textproduct: State College

WHAT HAS CHANGED

* Increased confidence in plowable snowfall across much of central Pennsylvania Sunday-Monday. Winter Storm Watches have been issued for the Laurel Highlands and parts of the Lower Susquehanna Valley.

KEY MESSAGES

1) A few rain showers possible this afternoon across northwest PA, before transitioning to snow showers overnight. Ice jam risk continues into the weekend.

2) West winds gusting over 45 mph at times this afternoon through this evening across the Laurel Highlands and Scent Mtns.

3) Plowable snowfall increasingly likely on Sunday and Monday.

DISCUSSION

KEY MESSAGE 1: A few rain showers possible this afternoon across northwest PA, before transitioning to snow showers overnight. Ice jam risk continues into the weekend.

A few showers and thunderstorms have developed this afternoon across northwest PA in a weakly unstable environment ahead of an occluded boundary. Any rainfall with these should be very light and they will likely come to an end by the early evening as the front passes through. Cooler air moves in this evening as winds become westerly, allowing for a few snow showers over the north and west.

The ice jam threat continues into the weekend as daytime highs in the 30s and 40s will support additional ice breakup, The threat may not be as high as it was earlier as we are not anticipating any significant rainfall.

KEY MESSAGE 2: West winds gusting over 45 mph at times this afternoon through this evening across the Laurel Highlands and Scent Mtns.

Sharp pressure rises and deep vertical mixing will be accentuated by favorable descending branches of a 100+ kt upper jet across Southern PA to rapidly increase a westerly sfc wind/gusts behind an occluded/cold frontal passage during the early to mid afternoon hours.

Gusts across the Laurels and areas immediately downwind along the I-99 corridor could exceed 45 mph at times through midnight. A Wind Advisory remains in effect for Cambria, Somerset, Bedford, and Blair Counties.

KEY MESSAGE 3: Plowable snowfall increasingly likely on Sunday and Monday.

Confidence continues to increase in an area of low pressure developing off the east coast on Sunday as a potent upper trough moves offshore. While there are still some differences in the exact track of the surface low after it develops, largely due to differences in the timing of when/where the phasing of the northern and southern branches of the jet stream occurs, most of the 12Z guidance has continued the recent northwest trend that has been observed over the last few model cycles. This would likely bring heavy snowfall into portions of the Lower Susquehanna Valley. The WPC Winter Storm Outlook continues to tick higher, with much of the eastern half of the forecast area now seeing a 50 to 80 percent chance of seeing snowfall exceed warning criteria. We have issued a Winter Storm Watch for portions of the Lower Susquehanna Valley where we currently have the highest confidence in seeing 4 to 6 inches of snowfall.

A couple of things to keep in mind regarding this snowfall forecast: * There is still considerable uncertainty regarding the eventual track of the low. A more westerly track will favor heavier snowfall over parts of Central PA while an easterly track may shift the heaviest snow to the east of our area. * As the low quickly deepens off the east coast, expect areas of enhanced frontogenesis to develop in the western periphery of the system. This will create areas of enhanced snowfall rates, but there is currently low confidence on where these heavier bands set up.

A prolonged period of upslope snow is also anticipated in the Laurel Highlands as flow behind the surface low becomes northwesterly. Most guidance continues to show a 48 hour period from early Sunday morning through early Tuesday morning where snow should fall, with 4 to 8 inches of snow expected along the ridges of Cambria and Somerset Counties. As a result, we have issued a Winter Storm Watch for those two counties as well.

Significant uncertainty still remains regarding snowfall over the rest of Central PA, but snow associated with the upper low is likely Sunday afternoon into Monday morning. Some models continue to show an area of enhanced surface convergence associated with an inverted trough extending northwestward from the coastal low, but it remains to be seen where this feature will set up.

AVIATION /19Z FRIDAY THROUGH WEDNESDAY/

A cold front will move west-to-east across the Commonwealth this afternoon, transitioning locations to IFR/MVFR in its wake. Westerly wind gusts in excess of 30 knots are possible across the western half of PA through this evening, particularly across the Allegheny ridgetops. Elsewhere, peak wind gusts are expected to be around 20-25 kts.

A stratocumulus cloud deck is expected to develop along and west of the Alleghenies this evening as an upper-level low moves overhead, bringing generally MVFR CIGs to KBFD, KJST, and KUNV. Periods of IFR CIGS are possible at KBFD, and at KJST to a lesser extent, which may coincide with lake effect flurries or snow showers. Stratocumulus clouds will develop later tonight and early tomorrow morning across eastern PA as the upper-level low progresses eastward. Cloud coverage was kept lighter over KAOO due to downsloping effects east of the Alleghenies, which is suggested by models such as the HRRR/RAP/NAM.

Outlook...

Sat...Lingering rain and snow showers.

Sun-Mon...Potential for restrictions in snow showers.

Tue...No significant weather expected.

Wed...Potential for restrictions in light rain/snow showers.

CTP WATCHES/WARNINGS/ADVISORIES

Wind Advisory until midnight EST tonight for PAZ024-025-033- 034. Winter Storm Watch from late Saturday night through late Monday night for PAZ024-033. Winter Storm Watch from late Saturday night through Monday afternoon for PAZ057>059-064>066.