Area Forecast Discussion
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403 FXUS61 KPBZ 082306 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 606 PM EST Thu Dec 8 2016 .SYNOPSIS... Periodic snow showers are expected into the weekend, especially north of Pittsburgh. All locations are likely to receive accumulating snow on Sunday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... Flurries / very light snow associated with the upper trough transitioning over the region will taper off this evening. 850mb flow will veer northwest overnight on the backside of the trough but without good wind alignment vertically, inversions remaining below 5000 ft, and best lift outside of the dendritic growth zone, confidence remains high that accumulations overnight should not be much more than an inch or so north of Interstate 80 and lesser totals to the south. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/... Surface winds finally take on a northerly component on Friday, particularly during the late morning and early afternoon. This is expected to be the period when snow accumulations would be the highest. Continuing the trend of the previous forecast shift, only minor changes were made to snowfall accumulations. While there will be an extended period of snowfall, there is no period with especially high snowfall expected. Although accumulations across northern portions of Mercer, Venango, and Forest counties could reach advisory levels of three inches per 12 hours on Friday, do not think that county averages will be high enough to justify any advisories. Perhaps the one exception might be in Forest county, with the 18Z RAP suggesting that a dominant band may develop to the east and move into the county by 15Z, a forecast that is also suggested by the 12Z GFS. This will bear watching by future shifts. With the arrival of today`s trough, an extended period of below normal temperatures has begun, with temperatures about ten degrees below normal. As winds begin to develop a southerly component once again Friday night, snowfall accumulations should begin to diminish. As high pressure builds in from the southwest Saturday, coverage of snow showers should really decrease, with any minor accumulations limited to locations north of Interstate 80. The break in precipitation will be short lived as more precipitation will lift across the region Saturday night into Sunday. This is expected to bring snow to all locations, although models are still differing in how far south precipitation will extend. So far it appears that snowfall totals would be highest in the north, with minimal snowfall accumulations to the south where rain may mix in. Below normal temperatures will continue through this period. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... 12Z GFS/ECMWF are in relatively good agreement that a warm front will lift across the region Sunday night into Monday, and until the entire region is in the warm sector, a period of freezing rain could be possible before snow changes over to rain. Did not have enough confidence in this to include freezing rain in the forecast at this time. The surge of warm air should bring temperatures back towards normal on Monday. Model agreement drops after the cold front moves through Monday night, and while models both show high pressure developing in the Plains and a storm system moving up the East Coast, the differences in where those systems track result in little agreement on precipitation, and have stayed close to the SuperBlend with chance pops through much of the rest of the forecast. After Monday`s near normal temperatures, temperatures will drop back below normal, with another reinforcing shot of cold air likely to drop many locations into the single digits Wednesday night and keep some locations in the teens on Thursday. && .AVIATION /23Z THURSDAY THROUGH TUESDAY/... MVFR to low VFR stratocu ceilings are expected through Friday under broad upper troughing and cold W-NW flow. MVFR to local IFR snow showers are expected overnight and Friday as a weak surface trough crosses. This trough should veer boundary layer winds to the WNW, resulting in some lake enhancement and localized IFR restrictions for ports N of I 80. A tight pressure gradient should result in gusty west winds through the period. .OUTLOOK... Restrictions are likely through early Saturday under upper troughing, and again Sunday and Monday with crossing low pressure. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$

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