Area Forecast Discussion
Issued by NWS Pittsburgh, PA
Issued by NWS Pittsburgh, PA
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993 FXUS61 KPBZ 250504 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 104 AM EDT Wed Sep 25 2024 .SYNOPSIS... Scattered showers and a few thunderstorms will move across the area today, ahead of a slow moving cold front. Unsettled weather continues into the weekend, although no day looks like a washout. && .NEAR TERM /THROUGH TODAY/... KEY MESSAGES: - Lull in the activity overnight. - Scattered showers and thunderstorms possible today. ------------------------------------------------------------------- Lowered PoPs overnight as the main moisture plume, and associated shortwave trough, has shifted eastward. Temperatures remain mild overnight. Area will be stuck between an upper low digging southward over the midwest and an amplifying ridge just off the east coast. As the 500mb low drifts southward, minor height rises are expected here today. This will actually direct another moisture plume on the eastern side of the upper low back over the region. Additionally, another wave of energy rotating around the 500mb low, will get caught up in the southwest flow aloft and move northeastward with the plume. This will mean another day with the threat for scattered showers and possibly a thunderstorm. Locating the areas with the highest risk for rain today will be predicated on where the moisture plume ends up and the track of the shortwave. Seeing some disagreement in the model solutions because of varying outcomes on the path of the large upper low to the west and the amount of amplification of the Atlantic ridge. Latest ensemble data if focusing higher QPF over the eastern half of the region, while the latest CAM models are pushing the main axis of QPF a bit further west. Will lean on the NBM which is providing a solution closer to that of the GEFS and EPS. Cloud cover, warm air advection aloft, and limited instability will keep the risk for convection lowered today. Temperatures today will again be above normal. && .SHORT TERM /TONIGHT THROUGH FRIDAY/... KEY MESSAGES: - Decreasing rain chances Thursday and Friday. - Still plenty of clouds and warm temperatures. ------------------------------------------------------------------- As we keep an eye on the southward drifting 500mb low that will sink from the midwest toward the lower Mississippi valley on Thursday, another large upper low will slowly move across southern Canada, just north of New York. The focus for the southern stream energy will be how much of the moisture from Helene will be absorbed into it, and the focus of the northern stream of energy will be how much will it deflect this moisture from the upper Ohio Valley. Models are actually in good agreement that these two systems will not phase which will work to limit rainfall potential through Friday. A surface cold front, associated with a surface low that will eventually be absorbed into the northern upper low, will cross the region on Thursday. The front will work like a barrier to the deep, tropical moisture streaming northward on the eastern side of the inland upper low. As the plume of moisture encounters the front and the northwest flow aloft on the southwest flank of the Canadian low, it will be suppressed and spread westward back toward the inland 500mb low. As this battle is ongoing, the remnants of Helene will move inland over the Gulf coast states and eventually the southeastern US and head northward. However, the remnant low will then begin to turn westward back toward the upper low keeping all of the tropical rainfall well south of the region. Additionally, as the remnants of Helene get absorbed into the inland upper low, a massive dry slot will develop over a large portion of the eastern CONUS and drive northward over our neck of the woods sometime Friday. As always with such a complex setup, which is made even more so by the intrusion of a tropical system, there remains uncertainty in the details. The pattern on Thursday, area resting between the two large upper level systems and the surface cold front drifting south of the region, would suggest that much of the day should be dry with lower chances probabilities for rainfall over the far south which will be closer to the stalling cold front. The surface front will likely be shoved northward on Friday as the northern 500mb low moves eastward over New England and the remnants of Helene move over the Tennessee Valley. This could lead to showers developing along the front and drifting northward. However, the large dry slot will also be moving northward over the east on Friday and drier air to our north, advecting southward on the back side of the exiting northern low, will work to decrease overall moisture associated with the cold front. High levels of uncertainty for Friday with how far north any potential showers will push and how long they will the be sustained as drier air becomes more prevalent. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... KEY MESSAGES: - Rain chances will continue through the weekend, but no day looks like a washout. ------------------------------------------------------------------- Large upper low to the southwest, which by the weekend will have completely absorbed the remnants of Helene, will remain basically stationary and weaken as it gets squeezed between a developing ridge over the Gulf of Mexico and another ridge over the Great Lakes region. The area will likely remain under the influence of the filling upper low, but this no sure bet. It will keep our area unsettled through the weekend with the risk for showers both Saturday and Sunday. By Monday and Tuesday, the remnants of the upper low will open up into more of a weak, but broad trough, that will eventually deepen as energy dropping southeastward through western Great Lakes phases with it. This would likely mean a continuation of the unsettled pattern and cooler temperatures. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/... Deteriorating conditions are expected early this morning with areawide MVFR/IFR cigs expected by 12z. Light showers are expected to become more widespread near dawn, with iso/sct thunderstorm chances increasing in the afternoon. Cigs will begin to improve after 18z, with an 80% chance of VFR at all terminals except FKL/DUJ. Afternoon rain will be sporadic, favoring locations northwest of PIT. Cigs will once again lower Wednesday night, with fog development also possible in areas that receive heavy rainfall. .Outlook... Widespread restrictions will linger into Thursday before VFR is expected to end the week. Periodic rain chance is then expected through the weekend. && .PBZ WATCHES/WARNINGS/ADVISORIES... PA...None. OH...None. WV...None. && $$ SYNOPSIS...WM/22 NEAR TERM...22 SHORT TERM...22 LONG TERM...22 AVIATION...Rackley