Scientific Forecaster Discussion
000 FXUS61 KPBZ 212005 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 305 PM EST Thu Nov 21 2019 .SYNOPSIS... Generally light rain will accompany a cold front which will cross late tonight and Friday morning. Another system will provide rain and snow shower chances over the weekend. Seasonably chilly temperatures can be expected. && .NEAR TERM /THROUGH TONIGHT/... A warm front continues to lift across the CWA this afternoon as a mid-level ridge continues off to the east. Mid-level cloud cover has overspread the region, but the low levels remain quite dry as noted on morning soundings. So, despite increased isentropic lift, it will take a bit for the boundary layer to moisten sufficiently to allow rain to reach the surface. Thus, only sprinkles or very light rain is expected through sunset, with PoPs increasing thereafter as condensation pressure deficits lower and allow more precipitation to reach the ground. Overnight, upper support for precipitation remains fairly weak initially ahead of the advancing cold front. Towards morning, the approaching mid-level shortwave will provide a better opportunity for lift. Areas near and south of the Mason-Dixon Line will also be closest to right entrance region jet dynamics, enhancing precipitation here a bit, and thus the highest PoPs were place here towards morning ahead of the cold front, which will lie somewhere in the CWA at 12Z. Temperatures will remain quite mild initially ahead of the front, before dropping into the upper 30s to mid 40s toward sunrise. && .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... Precipitation coverage will begin to decrease Friday morning as the cold front exits along with the support for lift. Low-level ridging quickly builds in behind the front with moisture evacuating as well, so despite low-level NW flow, there will not be much opportunity for lingering lake-induced showers. The boundary layer may cool in time to allow a few snowflakes to fall just before precipitation ends, but this is not expected to have any impact. All precipitation will cease by later Friday afternoon, with temperatures dropping a bit below normal once again. A brief period of ridging will provide dry weather Friday night and into Saturday morning. The next mid-level wave will then ride up the Ohio Valley through Saturday night. The NAM keeps this wave further south than most other guidance and is discounted. This system will be able to scoop up better moisture than the previous one as cyclogenesis occurs near the Gulf Coast, with southwest flow bringing a 0.75-1.0 inch PWAT air mass into our southern counties by Saturday, which will help push QPF to around a quarter-inch. Colder air wrapping in from the west behind the surface low will allow a mix with or change to snow Saturday night. However, with the dendritic growth layer remaining in the mid-levels and with a relatively warm ground, accumulation should be limited to a dusting in scattered cases. Overall, temperatures will remain fairly seasonable for Saturday and Saturday night. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... Some precipitation will linger into Sunday before ridging takes hold. Minor upslope enhancement could occur in WNW flow during the morning, but lake enhancement appears unlikely due to insufficient lake/air delta-T values. Any additional very minor snow accumulation will be limited to the higher ridges. Surface ridging will provide mainly dry weather Sunday night through Tuesday. Temperatures will rebound back to warmer-than-normal levels in southwest flow. The next system will likely impact the region during the Tuesday night-Wednesday period, with a low pressure center crossing the Great Lakes. Much uncertainty remains with the track and intensity of this low, with the ensembles providing a wide range of solutions. Relied on the model blends for now, with precipitation chances lingering into Wednesday night. A mostly dry and seasonable Thanksgiving Day is currently in the forecast, with high pressure returning. && .AVIATION /19Z THURSDAY THROUGH TUESDAY/... VFR is expected through the day with increasing mid-level clouds as a trough approaches from the west. A warm front will lift through the region late this afternoon/evening, with some very light showers possible along the front and stronger S/SW winds behind it. A low-level jet ahead of the approaching trough will create a period of LLWS at all sites, primarily btwn the warm and cold frontal passages. MVFR/IFR conditions and mostly light rain showers is expected with approach and passage of the sfc cold front Friday morning. The passage of the shortwave trough and increasing sfc high pressure in the wake of the cold front Friday will end showers and bring a return to VFR conditions. .Outlook... Restrictions are likely with the next low pressure system this weekend. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None.