Area Forecast Discussion
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000 FXUS61 KPBZ 280748 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 348 AM EDT Wed Sep 28 2016 .SYNOPSIS...
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An upper level low pressure system will bring rain to our region for the second half of the week, with heavy rainfall totals possible in some areas.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Main event this period is the advance of the deep upper low from Lake Michigan this morning to near Louisville by 12Z Thursday. Models are in good agreement with this general scenario. PoPs today will increase from both the west and east with time. Steep low-level lapse rates plus a ribbon of isentropic lift will help to promote shower development in Ohio this afternoon, while the development of moist onshore flow will help to increase precipitation coverage along the ridges. The lowest PoPs end up being in the south-central CWA, furthest from these influences. Tonight, the loss of instability will allow shower coverage to decrease across the southwestern CWA. However, continued moist southeast flow and column saturation will necessitate likely PoPs north of I-76. The NAM is more bullish with precipitation in the eastern CWA as it develops a fairly significant deformation zone and divergence aloft, while other models are more subdued overnight for us, keeping the better forcing to the east. While the exact NAM scenario is not expected, and the heavier rain should remain to our east tonight, the general idea of an enhanced band of precipitation cannot be ruled out. Temperatures were adjusted to depict a bit less of a diurnal range between today and tonight, given the moistening pattern.
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&& .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
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Cut-off low pressure system will stall out near the middle Ohio River Valley Thursday, pin-wheeling through the latter part of the week. Initially kept pops a little lower across the southern half of the forecast area, as a dry slot and strong downsloping flow will keep showers scattered. Later Thursday afternoon, the low retrogrades to our southwest, bringing a frontal zone over the area that will result in more organized rain production. Model guidance continues to depict a strong 850mb jet developing through the forecast area Thursday night. While the bulk of this 50kt+ jet will remain just above the the inversion, some stronger wind gusts should scrape the ridge tops, and potentially impact the western downslope areas of the Laurel and Chestnut Ridges. Additionally, this strong jet will transport a plume of anomalously high PWATS over the area. This could bring efficient rainfall production Thursday night into Friday morning, primarily to our southeastern zones. Exact placement of the frontal zone and subsequent rain production is still somewhat nebulous. This along with dry antecedent conditions is tempering flash flood concerns at the moment, but several inches of rain is possible over a 12 to 24 hour period. Rain will linger through much of Friday, before drier air wrapping around the system pushes into the southern zones. Diurnal temperatures spreads will be relatively compressed under the influence of the low. Highs will be near average values.
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&& .LONG TERM /SATURDAY THROUGH TUESDAY/...
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Stubborn low pressure system will begin to accelerate northward over the early part of the weekend. As it does, light rain chances will gradually cease with drier air continuing to feed in from the south. Some differences exist in timing the departing low, but by early next week, the low should be swept away by the upper-level jet. Building high pressure will restore dry conditions will slightly above average temperatures.
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&& .AVIATION /07Z WEDNESDAY THROUGH SUNDAY/... VFR conditions are expected through the day today, but clouds will be on the increase as a strong upper level low pressure center drops into the Ohio Valley. Showers are first expected this afternoon in southeast Ohio near the upper low, and along the eastern ridges as easterly flow allows moisture to pool. After sunset, many locations will see ceilings drop to MVFR levels and showers will slowly become more numerous. Cannot rule out a thunderstorm in southeast Ohio before sunset, but chances are too low to include in the TAF. .OUTLOOK /WEDNESDAY NIGHT THROUGH SUNDAY/... Periodic restrictions will remain through Saturday as the upper level low lingers over the region. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$

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