textproduct: State College

WHAT HAS CHANGED

* The Winter Weather Advisory has been expanded to include Cambria, Northern Centre, and Northern Clinton Counties. * Increasing potential for a significant winter storm to impact Central PA this weekend.

KEY MESSAGES

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

2) Monitoring the track of a significant winter storm this weekend; significant impacts possible in southern PA, but the northward extent of heavy snowfall remains uncertain.

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

DISCUSSION

KEY MESSAGE 1: Quick-hitting winter storm for northwest PA to bring briefly heavy snow this afternoon and into the evening, with total accumulations of 2-5" by Thursday morning.

A surface low passing well to our north today will bring a period of localized moderate to heavy snow, centered from the mid afternoon to early evening hours, as warm advection forcing increases with a 50 to 60 knot low level jet moving in from the west. The best snowfall rates, potentially around one inch per hour, 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.

Occasional light snow should persist late Wednesday night into Thursday morning, but the best forced lift and moisture depths should be north and east of the Commonwealth by then. Model soundings indicate the potential for the snow to mix with or completely change over to freezing drizzle as the cloud layer will almost completely be below the dendritic growth zone.

KEY MESSAGE 2: Monitoring the track of a significant winter storm this weekend; significant impacts possible in southern PA, but the northward extent of heavy snowfall remains uncertain.

Confidence continues to increase in a significant winter storm impacting the Mid-Atlantic this weekend, but there is still some uncertainty as to the extent of impacts in Central PA. Ensemble cluster analysis continues to highlight two potential evolutions with this system, the first being a scenario where the northern and southern stream waves remain slightly out of phase. This would result in the system taking a more southerly track, keeping the heaviest snowfall to our south and a plowable snowfall favored south of I-80. The second solution is one where the two branches of the jet stream interact favorably and produce a more amplified system. This would result in the surface low tracking farther to the north and bringing the heaviest snow into Pennsylvania with significant impacts possible in southern PA. The deterministic guidance is also split on the evolution of this event, with most runs of the GFS favoring the first scenario while the ECMWF has been showing something more in line with the second one.

That being said, NBM probabilities of seeing 6 inches or more of snow continue to increase across parts of the region. The most recent runs show a 60 to 80 percent chance of exceeding 6 inches of snow for areas south of the PA turnpike, and a 50 percent chance as far north as I-80. Details will become increasingly clear over the next day or two, but now is the time to prepare for a potentially significant winter storm that will affect parts of Central PA Saturday and Sunday.

KEY MESSAGE 3: Frigid temperatures return by this weekend and will likely stay around until the end of January.

On Friday, an initial wave of Arctic air will be in the process of building south across the Commonwealth. As a result, we should see a fairly large temperature gradient, with highs 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 /12Z WEDNESDAY THROUGH SUNDAY/

Widespread VFR conditions continue through the morning as mid and high clouds move in from the west 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 after 21Z. Some snow is possible as far as east as IPT, but low level dry air should limit its intensity. MDT and LNS will likely remain dry. Sites that see steady snow will see IFR/LIFR visibility and BFD and JST will likely have IFR ceilings as well.

Winds are light this morning, but will gradually increase to 10 to 15 knots. An approaching 50-60kt LLJ will lead to LLWS at all TAF sites this afternoon and evening.

Snow mixes with or changes to freezing drizzle this evening, leading to improving visibility. Wind gusts also increase overnight, with gusts of 20 to 25 knots possible. Heavier snow showers redevelop over northwest PA late tonight and may move through BFD prior to 12Z. These snow showers would likely cause brief periods of IFR visibility.

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.