textproduct: State College

SYNOPSIS

* A period of accumulating snow is expected from late Saturday morning into early Sunday, followed by another Arctic blast and locally heavy lake effect and upslope snow across NW PA. * Wind will become gusty on Sunday leading to potentially dangerous windchills when combined with the cold air. * Signs of a warmer pattern change on the horizon by the middle of next week.

NEAR TERM /UNTIL 8 AM SATURDAY MORNING/

Mainly clear skies are present over central Pennsylvania as of 9 PM this evening. Winds have slackened and become mostly calm after the gusts from earlier today. High pressure will slowly weaken and high clouds will build in from the west ahead of a clipper system progressing over the central US. Conditions will stay mainly dry tonight, but northwesterly flow ahead of the system could bring a few lake effect snow showers across the northwest by daybreak. Lows tonight will range from the low teens across the north to the upper 20s in the southeast.

SHORT TERM /8 AM SATURDAY MORNING THROUGH MONDAY/

The aforementioned clipper system will arrive Saturday afternoon supported by a digging 500mb trough and plume of moisture from the Pacific Northwest US. Snow will be the result of a fast moving cold front and strong jet dynamics moving with this trough. The deep trough and strong forcing will result in moderate snow over most of western Pennsylvania with widespread light snow over the remainder of the state. Upslope will aid in snowfall efficiency resulting in the highest snowfall amounts across the Laurel Highlands and in the northwest mountains. A Low pressure center will develop south of Pennsylvania by late Saturday afternoon, but will quickly move off the coast, allowing for additional snow to fall over the southeast and Lower Susquehanna Valley. Uncertainty is greatest with this low; however, and the speed and position could limit the snowfall totals.

By late Sunday morning the synoptic snow will largely be over, but upslope and lake effect precipitation will persist in the west along with a deep Arctic airmass. A tight pressure gradient is expected on the back side of the departing upper level trough, and an 850mb jet streak will transfer significant momentum to the surface. Wind gusts up to 40mph are possible over the Laurels, and with reinforcing cold air could bring dangerous windchills to the region on Sunday. Min wind chills near and below zero Saturday night through Sunday night/AM Monday are very likely. Max temps Sunday will drop by 10-20 degrees vs. Saturday with mean departures from climo on the order of 15 to 25 degrees. A few locations could see highs close to the daily record low max.

Lake effect/upslope snow showers continue through Sunday night into Monday before high pressure begins to build in from the south again.

LONG TERM /MONDAY NIGHT THROUGH FRIDAY/

Winds by Tuesday shift more to the west, then to the southwest by Wednesday, as a large high pressure system moves eastward to the southeast states. The large scale flow pattern becomes more zonal, as upper level heights rise to much higher values than we had seen lately. This will set the stage for temperatures climbing to above normal, at least for the period Wednesday into late Thursday.

A cold front by Friday will edge things back down some, but not the really cold that has been typical this month so far. The month has been nearly 10 degrees colder than normal up to this point, which is nearly half of December.

Earlier discussion below.

One more shortwave trough could brush the area with light snow early next week; otherwise there is growing confidence in a mid to late December thaw as the pattern transitions to a milder SW flow aloft with mean troughing shifted westward into the Central U.S. Dry wx is favored Tue-Thu with highs climbing toward the 40-50F range late in the week - possibly in advance of the next widespread precip event.

AVIATION /04Z SATURDAY THROUGH WEDNESDAY/

VFR conds, clear skies and widespread nearly calm winds overnight.

Wind will increase slightly on Sat. Clouds will thicken up from the west early in the AM, with light to mdt snow hitting the ground in the western highlands (esp JST) before noon local/17Z. L/IFR in SN will spread east and affect all of the area Sat aftn and night, but end in IPT/AOO/UNV first (by midnight local (05Z). JST will be LIFR for a 3-6hr period as they should have the steadiest snow of any of our terminals for the longest time Sat aftn/evening. IFR may (50%) linger closer to sunrise in the SE (MDT/LNS) with the worst conditions there between 04Z-09Z. LNS will likely be worse than MDT with the heaviest snow falling to our SE (Philly/DC). It will get windy on Sun and into early Sun night as very cold air moves in. The western terminals (BFD and JST may (40%) have a few hour break Sun AM before lake effect kicks back in, and cigs/vsbys drop again Sun late AM/aftn.

Outlook...

Sun...SHSN/IFC linger in western highlands. MVFR cigs/isold SHSN AOO/UNV. MVFR/VFR cigs elsewhere. Gusty west wind.

Mon...MVFR w/sct SHSN BFD, no sig wx elsewhere.

Tue-Wed...No sig wx.

CLIMATE

Astronomical winter (solstice) begins at 10:03 a.m. on Sunday, December 21st.

CTP WATCHES/WARNINGS/ADVISORIES

Lake Effect Snow Warning from 10 AM Saturday to 7 AM EST Monday for PAZ004-005. Winter Weather Advisory from 10 AM Saturday to 7 AM EST Monday for PAZ010-017-024-033.