Scientific Forecaster Discussion
000 FXUS61 KPBZ 260918 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 518 AM EDT Tue Sep 26 2017 .SYNOPSIS... High pressure will bring dry and warm weather through mid-week before the passage of a cold front heralds a return to seasonably-cool conditions. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... An increase in upper level moisture has been noted streaming over the area for the past 12 or so hours in response to a passing weak upper jet streak that is dynamically pulling a bit of the blow off from Maria northward into our region. This jet streak will exit to the east as the morning goes on today, yielding increasingly sunny conditions through the day after a start with a fair bit of high clouds. The aforementioned high clouds have been rather pervasive through much of the night. This has worked to keep our temperatures largely in the upper 60s for almost the entire area save for portions of central Ohio through 5 AM. A warmer start to the day combined with clearing with the exit of the weak upper jet streak will likely yield a day much like yesterday. Similar 850 mb temperatures, but a bit lower H5 heights should be largely offset by a far warmer start. Thus, highs in a few spots will again challenge or surpass 90F, while most others will still jump into the upper 80s. This is 15-20 degrees above normal. Fries && .SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY NIGHT/... The only real highlights of the short term period will be the passage of frontal boundary late on Wednesday and the attendant reduction in temperatures. While timing remains very similar to the last several runs of all model systems for a passage of the front through the area after 18z Wednesday, consensus on layer saturation is a bit more tenuous. Without saturation of the lifted layer with the front, rainfall will be tough to come by. The overly moist NAM has continuously projected widespread precipitation with the frontal passage, and the GFS has likewise indicated at least modest QPF. The ECMWF and Canadian, however, have continuously opted for a dry frontal passage. Given that the best mid-level forcing and moisture shear off to the east well to the north of the area, it still seems hard to believe that widespread organized rainfall will precede this front. As a result, just very low-end slight chance PoPs have been maintained with its progression. Regardless of the rainfall with the front, temperatures look to drop off quite a bit. While early in the week 18-19C will be common at 850 mb, by Thursday, northwesterly flow will have dropped those numbers back toward 6-7C, and this will yield a more seasonable diurnal. Fries && .LONG TERM /FRIDAY THROUGH MONDAY/... As an upper ridge builds into the Great Plains, a shortwave trough will move southeastward across the Great Lakes. Models do seem to be latching onto boundary layer moisture increases as it traverses the relatively warm Great Lakes. All models do now show QPF across the area, especially northern reaches of it, late Friday into Friday evening. PoPs were increased for this. In the wake of the wave, high pressure returns for Saturday, with seasonable temperature through the weekend. && .AVIATION /09Z TUESDAY THROUGH SATURDAY/... Other than localized morning fog, high pressure will maintain light wind and VFR through the TAF period. .Outlook... Restrictions are possible with cold fronts late Wed and again Fri. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None.