Area Forecast Discussion
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000 FXUS61 KPBZ 291040 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 640 AM EDT Thu Sep 29 2016 .SYNOPSIS... Upper level low pressure will bring rain to our region for the rest of the week. && .NEAR TERM /THROUGH TONIGHT/... 615 AM - Updated PoP fields in accordance with radar trends and HRRR run...with higher values near and north of Pittsburgh, and a lull in activity along the southeast slopes. Coverage will increase through the day as moisture deepens. Previous discussion... Models in good agreement in dropping upper low into western Kentucky today and stalling it there through tonight. Several issues to deal with during the near term period, including QPF, thunderstorm potential, and wind gusts tonight. Heavier rainfall totals to this point have occurred along the eastern slopes in Garrett/Tucker Counties, as well as in a small band from western Fayette into Allegheny, with 0.5 to 1.0 inch of rain. Expect best focus for precip will shift to the north of Pittsburgh and along the eastern slopes as the morning progresses, due to shifting of best upper diffluence and continued upslope flow. This is similar to HRRR depiction. Accordingly, lesser coverage is foreseen for southeast Ohio into northern West Virginia. Coverage should improve this afternoon in these areas as a few breaks in the clouds allow for instability showers and a few thunderstorms to develop. Cannot rule out a strong storm or two across the south given the high levels of deep shear, and even some small hail is possible. However, overall low levels of instability should keep this threat isolated at best. Still concerned about potential QPF levels along and just west of the eastern ridges as the Atlantic moisture tap continues through tonight. The period for heaviest rain appears to be from late this afternoon through 06Z tonight when moisture is deepest. Considered issuance of a small flood watch for several counties in our southeast. Given the dry conditions before tonight, the relative underperformance of QPF to this point, and the apparent lull in activity this morning, have elected to hold off on issuance at this time, thinking that expected rainfall will fall over a long enough period to prevent widespread issues. Will continue the HWO mention and monitor the situation closely. Final issue to consider is potential wind gust issues tonight along western slopes of the ridges. With potential stable layer above ridgetops and winds oriented favorably with ridgelines, there is the potential for strong downsloping winds. Kept gusts below advisory level for now, but this will also need to be monitored. && .SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/... Rain and gusty winds will linger through Saturday as the nearly stationary mature low pressure system gradually fills. A strong 60kt+ jet at 850mb is progged to stream across our southeast ridges through Friday morning. Still some questions as to the orthogonal flow and just how much of this wind will be able to break to the surface, so at the moment, the near Advisory gusts and HWO mention were maintained from previous forecast. Strong downsloping flow will continue through the day Friday as the parent low moves very little. This, along with the deep Atlantic moisture feed and convergence zone lifting north will result in a relative minima in shower activity just west of the ridges through center of the forecast area. This rain shadow effect is reflected in most operational model qpf fields and PoPs and WPC collaborated qpf have been trimmed back accordingly. As the low dislodges and slowly creeps northward Saturday, the dry slot wrapping around the eastern edge of the circulation will push into our area. This will bring an end to any organized rain bands, leaving only scattered, light rain showers mainly across the north. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... Lingering low pressure system will slowly drift over the southern Great Lakes Sunday into Monday. While this will keep rain chances in the forecast across the north, the aforementioned dry air will erode residual moisture. By Tuesday, the low will finally move east of the area, and high pressure will build into our west. This will bring a return to drier and warmer conditions with temperatures returning to above-average through the end of the week. && .AVIATION /10Z THURSDAY THROUGH MONDAY/... An upper low will continue to affect our region through the period. Most precipitation will occur north of PIT early in the TAF period, but activity is expected to fill in through the day as moisture increases. Southwest PA/northern WV will start with VFR ceilings, but those locations should join the rest of the terminals with MVFR conditions as time goes on. Brief IFR cigs/vis remain possible in any of the heavier showers. Best chance of afternoon thunderstorms lies south of PIT where some breaks in the clouds/better instability are possible, and have included VCTS in a few forecasts. The heaviest rain is expected from early evening through 06Z or so. Another concern, especially tonight, will be gusty winds along and just west of the ridges. Some gusts of 30-40 knots cannot be ruled out near LBE. .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. && $$

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