textproduct: State College

SYNOPSIS

* Rain will overspread much of the region today, with a period of freezing rain for parts of north-central and northeast PA * Briefly milder temperatures are anticipated overnight into Monday morning, before very windy and much colder conditions set in from Monday afternoon into Tuesday * Accompanying the colder weather this week will be the development of lake-effect snow bands into northwest PA

NEAR TERM /THROUGH TONIGHT/

KEY MESSAGES: - Steadier rain will develop by midday to early afternoon, with surface temperatures cold enough to support a period of freezing rain for the Endless Mountains, the Middle Susquehanna Valley, and the southwestern Poconos - Temperatures trend upward overnight, out ahead of an approaching and strong cold front -----------------------------------------------------------

Model guidance has remained consistent in tracking a rapidly deepening surface wave from the Upper Midwest into Ontario by early Monday. A strengthening low-level jet will increase moisture and thermal advection into far northern PA and NY State this afternoon and evening, where most of the steadier rain is anticipated. In fact, a tight south-north QPF gradient is foreseen, with rainfall amounts trailing off pretty quickly south of I-80.

Given that surface readings have fallen into the upper teens- lower 20s early this morning over many parts of north-central and northeast PA, residual low-level cold air should be sufficient to support a 3-6+ hour period of freezing rain. Even a bit of sleet on the front end is possible, but given very strong warm advection above the boundary layer, rain or freezing rain should be the dominant precipitation types. We did expand our existing Winter Weather Advisory westward and southward a bit, and now includes Potter, Clinton, Union, Snyder, Montour, Northumberland, Columbia, and Schuylkill counties.

Steadier rain and freezing rain could well taper off for a time late tonight, as the best warm advection forcing shifts farther north and east. Temperatures are expected to be on the rise overnight and should ultimately climb above freezing throughout the forecast area. A strong cold front trailing from the aforementioned surface wave should be entering western PA in the pre-dawn hours and could approach the Allegheny front towards daybreak, possibly with a narrow line of rain showers.

SHORT TERM /MONDAY THROUGH TUESDAY/

A strong cold front will charge across the Commonwealth Monday morning, perhaps with a broken band of rain showers.

Behind the front, strong low-level cold advection will commence, along with very windy conditions. Regarding high wind potential, although the strongest isallobaric forcing is progged over upstate NY and southern Canada, very impressive pressure rises, good post-frontal downward momentum transfer, and an overall strong wind field into the upper part of the boundary layer across PA, argue for widespread wind gusts of 40-50 mph, and isolated gusts of 55-65 mph over the Alleghenies and points just downstream. Based on this, we've added Clearfield and northern Centre counties to the existing High Wind Watch (formerly including Somerset, Cambria, Bedford, and Blair counties). Wind Advisories seem probable elsewhere. The period of strongest winds is anticipated from early Monday afternoon through the evening hours. High Wind Warning vs. Wind Advisory upgrade decisions will likely be made later today.

A non-diurnal temperature trend is expected on Monday, with morning highs in the 40s-mid 50s, plummeting into the 20s-mid 30s by late afternoon.

The other primary issue is the expected development of lake- effect snow bands Monday, Monday night, and Tuesday. Flow orientation is expected to slowly veer from about 260 (west- southwest) to 290 (west-northwest) with time. 75th to 90th percentile snowfall totals (a good bet where the most persistent bands reside) argue for 3-8" of snow from northern Warren into northwest McKean county, justifying the Winter Storm Watch. There is also potential for a more southern band to track into Elk and northern Clearfield counties, but there is much more uncertainty in this regard.

Frigid wind chills will also become an issue Monday night and Tuesday morning in particular (apparent temperatures +/- single digits). These values don't quite reach cold weather headline criteria (-10f).

LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/

On the large-scale, a cold, cyclonic NW flow pattern will reside over much of the eastern CONUS this period, underneath a deep upper low in eastern Canada. This will support a reinforcing shot of arctic air in the Thursday-Friday time frame, followed by gradual moderation next weekend, as heights aloft slowly start to build.

A prolonged period of lake enhanced/upslope snowfall may well continue across northwest PA and the Alleghenies through at least Thursday-Friday. However, it does appear that the ambient air mass could be fairly dry later in the week vs. earlier, which could eventually cut down on snowfall potential. However, there is considerable uncertainty in this regard, and we'll continue to monitor over time.

Of high confidence, though, is very cold temperatures (highs only in the teens-mid 20s later this week), along with frigid wind chills. As alluded to above, readings should gradually moderate by later next weekend.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Continued LIFR conditions are currently being observed across central Pennsylvania's western terminals (BFD/JST/AOO) with all model guidance pointing at continued IFR and below conditions for these three airfields through 00Z Monday. Widespread precipitation stations near a KPKB-KTOL will gradually continue moving eastward this morning with little in the way of timing changes for the 12Z TAF package from the previous cycle. There remains some uncertainty with what will occur within a weaker set of reflectivity returns across western Pennsylvania in the near-term with recent observations at FKL outlining UP/FG. Given that the bulk of model consensus and observations under current precipitation, LIFR continues to be the most likely scenario moving forward through 00Z Monday.

Further east, recent observations outline IFR at UNV with MVFR prevailing otherwise as of 11Z/6am EST. Deteriorating trend is expected with IPT/UNV increasingly likely to experience a longer duration of IFR/LIFR conditions based on a combination of GLAMP/RAP/HREF model guidance through the entire 12Z TAF package. HREF probabilities do outline a brief window for UNV/IPT to prevail low-end MVFR in the 14-16Z window ahead of precipitation and these seem to be well-founded based on RAP model soundings, so have went ahead and introduced this potential in the 12Z TAF package. Further east, higher chances outlined in GLAMP/HREF model guidance for MDT/LNS to have slightly more optimistic chances at MVFR, before IFR/LIFR becomes probable after ~20Z Sunday.

Winds start out light for the bulk of the 12Z TAF package before LLWS concerns begin to ramp up after 18Z Sunday as a surface low pressure and associated cold front well west of the area moves eastward, with a nose of a 50+kt 850mb LLJ entering western Pennsylvania between 18Z Sunday and 00Z Monday. This LLJ influence continues to expand eastward through the end of the 12Z TAF package. High confidence on the overall presence of LLWS impacts and timing at this juncture, with the main uncertainty coming with respect to the magnitude of LLWS at onset.

Outlook...

Mon...Mild early with widespread rain showers, then colder and windy later in the day with with snow showers across the N/W.

Tue...Windy and cold with snow showers N/W. Dry southeast.

Wed...Chance of snow showers across the north and west, dry southeast.

Thu...Chance of snow showers.

CTP WATCHES/WARNINGS/ADVISORIES

Winter Storm Watch from Monday morning through Tuesday evening for PAZ004-005. Winter Weather Advisory from 1 PM this afternoon to 1 AM EST Monday for PAZ006-012-037-041-042-045-046-049>053-058. High Wind Watch from Monday morning through Tuesday afternoon for PAZ017-018-024-025-033-034.