Area Forecast Discussion
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000 FXUS61 KPBZ 180207 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 907 PM EST Sat Feb 17 2018 .SYNOPSIS... A quick moving system brings snow to most locations this afternoon and evening. The beginning of next week should be warm and wet. && .NEAR TERM /THROUGH SUNDAY/... Update for 9 PM...The main frontogenetical band of snowfall has exited. Behind it, a dry slot aloft has allow for a chasm to form in the radar mosaic imagery along the Ohio Valley generally downstream from Pittsburgh. In this area, where precipitation is still falling, it is coming in the form of drizzle or freezing drizzle, depending upon temperatures. That said, road temperatures really still are not conducive to ice accretion, so advisory expansion is not anticipated. Behind this chasm of dry air aloft, upper divergence again increases and the lifting profile deepens back up toward the -10C isotherm. As such, snow redevelops with the northern stream wave for a short period with light additional accumulations, especially north of Pittsburgh and in the terrain. Fries Previous discussion follows...Regional radar mosaics indicate a rather obvious bright band that has now sagged just south the south of our area. As a result, it can be inferred that our entire CWA has changed over to snow due to a combination of wet bulb effects and dynamical cooling. This is in response to a band of strong frontogenesis that has become somewhat bifurcated into a band north of the low that is translating over the Appalachians to the south of the area and affecting most northern WV and the extreme SW portion of PA and a second band in conjunction with what amounts to a bit of an inverted trough going up the areas west of the terrain. The combination of these two is allowing ascent to reach a deep enough portion of the column to facilitate efficient snow growth. While snow growth is efficient, boundary layer processes as well as warm ground temperatures are working against accumulations. In urban areas, most roads are still primarily wet, and even in rural spots, road accumulations are inconsistent. That said, surface temperatures are still above freezing generally south of Morgantown in the lower elevations, and these area the areas that while snow will be falling at a great clip early this evening, that accumulation may be least efficient. However, in a band from roughly Monroe County, OH, through southwest PA and the Morgantown area, even the lower elevations should yield a swath of wet 2 to 4 inch snow accumulations. Winter weather advisories have thus been expanded for this, yet have opted to exclude Pittsburgh due to urban effects that should preclude substantial road accumulations and thus impacts. In the higher terrain, with colder surface temperatures, roughly similar QPF will simply yield a more efficient snow. Thus, warnings for most of the higher terrain seem warranted. Fries Once the low moves offshore by sunrise, a mostly sunny day is expected on Sunday. Sunday appears to be the only day in the forecast when sun will be prevalent. Temperatures will be near seasonal values. && .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/... Latest model runs slowed up arrival of precipitation from the inherited forecast, and it now appears that it will take until Monday for most locations to begin receiving rainfall. The rain will be in advance of a low pressure system moving into the Plains. The warm front will waver through the period, leaving most locations with only a chance of rain Monday night and Tuesday. Temperatures will be well above normal through this portion of the forecast, with temperatures in the 50s/60s on Monday and most locations reaching the 70s on Tuesday. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... While the GFS and ECMWF have wavered on the timing for the cold front passage over the last few days, there is general consensus that this will happen Wednesday into Thursday. This will bring the greatest chance for rain, but also an end to the very warm temperatures. The front will linger to the southeast, keeping a chance of showers in the forecast. Even after the cold front passes, temperatures should still remain slightly above normal. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/... Snow will continue late this evening as low pressure finalizes it`s pass across our region. Snow will then taper down west to east near midnight, so visibilities should greatly improve. Ceilings will improve little through the night but could lift to the IFR/MVFR interface. Daybreak Sunday will bring mixing and further lifting of the ceilings. By midday, surface flow backing to the south will push drier air in and scatter out the cloud deck from south to north. VFR is expected for the latter part of the current TAF period. OUTLOOK...VFR prevails until Monday afternoon when precip with a lifting warm front clips the region. && .HYDROLOGY... Flooding continues on the Upper Monongahela, Ohio, and at Stillwater Creek. Precipitation tonight should not pose much of an issue beyond slowing the fall through Sunday. All points are forecast to be below flood stage by Sunday night. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...Winter Storm Warning until 4 AM EST Sunday for MDZ001. OH...Winter Weather Advisory until 4 AM EST Sunday for OHZ069. PA...Winter Weather Advisory until 4 AM EST Sunday for PAZ016-022- 023-031-073>075. Winter Storm Warning until 4 AM EST Sunday for PAZ076. WV...Winter Weather Advisory until 4 AM EST Sunday for WVZ004-012- 021-509-511-513. Winter Storm Warning until 4 AM EST Sunday for WVZ510-512-514. && $$

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