textproduct: State College

WHAT HAS CHANGED

* Increased expected snowfall amounts for Wednesday night into Thursday, prompting the issuance of a Winter Storm Watch for Cameron, Clearfield, Elk, McKean, Potter, and Warren County from 12PM Wednesday to 10AM Thursday.

KEY MESSAGES

1) Dangerous, sub-zero wind chills gradually rise this morning; warming trend continues through Thursday afternoon.

2) Quick-hitting winter storm to bring a heavy thump of snow and localized accumulations >4-6" in northwest PA on Wednesday afternoon into Thursday morning.

3) Monitoring the track of a significant winter storm this weekend; highest impacts will remain south of PA, but northward extent of accumulating snowfall remains uncertain.

4) Frigid temperatures return this weekend and likely stay around until the end of January.

DISCUSSION

KEY MESSAGE 1: Dangerous, sub-zero wind chills gradually rise this morning; warming trend continues through Thursday afternoon.

Numerous locations across the Alleghenies have reported wind chills of -10F or lower this morning, with temperatures near zero west of I-99 and north of I-80 and in the single digits southeast toward the I-81 corridor. Wind will remain a bit gusty today before high pressure builds in tonight. A Cold Weather Advisory remains in effect until midday with dangerous wind chills expected to continue this morning.

As high pressure builds in and moves just southeast of PA tonight, winds will taper off and one more cold, clear night is expected with lows in the single digits areawide. High pressure moves off the coast on Wednesday, bringing gusty southerly winds. In the summer time these winds would be refreshing, but - despite bringing moderating temperatures - they'll keep wind chills in the teens and 20s all day on Wednesday. Additional "warming" is expected for Thursday ahead of a cold front as highs trend a few degrees above normal in southeast PA.

KEY MESSAGE 2: Quick-hitting winter storm to bring a heavy thump of snow and localized accumulations >4-6" in northwest PA on Wednesday afternoon into Thursday morning.

After a brief dry period, snow chances return to parts of the region late Wednesday into Thursday as deep layer WAA develops at the nose of a potent 50-60 kt llvl southwesterly jet and beneath the right entrance region 80-90 kt 500 mb jetlet - ahead of the next shortwave rotating through the persistent trough over the eastern US. The typical lake effect and upslope snow regions of the Alleghenies in Western PA will once again be favored as the surface low is expected to pass well to our north.

The latest medium-range guidance highlights the potential for several hours of snow rates of 1 inch per hour or greater Wednesday evening in the northwest mountains. This could lead to rapidly deteriorating road conditions and a quick accumulation of 4"+ of snow. Based on an increasing probability of snowfall amounts over 6", we decided to issue a Winter Storm Watch for the northwest batch of counties. Snowfall amounts will cut off rather quickly with southeastward extent, and little to no snow is expected east of I-99 or south of I-80.

There may be a lull in the snowfall early Thursday morning before another strong Arctic front helps invigorate scattered to numerous snow showers and squalls on Thursday. The setup may end up being similar to Monday, though current guidance keeps the threat a little bit farther north. Regardless, anyone one with travel plans in northwest PA on Wednesday afternoon through Thursday morning should monitor the forecast for additional updates. Temperatures will drop off quickly Thursday night, dipping back into the single digits in the northwest mountains.

KEY MESSAGE 3: Monitoring the track of a significant winter storm this weekend; highest impacts will remain south of PA, but northward extent of accumulating snowfall remains uncertain.

This weekend, a strong 500mb trough will dig into the Central US over top of anomalously frigid Arctic air. Meanwhile, a surface low will develop across the southern Plains ahead of an advancing southern stream trough paving the way for a potentially significant winter storm from Oklahoma to Virginia. Importantly, the right entrance region of the 500mb jet is progged to overlap with the left exit region of a jetlet ejecting from the aforementioned southern stream shortwave, creating ideal lifting conditions for heavy snowfall in the mid Atlantic Saturday night into Sunday. There are subtle differences in timing among the suite of ensemble guidance, but confidence is increasing in at least light snowfall accumulation all the way up to I-80. The highest snowfall amounts in our area will likely be in southeast PA, with specific totals contingent on the track of the storm. Stay tuned for updates to the forecast!

KEY MESSAGE 4: Frigid temperatures return this weekend and likely stay around until the end of January.

A sprawling ridge of high pressure will move into the lower 48 on Saturday in the wake of a strong Arctic front. This system will usher in a period of frigid air with temperatures as much as 30 degrees below average in the climatologically coldest part of the year. Highs on Saturday may struggle to make it out of the single digits at higher elevations across northern and western PA. The long range pattern favors a persistent eastern US ridge, which should keep below normal temperatures firmly in place through the end of the month. The Climate Prediction Center has placed much of Pennsylvania in a rare High Risk of Much Below Normal Temperatures for Tuesday January 27 - Friday January 30th. A couple storm systems may bring gusty winds, which would exacerbate wind chills and could necessitate additional Cold Weather Advisories or Extreme Cold Watches/Warnings.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

VFR conditions continue across all of Central PA outside of the far northwest where borderline MVFR/IFR ceilings persist through the morning. These clouds slowly lift through the day, but most guidance suggests that it takes until 00Z Wednesday for VFR to return. Wind gusts of 20 to 25 knots continue through the day before decreasing overnight. Mid and high clouds increase from the west after 06Z ahead of an approaching low pressure system.

WED...Snow/IFR NW in the afternoon, MVFR-VFR elsewhere.

THU...CFROPA, gusty. LIFR/IFR cigs/vis in SHSN N & W, MVFR cigs SE.

FRI...Gusty W wind. Lake effect BFD (ocnl IFR). Mainly MVFR elsewhere.

SAT...Widespread SN/IFR possible, esp S.

CTP WATCHES/WARNINGS/ADVISORIES

Cold Weather Advisory until noon EST today for PAZ004>006- 010>012-018-019-025-034-037-041-042-045. Winter Storm Watch from Wednesday afternoon through Thursday morning for PAZ004>006-010-011-017. Cold Weather Advisory until 1 PM EST this afternoon for PAZ017- 024-033.