Area Forecast Discussion
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000 FXUS61 KPBZ 101010 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 510 AM EST Sat Dec 10 2016 .SYNOPSIS... Lake effect snow will wind down north of I-80 this morning with a few breaks in the clouds across the area. Cold air will remain in place in advance of the next system that will affect the area tonight through Monday with snow, rain, and the potential of a wintry mix before much colder air arrives late in the week. && .NEAR TERM /THROUGH TONIGHT/...
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Fairly unstable lapse rates remain through the dendritic layer this morning with near saturation also noted in RAP soundings through the layer. This is occurring as a weak wave seems to be passing through on water vapor imagery as well. The net result is flurries pushing eastward through the area, which should dissipate as the morning goes on and drier air encroaches. Likewise farther north, lake effect snow bands that had resided over Mercer and Venango County all night due to persistent NW flow through the dendritic layer even with southwesterly surface flow have gradually moved northward as 850 mb flow turns more westerly and eventually southwesterly today. This should end accumulating snow in these areas, and the previous lake effect snow advisory was thus dropped. Some breaking up of clouds will be likely during the daytime today as time height sections reflect a drier column with surface flow favoring a southerly component. With that in mind, high temperatures may manage to muster a bit more upward mobility than in previous days even as no areas seem poised to challenge freezing. The next system starts to affect the area after midnight tonight with isentropic ascent increasing in the 285-295K layer. GFS/NAM/SREF reflections of lift all favor a band of modest moisture transport in the layer focused first over the southern periphery of the area, then transiting slowly northward as warmer air advances northward into the Ohio Valley. Consensus QPF favors roughly a tenth of liquid equivalent as this crosses with a solidly below freezing column and an impressive depth of saturation and lift through the dendritic layer. Thus a general inch or two of snowfall seem likely as this area of isentropic ascent in advance of a northward migrating warm front seems likely overnight tonight into Sunday morning for most areas. Fries
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&& .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... A complex weather scenario will unfold over the latter part of the weekend, stretching into Monday. While there are still some unknowns with this system, confidence in several features including the warm trend is growing. Increasing southerly flow on the back of the surface high will shift the warm advection snow band northward Sunday. Models still differ on just how far north this band gets, with the NAM bullish on fully clearing the northern periphery, which seems a bit hasty, given the strongest southerly flow is still well back to the west. Regardless, most operational models show a carved out warm sector drawn into the area, so PoPs south and east have been trimmed accordingly. The result is categorical light snow continuing north of Pittsburgh Sunday afternoon, while what lingering precipitation that exists south of Pittsburgh transitions to a rain/snow mix as the warm push begins. By Sunday night, the nearly stacked low will move through the Great Lakes. The center of the low this far west favors strong warm advection overnight into Monday. Pushed by a stout low-level jet in the warm sector, 800-900mb temperatures will increase quickly. Surface temperatures will gradually warm through the pre-dawn hours, but will lag behind the warming mid-levels. The result will be a relatively complex precipitation type forecast. Current forecast calls for pure rain making it to Pittsburgh by the morning rush, rain/snow north to I-80, and snow persisting north of that. Southeasterly flow near the ridges will hold the cold air in the eastern facing slopes, ultimately favoring a freezing rain or sleet profile before flow veers southerly. Given the recent warm trend seen in the models, will carry just a chance of freezing rain, and hold off on any FZRA headlines. In terms of snowfall, the best estimate for totals from Saturday night through Monday morning is still about 4-6 inches north of I-80 in PA, with only an inch or two across Pittsburgh. Length of snowfall duration and melting of accumulating snow will preclude any snow related headlines at this time. The warm push continues well into Monday, transitioning all precipitation south of I-80 over to rain. This should quickly cut into any laying snow that had accumulated in the previous 24hrs. Strong moisture transport along the trailing cold front will bolster rain efficiency through Monday afternoon. Will continue to carry categorical PoPs with the frontal passage. Showers will taper down Monday evening as the front clears our eastern edge and upper-level moisture evacuates. A mostly west wind behind the cold front will be detrimental to any continued lake-effect or upslope snow, but enough low-level moisture could maintain patchy drizzle. && .LONG TERM /TUESDAY THROUGH FRIDAY/... High pressure in the wake of the early week system will continue the dry trend through Tuesday. An arctic cold front will re- enforce the cold west-northwest flow Tuesday night. Dry high pressure at the surface will cut-off any moisture feed along the front, thus limiting precipitation potential with it`s passage. Another shot of cold then comes with a passing shortwave trough Wednesday night. All told, by Thursday temperatures will be well- below average with high temperatures expected to be 5-10 degrees below our average lows. Next best chance of widespread precipitation comes with a system next early next weekend. At this point, models showing fairly good agreement with the developing stages. Temperatures will begin to moderate by the weekend as the upper- level flow amplifies ahead of this next system. && .AVIATION /10Z SATURDAY THROUGH WEDNESDAY/... Lake effect snow bands will continue to impact KFKL and KDUJ into the dawn hours. Elsewhere, MVFR cigs will gradually improve to VFR today as drier air arrives from the SW. The next system will roll into the area tonight with snow overspreading all sites near the end of the TAF valid period. .OUTLOOK... Widespread restrictions are likely Sunday and Monday, and again Wednesday, with crossing areas of low pressure. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$

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