textproduct: State College

WHAT HAS CHANGED

* Focus shifting to tail end of winter storm, with light to moderate synoptic snow showers tapering off this morning while scattered lake effect and upslope snow showers move in from the west later today.

KEY MESSAGES

1) Light to moderate snow tapers off/exits east after sunrise with gusty northwest winds ushering in lake effect/upslope snow showers for the rest of the day.

2) Turning breezy and colder through Tuesday in the wake of the Nor'easter.

3) Multiple chances for rain and snow throughout the coming week.

DISCUSSION

KEY MESSAGE 1: Light to moderate snow tapers off/exits east after sunrise with gusty northwest winds ushering in lake effect/upslope snow showers for the rest of the day.

There are two main areas of snowfall affecting Central PA early this morning associated with the big coastal storm: the first is a quasi-stationary deformation band that has parked over the mainstem Susquehanna River valley overnight. This area of snow has weakened considerably as of this forecast update, though pockets of 0.5" per hour snowfall rates remain possible through sunrise, especially south of I-81. The second area of snow is located within an inverted trough that covers much of the Central Mountains from I-80 down to the Mason-Dixon line between I-99 and US-15. This batch of snow has been producing snowfall at rates of 0.25-0.5"/hr and has also shown some signs of weakening with recent radar scans.

Mid and upper level winds will continue to veer over the next few hours, basically shutting off the easterly moisture advection and bringing an end to the synoptic snow showers. Storm total snow (which includes snow that fell and subsequently melted Sunday morning) is still on target to range from 4-8" from Schuylkill Co south into Lancaster County and southeastern York County, with locally higher amounts possible in northeast Lancaster County and the higher terrain of Schuylkill County. Totals of 3-8" expected in the higher terrain of the central ridge and valley region intersected by the north-south oriented snow band in the inverted trough. Lower amounts at lower elevations and on paved surfaces.

As deep layer winds shift to northwesterly by late morning, a classic lake effect/upslope regime will build into Central PA. Cool temperatures aloft will supply enough instability to support scattered showers/squalls in northwest to southeast oriented lake effect snow bands. Additional snow accumulation is possible from these lake effect/upslope bands, especially west of I-99 and north of I-80. The tallest ridges in Cambria and Somerset County will cash in on significant snowfall, with 8 to 12 inches possible through Tuesday morning. A plume of lake effect moisture is already directing a steady stream of snow showers into Somerset County this morning and veering winds will shift that band a bit farther norther later today.

The Snow Squall Parameter lights up across south central PA this afternoon, highlighting some instability and the potential for heavier bursts of snow. Overcast skies should limit the risk for flash freeze conditions and more significant impacts from snow squalls. We'll continue to message the threat for lake effect snow showers and ramp up wording to emphasize snow squalls if conditions trend more concerning later today.

2) Turning breezy and colder through Tuesday in the wake of the Nor'easter.

As the Nor'easter bombs out off the east coast, a tightening pressure gradient over Pennsylvania will cause wind gusts increase to 25 to 35 mph today, leading to blowing and drifting snow, particularly in southeast PA. The heavy and wet nature of the snow could mitigate the blowing and drifting potential to some extent, but motorists should be alert for changing conditions today and tonight, even if no snow is falling.

By this evening, a surge of colder air will move into Pennsylvania. Low temperatures by Tuesday morning will range from the single digits along and north of US-6 to near 20 south of the PA Turnpike. Add in some lingering gusty winds and wind chills will drop into the single digits above and below zero. Northerly flow again on Tuesday will ensure cold advection continues and temperatures remain well below average for this time of year. A bit more in the way of sunshine will be a small consolation for highs in the 20s and 30s, which are -10 to -15 below normal.

KEY MESSAGE 2: Multiple chances for rain and snow throughout the coming week.

A clipper system tracking to our north Tuesday night and Wednesday will bring light to moderate snow to parts of the region. An initial wave of snow is likely as warm advection overspreads the area with additional snow showers and potentially a few snow squalls possible along the cold front as it moves through during the afternoon on Wednesday. There is increasing confidence in at least a plowable snowfall along and north of I-80.

Most model guidance then shows a stronger low pressure system moving through the region Thursday night and Friday, but details remain uncertain. Rain or a mix of rain and snow would be favored for much of Central PA. Prevailing zonal flow aloft should mitigate a major system like we experienced on Sunday. Early indications are that the rain-snow line will set up from west to east. The big question yet to be answered is will that rain-snow line set up south of the Mason-Dixon line (all snow for PA), along I-76 or I-80 (snow north, rain south), or north of PA (all rain for PA)?

AVIATION /07Z MONDAY THROUGH FRIDAY/

MVFR/IFR conditions will continue in snow bands and snow showers through this morning, with locations east of the Alleghenies improving to VFR during the afternoon as snow diminishes and ceilings raise due to diurnal mixing.

Along and west of the Alleghenies, MVFR/IFR conditions are expected as rounds of upslope and lake effect snow showers bring ceiling and visibility restrictions. Snow showers will gradually taper off during the evening as drier air moves into the region, though MVFR ceilings could linger as mixing heights lower with the loss of daytime heating.

Otherwise, gusty northwesterly winds are expected statewide through Monday night, with peak wind gusts generally between 20 to 30 kts. These winds could result in blowing/drifting snow.

Outlook...

Mon...MVFR/IFR restrictions possible in snow showers at BFD/JST, otherwise improving to VFR with gusty northwesterly winds.

Tue...MVFR-IFR restrictions possible in snow showers at BFD/JST, largely VFR elsewhere.

Wed-Fri...Restrictions possible in rain/snow showers.

CTP WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory until 1 PM EST this afternoon for PAZ005-006-010-011-017-025-034-037-041-046-051>053. Winter Storm Warning until 1 PM EST this afternoon for PAZ012- 018-019-026>028-035-036-042-045-049-050-056>059-063>066. Winter Storm Warning until 1 AM EST Tuesday for PAZ024-033.