textproduct: State College

SYNOPSIS

* Gusty winds and lake effect snow showers will bring hazardous travel to Central PA once again today. * A weak clipper system will bring light snow to Western and even parts of southern PA tonight. * A more significant system and second surge of arctic air will bring several inches of snow to locations west of US-219 on New Years Eve, along with a threat of snow squalls along the frontal boundary.

NEAR TERM /UNTIL 6 PM THIS EVENING/

KEY MESSAGES: - Wind chills this morning will range from zero to 5 below across the Northern and Western Mountains of PA to the single digits and mid teens elsewhere. - Frequent wind gusts over 40 mph are expected at least through this morning, with high end gust potential over 60 mph over the Laurel Highlands and points immediately to the east. - Another round or two of significant lake effect snow accumulation is expected over northwest PA. --------------------------------------------------------------

Continued strong low-level west-northwest flow with vertical mixing between 3-4 KFT AGL will continue to tap 40-50+ KT wind this morning from just 2-3 KFT AGL and mix it down to the surface in wind gusts of similar magnitude.

Peak gusts should gradually decrease late this morning through this afternoon, but should still be gusting into the 30s (KTS) later today through this evening based on multi-model forecast sounding data.

Frigid wind chills will be an issue this morning (apparent temperatures +/- single digits). These values don't quite reach cold weather headline criteria (-10F).

A favorable WNW LLVL fetch off the lakes along with upslope effects (and pulses of omega within the DGZ) should support efficient snowfall accumulation in the Laurels through this afternoon as well. Extended the Lake Effect Snow Warning across NW PA through 12Z Wed, while Wint Weather Advisories further south will be allow to expire at 18Z today.

Outside of the Wint Weather Advisories and Warnings, narrow and slowly moving bands of LES will continue to bring mainly light accums of a coating to 2 inches.

Max temps today will range from about 20F across the NW Mtns and ridge tops of the Laurel Highlands to around 32F across the larger metro areas in SE PA.

SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/

Snow showers and areas of more persistent, heavier LES will retreat to the north and possibly out of our CWA temporarily as the mean flow within the well-mixed layer backs to about 240 deg later tonight through Wednesday.

Hi-res guidance depicts a compact shortwave drifting southeast across western PA on this evening into Wednesday morning. This will bring additional, mainly light, snowfall accumulation to the laurel highlands and southern 1-2 layer of PA counties, thanks to enhanced and deeper Uvvel beneath the left exit region of a 100-100 kt upper level jet that will be moving along/over the Mason/Dixon Line.

A local enhancement of snow in the northwest mountains could also necessitate a short-fused Advisory, but impacts should be fairly limited. In the wake of the shortwave, winds shift around to a more westerly fetch, which should also direct lake effect snow bands back into the NW Mtns, especially to the North of Route 6.

LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/

On the large-scale, a cold, cyclonic NW flow pattern will reside over much of the eastern CONUS this period, underneath a deep upper low in eastern Canada. On Wednesday afternoon (New Years Eve), a wave of low pressure will drift eastward across the Great Lakes, and drag an Arctic front south through the Commonwealth. Underneath a favorable left exit region of an upper jet, this front could produce a threat of snow squalls late Wednesday evening into the New Year. Anyone planning to travel for the New Year should be prepared for the risk of snow squalls. If a Snow Squall Warning is issued, delay travel or exit the highway at the next opportunity. A snow squall produces blizzard-like conditions with gusty winds, heavy snow, and significantly reduced visibility.

In addition to the threat of snow squalls, fairly robust moisture associated with this system will bring a more potent batch of snow to the region. Snowfall totals over 4" are possible west of US-219 with localized amounts >6" in the higher elevations of western Somerset County as well as in the typical lake effect zones of northwest PA. A Winter Storm Watch is now in effect for Somerset County from 4PM Wednesday through 10AM Thursday for the significant snowfall potential on the western ridges. Additional Watches/Warnings may be needed in northwest PA along with advisories along the spine of the Alleghenies if confidence continues to increase.

In the wake of this system, a reinforcing shot of arctic air later Thursday into Friday should support wind chills in the single digits and teens once again. Then, temperatures should gradually moderate this weekend, as heights aloft slowly start to build.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

Widespread VFR outside of lake effect and upslope snow showers that will continue to impact BFD/JST with high (~80-90%) confidence, AOO with moderate (~50-60% between 12-14Z Tue) and UNV/IPT with low (< 30%) confidence. Recent GLAMP guidance outlines IFR visibilities within snow showers through ~18Z Tue, which matches current 12Z Tue observations fairly well. Brief periods of LIFR will be possible, mainly at BFD/JST between 12Z-15Z Tue where locally heavy snow bands could make runs at the airfields. Even with snow showers subsiding at BFD/JST, a combination of NBM/GLAMP/HREF model guidance continues to outline a pesky low-level cloud deck (between 1500-2500ft AGL) continuing throughout the entire 12Z TAF package. Given this reasonable agreement based off of RAP model soundings, there is moderate-to-high (~70-90%) confidence in MVFR persisting. Model guidance does reintroduce snow shower chances across western Pennsylvania after 00Z Wednesday with best chances for snowfall (~50-60%) at BFD/JST. Further south/east, models show some disparities, thus have limited mentions to UNV/AOO/IPT where there remains approximately a 30% chance of snow making it into the airfield.

A slight decrease in winds have been observed overnight across central Pennsylvania with only isolated instances of gusts exceeding 40kts over the past couple of hours. Winds will continue to gust above 30kts, especially at higher elevation locations such as BFD/JST/AOO over the next couple of hours along with eastern terminals (IPT/MDT/LNS) before gradually decreasing after 18Z Tue. Gusts upwards of 20-25kts appear more plausible, with LLWS concerns minimized as the 850mb LLJ has shifted east of the forecast area.

Outlook...

Wed...Chance of snow showers across the north and west, dry southeast.

Thu...Chance of snow showers.

Fri...Mainly dry. Slight chance of a snow shower at BFD.

Sat...VFR.

CTP WATCHES/WARNINGS/ADVISORIES

Wind Advisory until noon EST today for PAZ004>006-010>012-019- 026>028-035>037-041-042-045-046-049>053-056>059-063>066. Lake Effect Snow Warning until 7 AM EST Wednesday for PAZ004- 005. Winter Weather Advisory until 1 PM EST this afternoon for PAZ010-017-024-033. High Wind Warning until 4 PM EST this afternoon for PAZ017-018- 024-025-033-034. Winter Storm Watch from Wednesday afternoon through Thursday morning for PAZ033.