Area Forecast Discussion
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365 FXUS61 KPBZ 081518 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 1018 AM EST Thu Dec 8 2016 .SYNOPSIS... A trough of low pressure will result in a return to cold temperatures and periodic snow showers through the end of the week. More widespread precipitation is likely Sunday with crossing low pressure. && .NEAR TERM /THROUGH TONIGHT/... With 10:15am update, the biggest change was to bring in pops slightly more quickly across northern counties today, as snow has already been reported as far south as Akron and Youngstown as of 10am. However, still think that little more than a dusting of accumulation would occur before sunset. Previous discussion... Mid-level clouds continue to stream across the area ahead of a trough in the upper levels. Some virga is likely falling from some of these, with the low levels remaining rather dry. The trough, currently swinging across the Great Lakes, will cross later today. Cold advection will be ongoing, picking up in intensity this afternoon. Temperatures may reach their maximum values during midday/early afternoon before dropping off thereafter. Snow shower activity this afternoon should be mainly confined to areas north of I-80, as low level W/WSW flow keeps most of the lake enhanced activity to our north initially. Even here, the dendritic growth zone is slow to saturate, so kept PoPs in the chance range for the most part. Low-level flow starts to take on a more northerly component tonight behind a weak surface-based trough. Snow shower chances pick up north of Pittsburgh, especially in the favored lake-effect area north of I-80. Increasing inversion heights and moisture point to better potential for accumulation, but the best setup will wait until towards 12Z Friday and thereafter. An inch or two of accumulation seems likely here. More scattered activity will occur elsewhere, driven by continued cold advection. Some modest upslope enhancement may allow for up to an inch or so of accumulation along the ridges in spots, with lighter accumulations elsewhere. Low temperatures will run some 5-10 degrees below climatology. && .SHORT TERM /FRIDAY THROUGH SATURDAY/... Ongoing snow showers north of I-80 Friday morning will spread southward through the day as the wind shifts slightly behind the passage of a surface trough. Boundary layer flow will veer to northwesterly as moisture depth increases and lapse rates steepen. For this reason, will keep the southward spread of likelies nearing the Pittsburgh metro by Friday afternoon. All areas could see at least some snow flakes, but areas south and west of Pittsburgh will see little to no accumulation. High pressure and backing boundary flow will quickly taper down snow showers southwest to northeast Friday night into Saturday morning. By daybreak Saturday, only the extreme northern edge of the forecast area will remain near the downlake moisture plume. Will linger chance PoPs in this area for now, but additional accumulation is not expected to be significant during the day Saturday. All told, total snow accumulation beginning tonight through Saturday morning has changed very little from the previous forecast. With the extended duration of the snowfall, will continue to hold off on any winter weather headlines. The re-enforcing cold air will hold temperatures 5-10 degrees below the averages. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... Operational models continue to stream a clipper type system through the Midwest, approaching our western fringes by Sunday. Despite this weak disturbance dampening upon arrival as the upper-level flow goes zonal, snow accumulation is possible Sunday afternoon/evening. With significant guidance differences extending into next week, the latter part of the long term forecast is offered with lower confidence. Models continue to slide a deepening low out of the northern Rockies early next week. Ensemble guidance fairly consistent in the depiction of an anomalously deep upper-level low tracking through the northern Plains. Their operational brethren carry this low much further north. These differences impact the track and intensity of a system arriving Monday. Opted to utilize the Superblend, siding closer to the 00z ECMWF guidance for this system. As the bowling ball low pinwheels east across North America next week, the door to much colder air swings open. The middle of next week should feature temperatures that are well-below average. && .AVIATION /15Z THURSDAY THROUGH MONDAY/... Deterioration to MVFR ceilings are expected this morning under broad upper troughing and cold advection behind a crossing cold front. A brief improvement to VFR is possible toward evening as mixing lifts the ceiling heights, though MVFR should return by late evening under the broad upper trough. MVFR to local IFR snow showers are possible overnight mainly N of I 80 as the boundary layer flow veers to WNW, with some lake enhancement possible. A tight pressure gradient should result in gusty west winds through the period. .OUTLOOK... Restrictions are likely through early Saturday under upper troughing, and again Sunday and Monday with crossing low pressure. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.