Area Forecast Discussion
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000 FXUS61 KPBZ 151741 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 1241 PM EST Fri Dec 15 2017 .SYNOPSIS... Cold with another round of snow showers expected this evening, mostly north of Pittsburgh. Some temperature moderation is then expected over the weekend. && .NEAR TERM /THROUGH TONIGHT/... Mid morning update made to make only minor tweaks to the forecast as snow is already being generated by convergence along the terrain as a shortwave in the upper trough pivots toward the Ohio Valley. Expecting little accumulation from this but still thought it warranted an increase in PoPs through the afternoon. Otherwise, shortwave trough over lower Michigan will make it`s approach this afternoon. Little change made to the analysis below, so will leave this as is. Previous discussion... The GOES-16 Simple Water Vapor RGB shows our next disturbance quite well, currently entering southern Wisconsin. This wave will cross the western Great Lakes today, deepening the upper- level trough and surface low under favorable upper-level jet dynamics. Although the system lacks deep moisture, it will acquire some low-level moisture on its journey through the Great Lake aggregate. Steepening lapse rates under mid-level cooling will help fuel snow showers as they enter our northwestern zones this evening. Most of this activity will stay north of Pittsburgh and probably north of I-80, but there will be potential for accumulating snow showers south to the PGH metro area. Current timing would be after the evening rush. A quick 1-2" is possible across the northern tier of counties, with a marked decrease expected traveling south. Snow showers will likely taper down after the passage of this wave near midnight tonight. However, a developing low-level jet and an injection of mid-level moisture will allow for some re- invigoration of snow showers Saturday morning. This evolution is represented well in the bevy of high-res models at our disposal. This activity will likely stay north of the developing mid-level warmfront, or roughly north of Pittsburgh. Another inch or so could be the result of these showers. All told, the roughly 24-30hr snow total will likely remain under Advisory levels, with the northernmost counties the most likely to flirt with the 3" mark. Elsewhere, and inch or less is expected. In addition to the snow, the aforementioned strengthening low- level jet will bring wind gust to the ridges that could near Wind Advisory levels tonight. Current forecast remains just below the Advisory threshold, although this will need to be monitored through today. Regardless, the increased wind tonight will bring a bite to the already cold air in place. Wind chills will commonly be in the low teens to single digits, colder in the higher terrain. && .SHORT TERM /SATURDAY THROUGH SUNDAY/... Warm air advection begins in earnest Saturday afternoon as high pressure builds into the region. This will quell any remaining shower activity that survives through midday. High temperatures may still be held below average values, especially further north where clouds will hold a little longer, but temperatures will certainly be warmer than the previous couple of days. High pressure will hold through Sunday morning, but another disturbance will be knocking on our doorstep to end the weekend. Isentropic lift along and just behind the lifting warmfront will provide modest ascent as a weak upper-level wave moves into the area. Will continue to carry chance PoPs for rain Sunday evening. A little snow could mix in further north where temperatures will be a few degrees cooler. No accumulation is expected. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... A brief spell of zonal flow aloft will give way to amplification once again heading into the middle of next week. First, a trough will dig into the central CONUS Monday night into Tuesday, bringing rain chances to the forecast area. In the wake of this system, another brief spell of mostly dry weather is expected Wednesday night into Thursday. Thereafter, models have consistently shown another, strong system moving through the eastern US late next week. Strong warm advection ahead of this system should mean precipitation at onset would be rain. Colder air is set to return next weekend. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
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MVFR ceilings continue to linger many south and east of Pittsburgh. Most locations should see a window of VFR conditions during the afternoon. The next upper disturbance will bring snow showers and MVFR cig/vis mainly north of PIT tonight. FKL, and to a lesser extent DUJ, have a chance of getting occasional IFR conditions in a couple heavier snow showers. After a lull overnight as the disturbance departs, a fresh injection of moisture will bring another round of snow showers and MVFR ceilings. Flight conditions will start to slowly improve from south to north by the end of the TAF period. .Outlook... The next chance for general restrictions arrives by Sunday with an advancing warm front.
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&& .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$

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