnight with lingering low level moisture resulting in restrictions to MVFR to IFR, primarily after 06z , with probability for MVFR at 60-90% and IFR around 50-80%. Even at any sites that do receive rain today, fog chances are lower with wind expected to remain elevated overnight. .Outlook... Periodic restrictions and showers and thunderstorms are possible through early Monday as low pressure slowly crosses the region. The greatest coverage of showers/thunderstorms will be in the afternoon and early evening. VFR returns by Tuesday under high pressure. && .PBZ WATCHES/WARNINGS/ADVISORIES... PA...None. OH...None. WV...None. && $$ SYNOPSIS...Frazier NEAR TERM...Frazier/Shallenberger SHORT TERM...Frazier LONG TERM...Frazier AVIATION...MLB/88