Scientific Forecaster Discussion
000 FXUS61 KPBZ 060333 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 1033 PM EST Thu Dec 5 2019 .SYNOPSIS... Dry and seasonable conditions are expected overnight under surface high pressure. A cold front will return precipitation chances Friday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... Evening Update: No major changes have been made to this forecast period. Previous Discussion: Zonal flow has mostly been re-established allowing skies to clear in several areas but high clouds will filter in gradually overnight ahead of an encroaching shortwave and its subsequent cold front. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY NIGHT/... Low level wrly flow should shift swly early Friday as surface high pressure advances eastward, with warm advection aiding in the recovery to near average temperature through midday. A Great Lakes shortwave trough is expected to advance across northern portions of the forecast area during the afternoon, increasing rain / snow chances through Friday evening. This disturbance appears weak as better forcing for ascent is progged across the Great Lakes. Forecast soundings also suggest the presence of low- level dry air hampering the development of a deep, moist boundary layer, resulting in generally light rain / snow amounts. Meanwhile, a southern stream shortwave is progged to increase rain / snow chances across northern WV and for the PA and MD ridges Friday. Similarly, forcing for ascent appears weak, thus precipitation accumulation amounts should be minor. Precipitation chances end Friday night with building sfc high pressure. Near-neutral temperature advection will maintain below average temperature given quasi-zonal flow aloft through Saturday. A pattern shift is then anticipated Sunday as a Hudson Bay low deepens, returning swly flow and warm advection aloft. Above average temperature is expected Sunday, with surface high pressure prevailing. && .LONG TERM /MONDAY THROUGH THURSDAY/... Stratiform rain chances increase Monday as a series of shortwaves impact the region on the van of the deepening Hudson Bay low and associated central Plains trough. Deep moisture transport is expected given strong jet support, with PWAT values in exceedance of the climatological 90th percentile. This should support widespread rain Monday into Tuesday. Long term model blends agree on the arrival of an arctic front by Tuesday / Wednesday, changing rain over to snow given cold advection in nwly flow behind the frontal passage. Wednesday high temperature should top out 15 to 20 degrees below average. Some moderation, albeit limited, is expected Thursday as flow aloft becomes quasi- zonal. && .AVIATION /04Z FRIDAY THROUGH TUESDAY/... Shortwave ridging has brought VFR to regional terminals. Cloud cover is expected to increase once again tomorrow as the next in series of shortwaves affects area ports. A brief period of low- level wind shear is likely in the morning before surface wind responds to the trough. In addition, scattered showers are possible tomorrow afternoon, but given limited moisture, should not bring widespread restrictions other than a MVFR cloud deck. .Outlook... More restrictions are likely early next week with the next disturbance. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None.