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Area Forecast Discussion
Issued by NWS Pittsburgh, PA
Issued by NWS Pittsburgh, PA
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185 FXUS61 KPBZ 161137 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 737 AM EDT Fri Aug 16 2024 .SYNOPSIS... An upper level disturbance will cross the region this afternoon increasing the risk for showers and storms. A cold front will cross the region late Saturday continuing the risk for showers and storms. The weather pattern will remain unsettled into early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... KEY MESSAGES: - Two shortwave troughs will cross the area today. Chance for showers and storms, with the most likely time this afternoon. - Temperatures remain near normal. ------------------------------------------------------------------- A couple of shortwaves, moving eastward on the southeastern flank of the large western Great Lakes 500mb low, will cross the region today. The first will be this morning and the second late this afternoon and evening. The first wave is weak and will be banging into a upper level ridge currently located over the region. The first wave will not be able to do much to the ridge as the second wave will dig into western Ohio this morning and cause additional height rises over the area. Any rainfall with the morning wave has been light and on the order of a tenth or two of an inch so far with plenty of dry air noted below 750 mb on the 12z PIT sounding. As the low levels saturate some more into this morning, some scattered showers will continue, but the main theme will be cloudy skies. The second, and stronger wave, will approach the western reaches of the forecast area early this afternoon and will reach the eastern edge as sunset approaches. This wave will be able to eject the ridge eastward and cause minor height falls as it crosses. The first wave will likely work to eliminate the drier air in the lower levels, so the second wave will have a more saturated atmosphere to work with. At least scattered showers and storms look like a good bet this afternoon, but the severe threat remains murky. Instability will be lacking with the HREF showing CAPE values of less than 1000 j/kg over the entire area, and less than 500 j/kg outside of Ohio. Shear will also be weak with 20 to 25 kts possible this afternoon. Additionally, we should have plenty of cloud cover today. Prob thunder on the NBM is generally 30 to 40% with the highest late this afternoon and evening. The latest CAMs do show a line of convection developing just ahead of the shortwave late this afternoon, but with a lack of shear, it struggles to remain organized as it heads eastward. The current marginal risk looks reasonable as an isolated strong to severe storm is not out of the question. PWATS will rapidly increase today, so any stronger storms could produce heavy downpours. High temperatures will be a bit tricky today with the threat of morning showers and ample cloud cover. Will go below NBM guidance today, which will put highs very close to seasonal averages. && .SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY/... KEY MESSAGES: - Slow moving upper low to bring unsettled weather through the weekend. - Strong to severe storms possible Saturday afternoon and evening with the passage of a cold front. ------------------------------------------------------------------- The Great Lakes upper low will slowly drift eastward on Saturday. Another short wave trough will cross the region late in the day and drag a surface cold front with it. Instability will be stronger on Saturday, but the shear remains marginal. The surface boundary will provide a better forcing mechanism, but we will still have to deal with ample cloud cover. The severe threat looks better on Saturday, but this is nowhere near a slam dunk. Models are hinting at some drier air moving in ahead of the front which could allow for breaks in the cloud cover, increasing the severe threat. The upper low moves little on Sunday, across southwest Ontario with upper trough in Ohio, so another day of diurnally driven showers and thunderstorms is expected. Shear and instability look to be weaker on Sunday which would decrease the severe threat. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... KEY MESSAGES: - Ensembles favor Great Lakes troughing into the middle of next week but with a drying trend. ------------------------------------------------------------------- Ensembles indicate that long wave trough will continue to linger in the Great Lakes and Northeast States Monday, Tuesday, and Wednesday. There are subtle indications that the trough starts to lift out a bit on Thursday. The standard deviation of 500 mb heights in this area gradually increase with time Tuesday and Wednesday as the details of short wave troughs become less and less evident. While diurnally driven showers are still a good bet under the upper trough on Monday, confidence has increased that a large Canadian high, and push of drier air, will move in for the Tuesday to Thursday time frame with sunshine and seasonable temperatures. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/... Mid and high clouds will stream across the area this morning as a warm front lifts through with lowering, though still VFR, CIGs in the 5-9kft range. Plenty of dry air is noted below 750 mb on the 12z PIT sounding, so only some light showers are expected early this morning. As the low levels saturate further, some restrictions to MVFR VIS are possible late morning in any moderate showers. Meager instability will build across the area by Friday afternoon limited by high confidence thick cloud coverage across most of the area save eastern OH where some breaks are possible headed into this afternoon. An initial shortwave trough, rotating around an upper low across the Midwest, 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 is decreasing given the limited instability and lack of surface forcing mechanism. Continued to carry only VCTS in the TAFs for the most likely time of occurrence (after 18z for ZZV, 20z for PIT, and 22z for LBE). Instability will diminish Friday 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 Saturday, 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...22 NEAR TERM...22/MLB SHORT TERM...22 LONG TERM...Craven AVIATION...WM/Rackley/MLB