Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 080623 AFDCTP Area Forecast Discussion National Weather Service State College PA 123 AM EST Thu Dec 8 2016 .SYNOPSIS... A cold front will sweep southeast across the area on Thursday. Colder air moving over the eastern Great Lakes will produce significant lake effect snow accumulation over the snowbelt region of northwest PA late Thursday through early Saturday. A weak low pressure system is likely to track west of Pennsylvania through the Great Lakes later in the weekend, bringing a chance for snow to the entire state. && .NEAR TERM /UNTIL 8 AM THIS MORNING/...
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Regional radar mosaic at 06Z showing some weak returns aloft assoc with lift along jet streak, which extends from the Ohio Valley through Pa. PIT 00Z sounding showing plenty of dry air in the lowest 5kft, which will likely preclude precip reaching the ground in most places. However, wouldn`t be surprised to see a few flurries toward dawn across the the northern half of the state, as mid level shortwave moves through. Plenty of mid level cloudiness and an active westerly breeze should keep temps from falling much from 06Z readings. Lows should generally range from around 25F across the Allegheny Plateau, to around 30F across the Susq Valley.
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&& .SHORT TERM /8 AM THIS MORNING THROUGH 6 PM FRIDAY/...
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Shortwave and assoc batch of mid level cloudiness should push east of the region by around midday. In it`s wake, arrival of colder air will result in developing lake effect snow downwind of Lake Erie today. All model data currently indicating the boundary layer flow will remain WSW for much of the day, keeping the bulk of the accumulating snow north of the border. However, a gradual veering of the flow is anticipated by late afternoon, bringing snow into Warren/Mckean counties. LE snow warning remains up for Warren and an advisory for Mckean beginning this afternoon. Confidence has increased to the point where we feel heavy lake effect snow bands are a given tonight through Fri night. Proximity to the lake and fetch for the first ~12 hours of the event point to NW Warren and far nrn McKean Co as most likely to see a few inches by sunset Thurs, and many more by sunrise Friday. Accumulations could be quick overnight with a gradual veering to the winds and multiplication of the bands as the cross- lake fetch becomes shorter. We may also see LH & Georgian Bay get involved and contribute moisture to our region.
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&& .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... The long term will begin with arrival of a deep trough extending from a low pressure system situated over eastern Canada. This trough will usher in cold northwesterly flow. Latest model runs have 850 mb temperatures in the -10 to -12 range, especially through the northwest mountains. The initial wave of precipitation from the front should generate decent snowfall however the main focus of this event will be the lake effect pattern that will set up after frontal passage. The latest ensembles have actually lessened the amounts into the NW mtns. From the model soundings there should be a subsidence inversion but bases should lift rapidly up to, then well over 7-10 kft agl Thursday night right through Friday night, as the mean low to mid level flow becomes well- aligned from the West- Northwest. The subsidence inversion base will drop gradually down to below 7 kft agl during the day Saturday as the mean 925-700 mb flow backs to more of a west, then swrly direction by around 12Z Sunday. This scenario of inversion heights near 700mb, combined with the anomalously cold air at 850mb and warm lake waters of around 50F, support locally heavy snowfall (mainly NW Warren Co) between Thu night and Saturday. Strong lapse rates combined with a fairly gusty wind should hold snow/water ratios to no more than 20 to 1. Based on QPF input from WPC and latest higher resolution models first guess is for 36-hour totals of 12+ across the snowbelt of NW Warren by Sat AM. Further south, scattered narrow bands of snow showers will stream SE into the Central Ridge and valley Region occasionally, with a coating to an inch possible in a some locations. Orographic forcing over the Laurels should lead to an inch or two of accum Thu night through Fri night. A generally westerly flow is unfavorable to draw lake moisture that far south. Diminishing Lake effect snow is expected Saturday as inversion heights fall upon approach of high pressure. A bubble of high pressure at the sfc with a weak ridge aloft will bring a brief period of tranquility, though very cold overnight min temps, as it slides east across the forecast area late Saturday into early Sunday. Expect partial clearing and perhaps mostly clear skies across the SE part of the CWA for the middle third of the weekend. Deep, mean-layer flow backs to the southwest with a slug of low-mid level warm advection sliding up and over the deep/retreating cold airmass. This could create an area of light snow or snow showers across mainly the northwest half of PA Sunday afternoon and Sunday night. The boundary layer could become warm enough by Monday to mix in or changing the scattered light precipitation to rain showers, throughout the Lower Susquehanna Valley. Seasonable temps indicated early next week, but both the 012Z ECENS and NAEFS indicate a significant cold front for mid week with below normal temps likely late next week. && .AVIATION /06Z THURSDAY THROUGH MONDAY/... 03Z TAFS sent. Minor adjustments made. Earlier discussion below. Some mid level clds outside as of late aft. Main change at 00Z was to add some snow showers in for BFD later on Thursday. Time of day and dryness of airmass, did not see conditions going down before 00Z Friday. Skies VFR as high pressure builds across central PA...continuing tonight through the first part of Thursday. Some snow showers will likely work into the north and west late Thursday into early Saturday afternoon, as winds shift more to the west and northwest off the Great Lakes. A weak storm system tracking northeast across the Great Lakes on Sunday may bring a few snow showers to the area late. Outlook... Fri...Windy with periods of snow showers. Mainly MVFR/IFR west to MVFR/VFR east. Sat...Decreasing winds. Low cigs/-SHSN psbl NW. VFR east. Sun...Some snow showers possible. Mainly late across the NW. Mon...Some snow showers...mainly NW. && .CTP WATCHES/WARNINGS/ADVISORIES...
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Lake Effect Snow Advisory from noon today to noon EST Saturday for PAZ005. Lake Effect Snow Warning from noon today to noon EST Saturday for PAZ004.
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&& $$ SYNOPSIS...DeVoir NEAR TERM...Fitzgerald SHORT TERM...Dangelo/Fitzgerald LONG TERM...Dangelo/Fitzgerald/Ceru AVIATION...Steinbugl

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