textproduct: State College

WHAT HAS CHANGED

* Refined timing of snow onset/location for Friday.

KEY MESSAGES

1) Lake effect/upslope snow showers gradually tapering off tonight while wind chill stays in the single digits above and below zero.

2) Snow squalls are possible Saturday in the western half of the CWA as a strong cold front blasts through. Little to no impact east.

3) An arctic blast Monday will produce little snowfall, but lead to dangerous, sub-zero wind chills Monday night and Tuesday.

DISCUSSION

KEY MESSAGE 1...

SHSN ongoing. All signs continue to point to strongest LES/SHSN subsiding early this evening in the Laurels and overnight in the northern Alleghenies. While snowfall amounts thus far have been light, it has also been fairly fluffy/dry. I-80 corridor in western PA continues to get hit with a long-duration LES band. Thus far, this band is turning weaker, more-cellular/broken and is staying just a couple of miles N of I-80 in Clearfield Co. Expectation is that this band will sink south far enough as it slides into Clearfield Co that we could need SQWs at some point.

As we near sunset, the upper trough axis overhead will be sliding east of the CWA. However, continued WNW flow will keep the upslope and LES going for a few more hours along and N of I-80. Not enough support for widespread accums of 3+" to pull the trigger on an advy for the I-80 corridor nor Elk/Cameron Cos, though. Timing of LES subsiding over the NW still looks like midnight, but some more-spotty and light SHSN will linger into the wee hours.

As the wind subsides through the night to 5-10 with G15-20KT, wind chills should only dip a little at first, then level off or even rise 5F in the second half of the night.

KEY MESSAGE 2...

Only light snow due to WAA is expected Friday aftn in the W with an inch or so poss in the far NW as the highest SF totals. After a brief break, Friday night and early Sat, large-scale lift increases very slightly, but we'll be under the left- entrance region of a 150KT jet. The main forcing will be with a broad/channeled vort max caught in the SWrly flow aloft. The heights will actually be rising overhead. But, enough energy in the mid-level flow and a sharp cold front will provide an active Saturday. A little lift ahead of the front could make a very light snow patch during the morning, but timing looks favorable for SNSQs in the aftn. Sunlight and modest heating could make them stronger. Daytime maxes will be in the 40-45F range in the Lower Susq and s-cent mtns. The orientation of the flow will still be SW- NE and may confine the SHSN/SNSQs to the NW half of the CWA. The dry air and negative influence of the upper pattern could keep the SE third to half of the CWA dry/snowless.

KEY MESSAGE 3...

Another lull on Sat night into Sunday night will be followed by another - colder - cold front. 8H temps drop into the -20/-23C range for Monday. Despite the blast of bitterly cold arctic air, it will already be dry over the CWA. The front will likely generate little snowfall outside of lake effect. Min wind chills Monday night/Tuesday morning range from 0F in the LSV to minus 20F over portions of the Alleghenies which suggests a cold weather advisory may be needed. We have highlighted the extreme cold risk in the HWO.

Fcst highs in the + single digits to ~20F on Tuesday are 15 to 25 degrees below the historical average. A long duration period of sub zero wind chills is likely from Monday night through Tuesday night/AM Wednesday. With all the arctic air in place and a flow out of the west, we should have LES when the flow is best favorable at the end of the forecast period, but waggles in the mid-level flow are expected. So, a broadbrushed and decreasing PoP strategy across western PA is best at this point due to the low certainty of the details.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

A strong surface cold front pushed east of the Delaware Valley around daybreak today, separating temps in the 30s just in its wake from 20s across the Central part of the state and teens in the west.

Satellite/Radar combo shows a well defined mid/upper level upper low over NW PA/SW NY with multiple, narrow bands of snow showers rotating around its south side and into the wrn mtns of PA.

For today, we'll see persistent IFR over the Western Mtns of PA including airfields KBFD, KFIG and KJST, while some improvement from MVFR to periods of VFR, BKN-OVC cigs will occur across the rest of the Central Ridge and Valley airfields in Central PA and the Susq Valley.

As the cold air pushes east/deepens, the depth of vertical mixing will top out at 4-5 KFT and help to transport stronger west/northwest winds of 30-40 KTS down to the surface in gusts.

Current gusts in the TAFs (in the 20s to near 30KT) may be 5 to 10 kts on the low side, especially during the second half of this current 06Z TAF cycle. Look for possible upward adjustments to sustained wind speeds and gusts with TAF updates today.

Snow will linger over the north and west with the system becoming more lake effect and upslope dominant under deep west-northwest flow. Airfields UNV and west have 30-50% chance of seeing snow throughout the day. Elsewhere, snow will tapper off by early morning. A blustery west/northwest wind will develop and then continue Thursday with gusts in the 30 to 35 kt range, possibly even exceeding 40 kts at airfields across the southern half of PA late Thursday / Thursday evening.

Outlook...

Fri...Snow and IFR most likely over the NW airspace. Breezy with gusts from 180 degrees 15-25kt.

Sat-Mon...Light snow/IFR possible.

CTP WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory until 1 AM EST Friday for PAZ004-005. Winter Weather Advisory until 1 PM EST this afternoon for PAZ006-037. Winter Weather Advisory until 5 PM EST this afternoon for PAZ024-033.