textproduct: State College

WHAT HAS CHANGED

* Small uptick in snowfall amounts tonight into Saturday, prompting the issuance of Winter Weather Advisory in the northwest mountains. * Added discussion about potential for plowable snow in the southeast on Saturday afternoon (low confidence - medium impact) * 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 Friday night 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 Friday night 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 Friday night across western PA as warm advection lift develops underneath the left exit region of a strong upper jet. With unidirectional southwest flow throughout all layers, the best moisture sourcing will remain west of the Laurel Highlands up into the northwest mountains. Precip will also develop in the Poconos up into New England late Friday night as the first shortwave races northeast. By daybreak Saturday, a couple inches of snow will accumulate along and west of US-219 where the most consistent snow is expected, while a coating to an inch is possible farther east.

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 brings confidence in Advisory-level snowfall amounts in the northwest mountains. A Winter Weather Advisory is in effect from 7PM Friday through 1AM Sunday there. Expansion may be needed to Cambria and/or Somerset County, but the orientation of snow showers/squalls should steer higher accumulations into Clearfield County and areas farther north.

Across southeast PA, the forecast is a bit more complicated. Marginal temperatures and less moisture content will limit snowfall accumulations somewhat, but the latest batch of 06Z guidance paints the potential for as much as a couple inches of snow Saturday afternoon as upper level divergence provides sufficient lift for an area of snow to develop. Low confidence at this time, but the forecast may need to be adjusted upwards in the next 24 hours if confidence increases.

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

Another lull on 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 the front. In its wake, surface and steering winds out of the southwest or west- southwest should keep lake effect steered into New York with 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. With all the arctic air in place and a flow out of the west, we should have LES when the flow is best favorable at the end of the forecast period, but waggles in the mid-level flow are expected. So, a broadbrushed and decreasing PoP strategy across western PA is best at this point due to the low certainty of the details.

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 /12Z FRIDAY THROUGH TUESDAY/

Areas of low clouds will continue to move across central PA into early this morning as upslope westerly flow gusts up to 20 mph. MVFR cigs are likely (80 pct or higher) at BFD and JST, and possible at AOO and UNV (50 pct chc). All sites should improve to VFR by 13-16z today. Winds will back through the day from westerly this morning to southerly this afternoon.

As warm air advection increases out ahead of the next cold front this evening, low level wind shear will develop with south to southwest winds near 40 kts at 2000 ft. Light snow will become possible first in the northwest mountains and Laurels by 21z Fri - 03z Sat. The forecast becomes more uncertain as the system pushes east. Hires guidance suggests a minimum in precip just east of the Allegheny Front, and the potential for a secondary max in precip (still relatively light amounts) over eastern PA, with some snow falling between 06z and 12z, and another round of snow potentially clipping MDT and LNS later in the morning or early afternoon on Saturday. Please continue to check the forecast for updates. Behind the cold front, scattered snow showers and a few snow squalls will be possible, especially in the northwest mountains Saturday afternoon and evening.

Outlook...

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

Mon-Tue...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.