Area Forecast Discussion
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623 FXUS61 KPBZ 282200 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 600 PM EDT Wed Sep 28 2016 .SYNOPSIS... Upper level low pressure will bring rain to our region for the rest of the week. && .NEAR TERM /THROUGH THURSDAY/... Southeasterly flow will continue to increase over the area as moisture starts to push northwestward toward the ridges from the Mid-Atlantic coast. An upper low poised to our west will move exceedingly little save for a bit of a southerly drift with the main change likely to be the tilt in the upper level system from positive to neutral. The change in the tilt of the system will allow for added upper level diffluence over western Pennsylvania later this evening, which should allow for shower development to spread northeastward from central Ohio toward northwestern Pennsylvania. Increasing southeasterly flow is likely to duke it out with dynamical developments aloft for areas generally from Pittsburgh southward to the west of the ridges overnight. With increasing downslope flow off the ridges and dry air in place, the chances of rain should be exceedingly low. However, an influx of middle and upper level moisture coupled with the development of the ridge entrance region of the jet will make precluding rainfall very difficult. Thus, through the night, while the chances of rainfall in Ohio, along I-80, and in the ridges become/remain very high, from Pittsburgh southward to the west of the ridges, activity will likely be much more scattered. Additionally, the increase in momentum off the surface as a result of the southward sag of the low should preclude inversion formation overnight to a degree. This will be particularly true in the ridges and their immediate lee. While soundings suggest the unstable layer should exist solely below the ridgetop high, gusts will be possible generally from Fayette County northward in the lee of the ridges. Momentum profiles do increase overnight, however the best momentum and profile for wind gusts looks to hold off until Thursday night/Friday morning, when wind advisories may be necessary. Fries && .SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/... The aforementioned low is forecast to briefly stall over the Middle Ohio River Valley on Thursday, before lifting northward and filling once again through the weekend. The concern for the period is for Thursday when an Atlantic moisture tap is forecast to stream across I 68 through Garrett County and over areas generally along and north of the PA turnpike, and deepen the overall moisture supply in the column. Currently-progged rain-supporting dynamics for that period are limited given the lack of convergence/mid level frontogenetical forcing in this mature stage of the low. The better probability for heavy rain off the immediate eastern upslope areas should thus be convection based and instability is not impressive. Have used general River Forecast Center QPF for todays prog of the event in conjunction with WPC QPF probability maps. The bottom line is no flash flood watch until precursor rainfall tonight at least defines a better threat area in conjunction with moisture convergence into the occluded frontal zone. As the Atlantic moisture tap in interrupted by Friday, the sustained rainfall threat will be alleviated with more scattered convection providing the rain chances. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... The low is forecast to lift northward and fill through the weekend with dry flow returning and diminishing rain chances for the region, and supporting seasonably warm temperature. && .AVIATION /22Z WEDNESDAY THROUGH MONDAY/... VFR will predominate into the evening, but deterioration can be expected tonight and into Thursday as rain becomes likely with moisture streaming into the eastern flank of the upper low. .OUTLOOK /THURSDAY NIGHT THROUGH SUNDAY/... Periodic restrictions can be expected into the weekend as the upper level low lingers, then slow lifts north of the region. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.