textproduct: State College

WHAT HAS CHANGED

* Slight increase in expected snowfall totals for tonight across the northwestern mountains and in east-central PA. The Winter Weather Advisory has been expanded to include Cambria County. * Updated minimum wind chills next week, which are approaching Extreme Cold Warning criteria (-20F) in the Laurel Highlands and Cold Weather Advisory criteria along and north of I-80.

KEY MESSAGES

1) Plowable snowfall tonight into Saturday with heavy snow showers and squalls Saturday afternoon.

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

3) Colder than normal temperatures continue through the end of the month with storm chances every few days.

DISCUSSION

KEY MESSAGE 1: Plowable snowfall tonight into Saturday with heavy snow showers and squalls Saturday afternoon.

A couple shortwaves rotating around the base of a prevailing eastern US trough will pair with an approaching low pressure system traversing the Great Lakes to produce multiple rounds of snow this weekend. The first arrives tonight across western PA as warm advection lift develops underneath the left exit region of a strong upper jet. Southwesterly flow will direct most of the moisture into the northwest mountains, though recent model runs have shown steadier snow making it as far south as Cambria County and the western ridges of Somerset County. These areas will likely see 2 to 4 inches of snow by daybreak.

During the day on Saturday, a couple reinforcing cold fronts and building instability will generate scattered to numerous snow showers and squalls. Storm motion drifting from southwest to northeast means the best chance for snow squalls will be generally west of the I-99 to US-15 corridor. High temperatures in the affected area will rise to near or slightly above freezing during the afternoon, but air temps will be below freezing for most of the day. With breaks in the clouds expected, road temperatures will likely rise above freezing, with a risk of flash freeze possible with any stronger/more persistent snow showers/squalls. Additional snowfall of an inch or two through the day on Saturday increases confidence in reaching advisory criteria in theses areas and we have issued a Winter Weather Advisory which is in effect from 7 PM tonight through 1 AM Sunday.

The forecast is a bit more complicated for eastern PA. Marginal temperatures and less moisture content should help to limit snowfall somewhat, but strong divergence associated with the right entrance region of a 150 knot upper level jet will support an area of moderate snow during the late morning/early afternoon. Most of the 12Z model runs support higher snowfall in the both the Middle and Lower Susquehanna Valleys, with 1 to 3 inches of snow expected. Slightly higher amounts are possible in the higher terrain of Schuylkill and Sullivan Counties and Winter Weather Advisories may be needed if confidence in higher snowfall totals continues to increase.

KEY MESSAGE 2: An arctic blast Monday will produce little snowfall but lead to dangerous, sub-zero wind chills Monday night and Tuesday.

A lull in the snow Sat night into Sunday night will be followed by another, colder, cold front. 8H temps drop into the -20/-23C range for Monday. Moisture will be lacking with the front, but light snow may accompany it. In its wake, surface and steering winds out of the southwest or west-southwest should keep lake effect snow bands directed into New York with mainly dry conditions in Pennsylvania. Min wind chills Monday night/Tuesday morning range from 0F in the LSV to minus 20F over portions of the Alleghenies. The current forecast would support a Cold Weather Advisory in the Laurels and northern mountains (approximately along, west, and north of the I-99/I-80 corridor) and possible an Extreme Cold Warning in the Laurel Highlands.

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. Low-level flow becomes more westerly by Wednesday, which will help to direct lake effect snow bands back into northwest PA.

KEY MESSAGE 3: Colder than normal temperatures continue through the end of the month with storm chances every few days.

A persistent west coast ridge and east coast trough pattern in the upper levels will ensure the continuation of below normal temperatures through the end of January. Periodic waves of low pressure traversing the High Plains and Great Lakes mean we'll see frequent opportunities for precipitation. Most of the storm systems appear weak and relatively disorganized at this point, which would favor the highest snowfall totals occurring in the typical upslope and lake effect snow regions of the Laurels and northwest mountains through the end of January.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

VFR conditions across central Pennsylvania are likely (~80-90%) to continue through 00Z Saturday as mid-level clouds continue across the area. Any restrictions in the near-term would likely come about due to low-level cloud decks at JST/AOO with current radar outlining some returns across the Laurels. Low-levels based on recent observations at JST/AOO outline 15F-20F dew point depressions, so lower probabilities (< 30%) in any precipitation at the airfields.

As warm air advection increases out ahead of the next cold front this evening, LLWS concerns increase south to southwest winds near 40 kts at 2000 ft. Recent NBM/NAM guidance has trended back on mentions; however, so have used more targeted windows across the western terminals. Light snow will become possible first in the northwest mountains and Laurels closer to 00Z Saturday with some potential in the hour-two ahead of this at BFD/JST. Uncertainty remains relatively high in the 6-24 hour window of the TAF package as the system pushes east and snow becomes more scattered in nature. The 18Z TAF package introduces longer durations of snow at the western airfields (BFD/JST/AOO) while limiting longer durations further east (UNV/IPT) with PROB30 groups in the area of most concern for visibility restrictions in light snow. Increased moisture return has allowed for increased confidence in an initial period of snow across MDT/LNS, but confidence still remains somewhat low, thus have outlined with PROB30s before a longer duration of heavier snow outlined in NBM/GLAMP model guidance. Lower restrictions will be possible in snow squalls with gusty winds across the western terminals (BFD/JST/AOO/UNV); however, ample uncertainty still exists so no mentions in the 18Z TAF package.

Outlook...

Sun...Scattered light snow/restrictions possible (low forecast confidence)

Mon-Wed...Light snow/IFR possible in the NW at times.

CTP WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from 7 PM this evening to 1 AM EST Sunday for PAZ004>006-010-011-017-024.