textproduct: State College

SYNOPSIS

* Lake effect snow will result in significant holiday travel disruptions across northwest Pennsylvania today while scattered snow showers and a few squalls will impact the weather across the Central Mtns, Laurel Highlands and possibly the Middle Susquehanna Valley. * A few periods of snow (Northern PA) and snow/or a wintry mix (Southern PA) are expected late Saturday Night and Sunday * A more significant/widespread winter storm is possible Tuesday and Tuesday night

NEAR TERM /THROUGH TONIGHT/

A prevailing upper trough across the eastern US will continue to bring well below normal temperatures and unsettled weather to the Commonwealth for Black Friday. 850mb temps around -10C will continue to support lake effect snow bands as steering winds veer to come out of the northwest. From this direction, snow bands will pick up moisture from Lakes Superior, Huron, and Erie before dumping snow in northwest and Central PA. Lake Effect Snow Warnings remain in effect for Warren, McKean, and Potter Counties with adjacent Winter Weather Advisories.

During the day today, a strong low pressure system centered in Central Quebec will drag a surface cold front through the northeast, helping invigorate ongoing lake effect snow showers and sustain them perhaps all the way to the I-80/I-99 corridor. Sufficient instability (50-100J/kg) could support snow squalls late this morning and this afternoon. Snow Squall Warnings may be needed along I-80, especially if there are breaks in the clouds (warmer road temperatures) and snow bands remain coherent down to I-80. These bands should waggle slowly east and northward through the day. That should improve conditions over the Laurels in the early aftn, and central mtns late in the day or at least by midnight. Unlike previous events this month, it seems the heaviest/most persistent bands should remain north of I-80, helping mitigate significant/long duration travel impacts there.

High pressure and upper level ridging will gradually move in from the southwest today and tonight. A lingering pressure gradient will support gusty winds again today, though gusts above Advisory criteria are less likely than they were the last two days). Snow showers will weaken in intensity and decrease in coverage later this evening and overnight as ridging builds. The flow doesn't look like it should quite take things all the way N of the CWA until well after midnight/closer to sunrise on Saturday. Thinning clouds and lighter winds means Saturday morning will be colder (by temperature) than Friday morning, but the wind chill will be comparably brisk.

SHORT TERM /SATURDAY THROUGH SUNDAY/

Saturday should be a chilly albeit tranquil day with high pressure briefly in control. Morning sunshine will give way to increasing high clouds with light winds. If you haven't put out your holiday decorations yet, Saturday looks like a better day than Sunday to get that done with highs in the 30s to low 40s and no rain/snow.

By Sunday, another low pressure system will move into the Great Lakes, bringing a wintry mix of precipitation to the Commonwealth. This system looks rather unimpressive from a forcing and moisture availability standpoint. With the low likely tracking through Michigan, the best forcing should remain well north of PA and we'll see scattered showers Sunday and Sunday night. Thermal profiles appear to be cold enough to initially support snow, then perhaps mixing with or changing to rain for the last day of November. Temperatures moderate somewhat, ranging from the mid 30s to mid 40s Sunday afternoon.

LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/

Unsettled weather will continue into early next week as cold air in the middle of the country sets up a strong baroclinic zone over the eastern US. High pressure in the Plains should keep cold air in place on the northern side of the precipitation shield from a wave of low pressure progged to move out of the Gulf of America on Monday. This system should move northeast around the base of the aforementioned high pressure system, bringing increasing moisture to the Mid-Atlantic Tuesday into Wednesday next week. Although it is still too early to pin down details of timing, intensity, and precipitation type delineation, the first widespread plowable snowfall of the season is possible.

The chance for significant (6"+) snowfall amounts appears limited at this time due the lack of a strong high pressure center that would help slow the northeastward movement of the center of low pressure. A relatively quick-moving system could still bring accumulating snow across a wide footprint, but high-end amounts would be lower. GEFS and ECENS probabilities of 6"+ have generally been below 15% over the last several runs. The track of the low pressure center will also be important for determining the exact location of the rain/snow transition line. Continue to monitor the forecast in the days ahead, especially if you have plans to travel.

High pressure will regain control of our weather in the wake of the aforementioned system for the middle to end of next week. Prevailing upper troughing brings high confidence in continued below normal temperatures and a chilly start to December.

AVIATION /08Z FRIDAY THROUGH TUESDAY/

Wind flow taking on a more northwesterly direction overnight. This will allow lake effect SHSN to pepper KBFD through mid day. Flight categories will waffle between IFR and MVFR as the snow bands shift.

Elsewhere, -SHSN will occasionally occur from central PA and points west...KJST and KAOO, and to a lesser extent...KUNV. Do not expect an appreciable drop in category as MVFR will persist with a stratus overcast. The exception being KJST where upslope winds and some influence from lake snow may briefly drop to IFR.

Northwest winds will continue to gust 25 to 30KTS overnight with occasional wind shear commensurate with those gusts.

Winds will remain gusty into Friday evening. A brief break in the adverse weather on Saturday, before more snow and rain on Sunday and again on Tuesday, as the weather pattern favors fast moving weather systems.

Outlook...

Sat...Improving conds; diminishing winds with lingering snow showers ending.

Sun...Light rain/wintry mix developing from west to east; restrictions possible. Precip may turn to rain across the south during the day.

Mon...Mixed/wintry precip likely early on. MVFR likely. IFR poss.

Tue...More snow possible, before a possible mix later in the day across the south.

CTP WATCHES/WARNINGS/ADVISORIES

Lake Effect Snow Warning until 1 AM EST Saturday for PAZ004>006. Winter Weather Advisory until 1 AM EST Saturday for PAZ010-011- 037.