textproduct: State College

WHAT HAS CHANGED

* The Winter Storm Watch for northwest PA has been changed to a Winter Weather Advisory from 12 noon Wednesday to 10 am Thursday.

KEY MESSAGES

1) Quick-hitting winter storm for northwest PA to bring briefly heavy snow later Wednesday afternoon into the evening, with total accumulations of 2-5" by Thursday morning.

3) Monitoring the track of a significant winter storm this weekend; highest impacts will remain south of PA, but northward extent of accumulating snowfall remains uncertain.

4) Frigid temperatures return by this weekend and will likely stay around until the end of January.

DISCUSSION

KEY MESSAGE 1: After dry, cold conditions into Wednesday morning, our next winter storm will take aim at northwest PA by early afternoon.

A robust short-wave trough, diving into the northern Plains this afternoon, will help develop an Alberta Clipper type surface wave, which will track across the upper Midwest and then into Canada later tomorrow and Thursday. As it does so, a warm front will cross PA Wednesday afternoon and evening. Pronounced warm advection forcing and a strong low-level jet (45-60 kt) will bring a 3-6 hour period of localized moderate-heavy snow, probably centered from the mid-afternoon to early evening hours. Snowfall rates could briefly reach an inch per hour. The best snowfall rates should concentrate from the northwest corner of the state onto the Allegheny plateau, with amounts trailing off farther east into the ridge and valley region.

Later Wednesday night into early Thursday morning, occasional light snow should persist, but the best forced lift and moisture depths should be north and east of the Commonwealth by then. In fact, it is not out of the question (although not strictly mentioned in the forecast) that patchy freezing drizzle could develop, if moisture depths decrease sufficiently to prevent ice formation in the cloud bearing layer.

At any rate, we expect just about all the precipitation to be over by mid to late morning Thursday.

KEY MESSAGE 2: Medium-range guidance remains consistent in depicting a large-scale winter storm this weekend, tracking from the mid-South across the Mid-Atlantic states.

The devil, however, will be in the details, with a tight northern edge snowfall gradient expected to be at play. Where exactly this gradient zone sets up is highly uncertain, with the potential for it lie somewhere across PA.

In all probability, the highest winter storm impacts will be south of the Mason-Dixon line, but again with a tight snowfall gradient and the anticipation of heavy snowfall where jet dynamics are maximized, this situation bears watching. Please stay tuned for the latest forecast updates.

KEY MESSAGE 3: An initial surge of arctic air is expected on Friday and Saturday, with a reinforcing shot behind this weekend's winter storm, lasting well into next week.

On Friday, the initial wave of frigid air will be in the process of building south across the Commonwealth. As a result, we should see a fair temperature gradient, with highs on Friday ranging from near 10 in northwest PA, to the lower 30s in the Lower Susquehanna Valley. By Saturday, it will be well entrenched, with daytime highs in the single digits and teens area-wide. As mentioned earlier, a reinforcing push of very cold air is anticipated by early next week. Sub-zero apparent temperatures are a solid bet, and subsequent Cold Weather Advisories/Extreme Cold Warnings may ultimately be required.

Given the prolonged nature of the cold, we may begin to see more infrastructure impacts (frozen pipes, for instance) by next week. At any rate, an extended period of well below normal temperatures for late January appears extremely likely.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Mostly clear skies are present across all of central Pennsylvania this afternoon outside of a broken deck of clouds across the northwestern airfields. Wind gusts of 20 to 25 knots continue through the day before decreasing overnight. VFR will dominate the airspace for the most part for the next 24 hours with little in the way of sky cover for the remainder of this afternoon. Mid and high clouds increase from the west after 06Z ahead of an approaching low pressure system.The next chance of perception will arrive late Wednesday afternoon across the northwest. A prob30 group has been introduced for BFD to outline the possible onset of snow.

WED...Snow/IFR NW in the afternoon, MVFR-VFR elsewhere.

THU...CFROPA, gusty. LIFR/IFR cigs/vis in SHSN N & W, MVFR cigs SE.

FRI...Gusty W wind. Lake effect BFD (ocnl IFR). Mainly MVFR elsewhere.

SAT...Widespread SN/IFR possible, esp S.

CTP WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from noon Wednesday to 10 AM EST Thursday for PAZ004>006-010-011-017.