textproduct: State College

WHAT HAS CHANGED

* Increased WSW sustained winds and gusts by ~5-7 KTS Monday afternoon and early evening, with peak gusts near Advisory criteria of 40KT across the Laurel Highlands * Increased chance for snow showers and isolated snow squalls over the Laurel Highlands and northwest mountains on Monday. * Expanded the Cold Weather Advisory to include all counties north of I-80 and along/west of I-99.

KEY MESSAGES

1) Scattered snow showers and a few snow squalls possible today ahead of an arctic front.

2) West-southwest winds will gust 30-40+ mph this afternoon, with the highest gusts expected in the Laurel Highlands.

3) Dangerous, sub-zero wind chills tonight into Tuesday.

4) Active weather continues through the end of the week with below normal temperatures and multiple chances for snowfall.

DISCUSSION

KEY MESSAGE 1: Scattered snow showers and a few snow squalls possible today ahead of an arctic front.

A warm front will move eastward across northern Pennsylvania Monday morning as an area of low pressure tracks well to our north. A few light snow showers (vsbys 3-6SM) will be possible as this front moves through, but any accumulation will be very light. Upslope flow over the Laurel Highlands will also support scattered snow showers, though the better chance for snow appears to be north of Interstate 80 ahead of an arctic front that moves across the region during the early afternoon. The snow squall parameter from both the NAM and the GFS highlight the northwest mountains for the best chance of heavier snow showers as weak instability develops ahead of the front. The flash freeze risk appears to be relatively low as surface temperatures will be well below freezing, but visibility drops and quick accumulations on roadways are possible.

Lake effect snow showers may linger across northwest PA for a few hours behind the front, but low-level flow backs to the southwest during the evening. This will help to direct the majority of the lake effect snow bands into New York.

Key Message 2: West-southwest winds will gust 30-40+ mph this afternoon, with the highest gusts expected in the Laurel Highlands.

Vertical mixing increases to 6-7 KFT AGL Monday afternoon/early evening and coincides with the passage of a westerly 850 mb jet max of 45 to 50KTS and fairly strong cold advection. 3 hourly surface pressure rises peak around 5 mb late Monday afternoon, and based on this scenario and accentuation of the llvl wind by the mixing and isallobaric component, we raised sustained and peak gusts by 5-7 kts for this afternoon/early evening. Held off on a Wind Advisory at this time for the Laurel Highlands, but will continue to assess as additional guidance comes in.

KEY MESSAGE 3: Dangerous, sub-zero wind chills tonight into Tuesday.

A frigid airmass characterized by 850 mb temperatures as low as -20C moves into Pennsylvania behind the arctic front tonight and Tuesday. Minimum temperatures tonight will range from near zero in the Laurel Highlands and across the northwest mountains to the low teens in the Lower Susquehanna Valley. These very cold temperatures will combine with winds gusting 25 to 30 mph to produce wind chill values as low as -10 to -20 for areas north of I-80 and west of I-99. We have issued a Cold Weather Advisory for those locations. Temperatures slowly rise to the teens and low 20s for Tuesday afternoon, but wind chills likely remain sub-zero for the northern mountains into Wednesday morning.

KEY MESSAGE 4: Active weather continues through the end of the week with below normal temperatures and multiple chances for snowfall.

Temperatures moderate slightly for the end of the week, though they will remain below normal with highs in the 20s and low 30s. After a brief dry period, snow chances return to parts of the region late Wednesday into Thursday as the next shortwave rotates through the persistent trough over the eastern US. The typical lake effect and upslope snow regions will once again be favored as the surface low is expected to pass well to our north.

Temperatures trend downward as we head into next weekend, though there is a lot of spread in the ensembles as to how low they actually get. The upper level pattern supports additional chances for snow with additional shortwaves moving through the trough, but the track and timing of these systems remains unclear. Recent guidance favors a southern wave with significant snow possible south of PA this weekend, but it is far too early for anything except continuing to monitor the forecast.

AVIATION /09Z MONDAY THROUGH FRIDAY/

A broad area of stratus with cloud bases of 2000-4000 ft was pushing across the Ohio valley at 10z Monday ahead of an approaching Arctic cold front. BFD, JST, and AOO have already seen cigs drop into the MVFR category, and UNV will likely be next (60 pct chc by 16z). Light snow coming out of this area of stratus may also bring vsby down to MVFR levels.

The Arctic cold front will push east through central PA between 17z and 23z. Wind gusts from the SW and W will pick up into the 30s and possibly around 40KT near KJST and KAOO thanks to some significant 3 hourly surface pressure rises of around 5 MB.

Solar warming will generate some llvl instability as the cold air rushes in. This will create isold- sct SHSN for the western terminals, and perhaps even a stronger squall (mainly near FIG and BFD). Outside of snow showers, cigs generally lift with drier air behind the cold front.

TUE...Gusty. MVFR/IFR NW, VFR elsewhere.

WED...Light snow/IFR NW, 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.

CTP WATCHES/WARNINGS/ADVISORIES

Cold Weather Advisory from 9 PM this evening to noon EST Tuesday for PAZ004>006-010>012-018-019-025-034-037-041-042-045. Cold Weather Advisory from 9 PM this evening to 1 PM EST Tuesday for PAZ017-024-033.