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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 /06Z WEDNESDAY THROUGH SUNDAY/

Widespread VFR conditions continue through the night under mainly clear skies. Mid and high clouds move in from the west during the morning and gradually thicken through the day ahead of a system that will bring snow to the western terminals. Most guidance shows snow beginning to fall at BFD between 16Z and 18Z, and at JST by 20Z. The HRRR and RAP also show steady snow impacting AOO and UNV for at least a few hours after 21Z. Farther to the east, very dry air in the low levels should prevent any snow from reaching the surface. IFR/LIFR conditions in snow are expected at BFD and JST, while AOO and UNV are expected to be MVFR. Snow either mixes with or changes over to FZRA/FZDZ after 00Z.

Winds will generally remain less than 5 knots through the rest of the night, but will gradually increase to 10 to 15 knots through the morning. An approaching 50-60kt LLJ moving in from the west will begin to bring increasing concern for LLWS, particularly west of UNV.

Thu...CFROPA, gusty. LIFR/IFR cigs/vis in SN N/W, MVFR cigs SE.

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

Sat-Sun...Dry to begin Sat. SN begins to enter south-to-north late Sat continue through Sun. IFR possible, most likely S PA.

CTP WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from noon today to 10 AM EST Thursday for PAZ004>006-010>012-017-018-024.