Area Forecast Discussion
Issued by NWS State College, PA

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155 FXUS61 KCTP 080316 AFDCTP Area Forecast Discussion National Weather Service State College PA 1016 PM EST Wed Dec 7 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 THURSDAY MORNING/... 10 PM update... No news is good news. Have been watching the mid/high clouds float over the area, but nothing making it to the ground as expected. Much drier air is behind the front/trough headed toward us from the Upper Great Lakes. Dewpoints now down in the lower teens in WI, and sub-20 on the MI shore/side of LM. Dynamics strongest to the north and moisture very very limited with this feature, so little/no precip is expected as it passes early Thurs NW and mid- day across the rest of the area. Winds may gust into the 20s, though, as is rather expected with a dry fropa. Prev... Near term is rolling along. Posted warning/advy in the NW for the LES to come. Dry air beneath our mid-deck is keeping the radar returns aloft/virga. Steady/gradual cold advection through the night will drop 8H temps into the negative double digits in the NW. Any developing lake bands will be hugging the lake shore over wrn NY overnight. Keeping the fcst dry for our CWA through sunrise. Prev... Rather pleasant early winter day over central PA in the wake of this mornings occluded frontal passage...as surface high pressure builds into the region. Downsloping westerly breeze is allowing maxes to warm through the upper 40s over the southeast while temperatures elsewhere range from the mid 30s north to the mid 40s. 5h shortwave streaks ENE from Ohio overnight increasing mid level cloud cover from west to east...along with an increase in lake cloudiness as colder air aloft works over the warmer waters of Lake Erie. Therefore...generally increasing clouds are seen west and north especially after midnight. This will keep min temps in the mid 20s north...ranging to the lower 30s southeast. && .SHORT TERM /8 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/... 7 PM Update... Confidence has increased to the point where we feel heavy lake effect snow bands are a given. 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. Prev... Lake clouds will continue to enhance on Thursday...with lake effect snow showers developing downwind of Lake Erie by late morning and afternoon. Model data continues to indicate boundary layer flow will remain WSW for much of Thursday, keeping the accumulating snow north of the border for most of the day. However, a gradual veering of the flow will likely bring snow into at least NW Warren County by late in the day. Lake effect watch currently begins at 18z Thu...but significant accums over my northwest counties aren`t expected until Thursday evening and lasting through Friday. && .LONG TERM /THURSDAY 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 /03Z THURSDAY THROUGH MONDAY/... 00Z TAFS sent. Not much change. Some mid level clds outside as of late aft. Main change 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. Marlier discussion below. 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... Lake Effect Snow Advisory from noon Thursday to noon EST Saturday for PAZ005. Lake Effect Snow Warning from noon Thursday to noon EST Saturday for PAZ004. && $$ SYNOPSIS...DeVoir NEAR TERM...Dangelo/DeVoir SHORT TERM...Dangelo/DeVoir LONG TERM...Dangelo/Fitzgerald/Ceru AVIATION...DeVoir/Gartner/Martin

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