Scientific Forecaster Discussion
000 FXUS61 KPBZ 201232 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 832 AM EDT Thu Jul 20 2017 .SYNOPSIS... Thunderstorm chances will escalate today with severe storms possible. Active weather remains in place through the weekend, with the risk of severe weather returning Saturday. && .NEAR TERM /THROUGH TONIGHT/... Only minor tweaks made to sky and hourly temperatures for this update. Forecast remains on track. With the midwest MCS diving towards the lower Ohio Valley and another shortwave tracking across southern Ontario, it appears that support for any significant convective threat will hold off until mainly after 21Z, when lift improves. Current PoP fields handle this well, with only scattered possibilities across the northwest counties prior to that time. Latest HRRR trends indicate that timing could be an hour or two later still...this will be monitored. Previous discussion... Thunderstorms will become likely over the I 80 corridor this afternoon, and those are expected to spread southward toward the I 70 corridor this evening. Those storms are anticipated with a shortwave that supported the MCS that pummeled the midwest overnight. Upper Ohio temperatures are again forecast to rise through the 80s/near 90 today, and those readings coupled with near 70 F dewpoints will not only be uncomfortable, but will fuel severe-healthy instability profiles. As mid level flow increases to near 50 kt with the approaching trough, deep layer shear will escalate and support an organized severe threat into the evening hours as the surface front and/or outflow ploughs into the warm, humid unstable airmass over the immediate area. Expect a thunderstorm line or short bowing segments to provide a fairly widespread damaging wind threat. In addition, with backed surface wind under west, to northwest flow aloft, the threat of the isolated tornado cannot be ruled out, especially in light of chaotic outflow boundary expectations. These threats have been mentioned in the morning Hazardous Weather Outlook. && .SHORT TERM /FRIDAY THROUGH SUNDAY/... The latest deterministic model trends are that this evenings convection and frontal passage will be decisive enough to drive the boundary south of the region for Friday. POPs for Friday were thus reduced, then revamped for the approach of another strong shortwave on Friday night, which may spawn more severe storms given the projected wind profile. Current timing does not favor that scenario, hence the marginal SPC outlook for the time frame. Models indicate a repetitive pattern for Saturday and cold frontal passage for Sunday. Likely POPs were thus maintained for the weekend along with the beginning of a downward temperature trend. && .LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/... The pattern of shortwaves crossing in zonal/northwest flow is expected to give way to Great Lakes-to-Northeastern-CONUS troughing during the first half of next week. Temperature will thus moderate back toward, and a few degrees under the average and rain chances will decline into mid week. && .AVIATION /13Z THURSDAY THROUGH MONDAY/... High confidence forecast save for the onset and duration of thunderstorms this afternoon and early evening. These times could vary by 2-3 hours and flight category between 6-12Z Friday. An upper level disturbance crossing this afternoon will bring a line of strong to severe storms moving from north to south affecting likely all terminals if it develops like many of the high resolution models indicate. Inserted VCTS for all terminals starting around 20Z north and 0Z south of the mason dixon line. Brief restrictions to IFR vis can be expected if an airport is impacted by a storm. In wake of the storms, last 6-9 hours of forecast its hard to tell if MVFR fog will develop or will there be a canopy of stratocu or just VFR? For now, rolled with an optimistic forecast no cigs below 030. .OUTLOOK... Additional rounds of storms are expected through the weekend, and morning fog is possible as well. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None.