textproduct: State College

WHAT HAS CHANGED

* The Winter Weather Advisory has been expanded to include Southern Centre, Southern Clinton, Blair, and Somerset Cos. * Increasing potential for heavy snow and moderate to major winter storm impacts Saturday night through Sunday night

KEY MESSAGES

1) Quick-hitting snowfall to disrupt travel through the Wednesday evening commute along & west of I-99/US-220

2) Increasing odds for heavy snow and moderate to major winter storm impacts Saturday night through Sunday

3) Arctic blast with frigid temperatures and sub-zero to dangerous wind chills expected from late week through the weekend into the first part of next week

DISCUSSION

KEY MESSAGE 1: Quick-hitting snowfall to disrupt travel through the Wednesday evening commute along & west of I-99/US-220

Radar and model trends would suggest and earlier end to the accumulating snow the NW mtns (tapering to light snow or patchy fzdz) with main surge of snow now focused along SWly LLJ/max pwat axis pointed across the Laurel Highlands into the south central Alleghenies. Snow was able to overcome very dry air near the ground and will likely make for a slippery late afternoon and evening commute particularly near and NW of the I-99/US220 corridor. We expanded the winter wx advisory a bit farther to the south and east earlier today to account for the shift in the snowfall and expected travel impacts around the evening commute.

Snow intensity/rates taper off into tonight with pockets of light snow or fzdz into early Thursday morning as higher level moisture peels off to the northeast. This could make for slippery spots overnight into the Thursday morning commute.

KEY MESSAGE 2: Increasing odds for heavy snow and moderate to major winter storm impacts Saturday night through Sunday

Confidence continues to increase in the potential for heavy snow and moderate to major winter storm impacts across central PA this weekend. As is often the case, the primary model uncertainty differences center around the interaction between the northern and southern branches of the jet stream which will ultimately steer the storm track and associated placement of winter wx/max snowfall. Latest models have doubled-down on the northward trend that was very evident in the previous 21/00Z model cycle. Big ? remains will that trend hold or will there be a reversion back to the south.

What we know at this juncture: 1. Ptype should be all snow as arctic air remains locked in place

2. Most likely timing for heavy snow is Saturday night through Sunday night

3. Odds of >6" and >12" of snow have trended higher (maximized over south central/southeast PA) with an increasing risk for moderate to major winter storm impacts this weekend

What we don't know yet: 1. Exactly when, where and how much snow will fall

KEY MESSAGE 3: Arctic blast with frigid temperatures and sub- zero to dangerous wind chills expected from late week through the weekend into the first part of next week

Initial shot of Arctic air will arrive on Friday and will be reinforced through the weekend into next week. The frigid to much below normal temperatures are expected to continue through the end of January.

Sub-zero to dangerous wind chills are most likely Friday night into Saturday morning (preceding the winter storm) and again Monday night into Tuesday morning. Cold wx advisory or extreme cold warnings may be needed.

The arctic air would favor a dry, fluffy snowfall over the weekend and be more prone to blowing/drifting impacts.

Given the prolonged nature of the cold, we may begin to see more infrastructure impacts (frozen pipes, for instance) by next week.

AVIATION /19Z WEDNESDAY THROUGH MONDAY/

Overcast skies are continuing to build in ahead of the arrival of snow from the west. VFR conditions will continue until precipitation begins to fall. Most guidance shows snow beginning to fall at BFD between 18Z and 20Z, and at JST by 21Z. Current radar is showing reflectivity returns over the northwestern airspace, but the air at the surface is too dry for snow to reach the ground just yet. The HRRR and RAP also show steady snow impacting AOO and UNV after 21Z. Some snow is possible as far as east as IPT, but low level dry air should limit its intensity. MDT and LNS will likely remain dry. Sites that see steady snow will see IFR/LIFR visibility and BFD and JST will likely have IFR ceilings as well.

Winds have increased into this afternoon to 10 to 15 knots. An approaching 50-60kt LLJ will lead to LLWS at all TAF sites this afternoon and evening.

Snow mixes with or changes to freezing drizzle this evening, leading to improving visibility. Wind gusts also increase overnight, with gusts of 20 to 25 knots possible. Heavier snow showers redevelop over northwest PA late tonight and may move through BFD prior to 12Z. These snow showers would likely cause brief periods of IFR visibility.

Thu...CFROPA, gusty. LIFR/IFR cigs/vis in SN N/W, MVFR cigs SE.

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

Sat-Sun...Dry to begin Sat. SN begins to enter south-to-north late Sat continue through Sun. IFR possible, most likely S PA.

CTP WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory until 10 AM EST Thursday for PAZ004>006-010>012-017>019-024-025-033-045.