textproduct: State College

SYNOPSIS

* Arctic cold front brings light snow, gusty winds and travel impacts to the Allegheny Front late tonight-Thursday morning * Plummeting temperatures into Thursday night with frigid to record challenging lows Friday morning * Reinforcing cold pattern locks in below average temperatures through the second week of December

NEAR TERM /THROUGH THURSDAY/

An arctic cold front will plow southeast across the Lower Great Lakes late tonight and reach northwest PA by early Thursday morning. Light snow/snow showers with minor accum (generally <1") will accompany the front over the western Allegheny Front (particularly along and west of Route 219) and could spill as far east as the I99 corridor into the peak morning commute. We issued a SPS in coordination with PBZ and LWX to highlight possible (sub-advisory level) travel impacts/disruptions.

SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/

Arctic front sweeps through CPA Thursday morning exiting the Lower Susq Valley by 18Z/1PM. Frontal lift combined with orographics and increasingly cold NW flow downwind of Lake Erie favors persistent snow showers over the NW mtns and Laurel Highlands through the morning hours.

Snow squall risk is still in-play to some extent with potential for linear development along the arctic front; however the best signal remains pegged to the north of CPA in update NY. So overall confidence remains low, but threat is non-zero.

Fcst snow amounts over the western high terrain remain light (1" or less) with locally higher amounts up to 2" possible in the favored lake effect belt in Warren County and ridgetop ski resorts in the Laurel Highlands.

Temps will plunge behind the arctic front as blustery NW flow sends readings falling NW->SE by Thursday afternoon. High pressure building over CPA will decrease the wind Thursday evening, setting up a frigid night with record challenging min temps fcst in the single digits to lower teens (15-20F below the historical average). It won't take much wind to drop wind chills below zero especially over the western mtns as gradient picks back up by Friday morning.

Quite cold, mid-winter feel to end to the week with fcst highs below the freezing mark in most of the CWA.

Low amplitude, jet-induced low pressure system tracking well south of PA and developing off the NC coast could brush the southern tier of CPA with light snow on Friday. Latest NBM probs for >1" are generally less than 30% for areas south of the PA Turnpike (slightly higher toward 50/50 odds southeast of US-30 from Chambersburg to Lancaster). This could impact travel early Friday so we will continue to monitor; but for now the most likely outcome is for snow accum to stay south of the MD line. Weak sfc ridging Friday night holds low cloud cover beneath the subsidence inversion with min temps trending 5-15 degrees warmer vs. Thursday night (albeit from near record lows!).

LONG TERM /SATURDAY THROUGH WEDNESDAY/

Series of reinforcing shortwaves offer opportunities for snow late in the weekend and early next week; however the main theme in the long term will be the cold pattern with below normal temps expected to last through the second week of December.

AVIATION /00Z THURSDAY THROUGH MONDAY/

Stratus with MVFR or lower cigs will persist over the NW mountains and Laurels (BFD and JST) for much of tonight. UNV and IPT are right on the edge of the stratus deck, and thus lower confidence for MVFR conditions (both VFR as of 01z). Latest satellite imagery also shows a bit of clearing around JST, but much of the model guidance shows a trend towards IFR and even LIFR by 06z Thu with the potential for light freezing drizzle or flurries from that point until cold FROPA Thursday morning.

Main issue for late tonight into Thursday will be the passage of a strong cold front. This will bring the potential for heavy snow showers, mainly at BFD and JST, but perhaps also into UNV, AOO, and maybe IPT between late tonight and mid day Thursday. The cold air and low level moisture is not real deep, but the airmass behind the front is abnormally cold, which has the potential to compensate by providing low level instability. There is also some potential for marginal LLWS right ahead of the cold FROPA, but in general the shear will be dissipated by mechanical mixing via wind gusts.

The weather for Friday into Monday will feature weather systems to the southeast and northwest of our area, so while low CIGS and snow showers may prevail at times, not seeing any big storms at this point.

Outlook...

Fri...Cold start to the day with temps 0-15F. Slight chance of light snow across the southern half of central PA with restrictions possible.

Sat-Mon...Restrictions possible, chance of snow showers in the NW.

CLIMATE

Cold temperatures in the wake of the Arctic Front will bring frigid temperatures to central Pennsylvania, the coldest of this winter thus far. Multiple locations across the area will be challenging record low temperatures on December 5th:

Location|Record Harrisburg | 12 (1926) Williamsport | 8 (1926) Bradford | 0 (2007) Altoona | 13 (1966)

CTP WATCHES/WARNINGS/ADVISORIES

None.