textproduct: State College

WHAT HAS CHANGED

* Slight increase in expected snowfall totals for late tonight (and primarily Saturday morning) with a Winter Weather Advisory issued for much of the Lower Susq Valley and Western Poconos for Sat Morning. The Winter Weather Advisory continues for the NW Mtns of PA and part of the Laurel Highlands.

KEY MESSAGES

1) Plowable snowfall tonight into Saturday with scattered heavy snow showers and squalls Saturday afternoon, mainly across the western half of the state.

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 (light to moderate snowfall) every few days.

DISCUSSION

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

A couple shortwaves (favorable quadrants of an upper level jet and nose of a low-level jet - for UVVEL) rotating around the base and eastern side 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 overnight 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 overnight, but strong upper level divergence associated with the thermally direct right entrance region of a 150 knot upper level jet (and band of enhanced 850-700 mb FGEN) will support an area of steady, light to moderate snow during the second half of Sat morning into the early afternoon near and to the east of the I-81 corridor.

Most of the latest high res model runs support higher snowfall in the both the Lower Susquehanna Valley and Western Poconos, with 1 to 3 inches of snow expected. Slightly higher amounts are possible in the higher terrain of Schuylkill (and perhaps even parts of Sullivan) County. A Winter Weather Advisory may be needed across Sullivan county 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 (light to moderate snowfall) 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 /09Z SATURDAY THROUGH WEDNESDAY/

Gradually deteriorating conditions will occur into early Saturday morning as large scale lift increases from SW to NW across the Central and Western PA airfields, promoting light snow and increasing coverage of low clouds. IFR cigs and vsby are likely to develop at BFD and JST between 09z and 13z. VFR will likely transitions to MVFR CIGS this morning over the Central Mtns and Susq Valley.

As a secondary area of snowfall develops farther east this morning, IFR Vsbys are likely for several hours across and to the east of the I-81 corridor from KMDT to KAVP between 12-17Z Sat.

Although conds will generally improve from IFR to MVFR across this west this afternoon, periods of lower restrictions will be possible in snow squalls with gusty WSW winds across the western terminals (BFD/JST/AOO/UNV) Sat during the afternoon and evening.

Outlook...

Sun...Scattered light snow/restrictions possible mainly in the east (low forecast confidence)

Mon...Cold fropa. West winds gusting over 30kts. Restrictions likely with snow showers especially west.

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

CTP WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory until 1 AM EST Sunday for PAZ004>006- 010-011-017-024. Winter Weather Advisory until noon EST today for PAZ057>059- 065-066.