Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 042123 AFDCTP Area Forecast Discussion National Weather Service State College PA 423 PM EST Sun Dec 4 2016 .SYNOPSIS... High pressure will slide quickly off to the east of the region this afternoon and evening. A weak frontal boundary but rather potent upper air disturbance will pass through tonight. Another storm system will affect the area during mid-week. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... 850 mb ridge will be drifting across central and eastern PA late this afternoon and early this evening. The waning boundary layer northwest flow around that level will mean a brief erosion of the lower layer of stratocu beneath an increasing/thickening shield of mid and high level clouds. So, mostly cloudy to ovc skies, dry conditions and light variable sfc winds will persist for the rest of the day - through a few hours after dusk. Temps late this afternoon will range from the mid and upper 30s across the Alleghenies...to the lower-mid 40s in the central and southern valleys. Latest 09Z/12Z Ensembles and Oper Models have been quite consistent with the onset timing (and type) of precip that will advance quickly NE later this evening within an area of robust llvl warm air advection aloft (and along/ahead of a potent mid/upper level vort max). The first few hours of the precip (after it arrives in the west this evening...and across the east around midnight) will be snow in most areas (with perhaps some sleet/rain mixed in across the south where the wet bulb 0C level will be AOA 1500 ft AGL). A longer period of snow will occur across the central and northern counties of the state...leading to a light accum of a coating to 2 inches. The Laurels may drop near or just below freezing for a period in the middle of the night as the temps begin to rise above 0C aloft and turn the precip to liquid form. This could allow for some FZRA in the southwest - most likely confined to the (highest) elevations. Still not enough confidence in temps being below freezing for very long where it does turn to rain in order to post a FZRA (or winter wx) advisory. Have mentioned it in the grids, and HWO but kept out mention of specific FZRA accretions. There will be enough precip falling through the initially deep layer of dry air and decent wet-bulbing via the 20-something dewpoint air to make a light coating of snow/sleet across much of the area. Low temps will be mainly in the 28-32F range across the north where the cold air is deep enough to support mainly snow...while readings in the l-m30s are expected across the central and srn valleys Will maintain a general 1-2" in the north and an inch or less in the central and a little less still in the south. The precip could turn to rain at the very end, but will likely just fizzle into some patchy drizzle. This, too, could spell trouble if it lingers into rush hour Monday morning and if temps can stay below freezing. && .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... The fast zonal flow aloft takes the best forcing for precip off to the east of the area early Monday. Will linger some sct -SHSN or --RA at best in the east and along the west/high elevations through about 15z Monday, but dry elsewhere. Low clouds and the patchy, very light precip early will break for some afternoon sunshine as a bubble (1020mb) of sfc high pressure slides quickly east from the Ohio Valley. High temps Monday afternoon will vary from about 40 across the higher terrain of the north and west to near 50F in the SE. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... Several successive systems will be moving through the region this upcoming week. This will allow for a more tumultuous weather pattern. The first will have a former cutoff low from old Mexico as it`s forcing mechanism. The low will cross the border early Monday and open up as it slides NE into the OH valley Mon night and overhead on Tuesday. A broad but weak low will form over the lower MS valley and perhaps do a split with one spot of lower pressure going up the OH river and another one more to the south which would eventually move over the DelMarVa peninsula. The 850 MB temperatures show warm air advection aloft. The big forecast question is the timing of frozen precipitation Monday night through Tuesday. The current models have a weak upper level trough that should only provide weak forcing. Without a very solid low moving to our south, there still remains some question whether all the precip will remain frozen. Similarly, without a distinct low going to the west of us, there is a question that the warm air advection will push over the entire area and turning things to all rain. Have transition periods of change precipitation from 09Z Tuesday to 18Z Wednesday. The main area of mixed precipitation remains over the central mountains. The southeast should have a period of mixed precipitation before switching to rain. The NE third of the area may stay cold enough to allow for an inch or so of snow. There is a chance for freezing rain, though the precip will be falling during the day, and temps should rise above freezing after the precip begins but probably before noon passes. Even the northern mountains should get into the mid to upper 30s. The end of the week system brings another chance for precipitation. This system is out ahead of a deep upper level low moving through the Upper Great Lakes. There is some model disparity on the timing however have increased POPS into Friday night as the precip looks like a good bet around the Thu/fri period and all solutions eventually bring this trough through. So predictability is increasing, but timing issues remain. Best bet for precip at this point is mainly rain over the SE as decent push of warm air lifts ahead of the system, with a rain/snow mix then a switch to snow as 850 mb temps are trending from -8 to -12 C. Behind the trough, looks like a chilly weekend in store with a potential sharp drop off in temps. && .AVIATION /21Z SUNDAY THROUGH FRIDAY/... Mean ridge in the 3-6 kft agl layer was over central PA at mid afternoon. A few lingering areas of MVFR ceilings across the Western and Northern Mountains, with VFR ceilings across central and eastern airfields will be replaced by a thickening layer of mid and high clouds late today/early tonight. This cloud deck will quickly lower to MVFR between 03-06Z...then areas of IFR late. Light precipitation moves in early tonight west...and shortly after midnight across the east. Mainly rain in the southeast...a rain/snow mix in central areas...and mainly snow in the north and northwest. Outlook... Mon...No sig wx expected. Tue...Mixed precip/low CIGs possible, especially late. Wed...Lingering low CIGs possible. Thu...Mixed precip/low CIGs possible. Fri...Blustery and much colder with NW mtn and Laurel Highland restrictions in SHSN and areas of enhanced Lake Effect Snow. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Lambert NEAR TERM...Lambert SHORT TERM...Dangelo/Lambert LONG TERM...Ceru/Gartner AVIATION...Lambert

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