textproduct: State College

SYNOPSIS

* A period of accumulating, though mainly light snow is expected to begin this afternoon across the west and early this evening over the Susquehanna Valley. The snow will continue into early Sunday, followed by another Arctic blast and locally heavy lake effect and upslope snow across NW PA. * The northwest wind will become gusty on Sunday leading to potentially dangerous wind chills when combined with the cold air. * A period of milder conditions is expected for the middle of the week ahead.

NEAR TERM /UNTIL 6 PM THIS EVENING/

Sfc high pressure just east of the Delaware Coast early this morning has a ridge extending north across the Hudson Valley of New York State. Widespread mid and high clouds were spreading east and over this ridge, and precip we're seeing on regional MRMS radar Mosaic.

A large area of light snow associated with the left exit region of a 90-100 kt westerly jet at 500 mb will begin across the Laurel Highlands first - during the early to mid afternoon, before expanding northward across the Alleghenies for the late afternoon.

Temps to start the day today will range from 10 to 15 degrees across the north to upper teens and low 20s in the southeast. Afternoon highs will reach 25 to 30 acrs the Northern and Western Mountains and mid 30s to lower 40s respectively from the Central Valleys to the Greater Philadelphia area.

SHORT TERM /6 PM THIS EVENING THROUGH MONDAY/

The aforementioned surface low/clipper system will arrive Sunday 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 upper level jet dynamics moving with this trough. The deep trough and strong forcing/cold air aloft 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 /09Z SATURDAY THROUGH WEDNESDAY/

Clouds will thicken up from the west early this morning, with MVFR cigs spreading into UNV and IPT by 11-16z. Any snow during the daylight hours will likely be on the light side. JST has the highest chance of MVFR vsby restrictions and IFR cig restrictions prior to 21z. As steadier snow moves in after sunset, a 4-8hr period of LIFR vsby will be possible at JST. IFR vsbys are likely at BFD and AOO tonight, but chances are lower for IPT and UNV as snowfall intensity will weaken as precip crosses the Allegheny Front. However, another area of forcing is progged to initiate an area of steady light to moderate snow in the lower Susq Valley, likely impacting MDT and LNS with IFR vsbys after 02z Sun and continuing through 07-09z. LNS will likely be worse than MDT with the heaviest snow falling to our SE (Philly/DC).

It will get windy (gusts 25-35kt) on Sun and into early Sun night as very cold air moves in. The western terminals (BFD and JST) may have a few hour break Sun AM with little snow 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 this morning to 7 AM EST Monday for PAZ004-005. Winter Weather Advisory from 10 AM this morning to 7 AM EST Monday for PAZ010-017-024-033. Winter Weather Advisory from 7 PM this evening to 1 PM EST Sunday for PAZ065-066.