textproduct: State College

SYNOPSIS

* Partly cloudy and bitterly cold/frigid tonight with some clearing across NW PA * Not as cold Tuesday; quick shot of snow in the NW mtns * Windy with snow/rain mix followed by lake effect snow through midweek; additional snow accumulation possible Friday-Sunday

NEAR TERM /UNTIL 8 AM TUESDAY MORNING/

11 PM Update... Clearing has already allowed BFD to dip to -2F. They should go a couple more degrees colder with Td currently -5F. They could start to rise later tonight if the SE wind picks up at all. Have tweaked the near term to make it colder on the whole.

Prev... Recent surface analysis outlines high pressure at the surface with high clouds gradually streaming northeast across the forecast area as a 500mb trough continues to track south of the region this afternoon. In terms of sensible weather, a fairly quiet near-term period with the biggest concern being cold temperatures overnight. Temperatures continuing to gradually increase and could increase by another 2-3F over the next couple of hours push high temperatures closer to the upper teens to upper 20s across the area, ahead of an evening with radiational cooling prospects across northwestern Pennsylvania where clouds coverage is expected to decrease. Current low temperatures overnight trend near-zero across NW PA; however, cannot rule out negative single digit temperatures overnight under the aforementioned radiational cooling. Fortunately, wind/gusts are not expected to be too high overnight with the continued presence of high pressure, thus wind chills are expected to be near the low temperatures marks. It should be noted that some daily record lows could be within reach particularly if the min temps prints on 12/8 (before midnight) vs. 12/9 or after midnight. (See climate section for 12/8 and 12/9 record mins).

SHORT TERM /8 AM TUESDAY MORNING THROUGH WEDNESDAY NIGHT/

11 PM Update... Main worries would be for a little FZRA in the east Wed AM and enough SN to make an advy in the far NW. Both look around 50-70% likely at this point. NAM is very dry for the morning where temps in the east might be allowed to warm up before precip hits the ground. But, it is also a 3rd-4th period forecast at this moment. Will allow the mid shift to further refine things with the neighboring offices, as it will only be a 3rd period worry for them.

Prev... A complex short-term period begins as high pressure shifts east of the region on Tuesday, allowing for a quick- moving mid- level shortwave to bring a quick shot of snowfall across northwestern Pennsylvania Tuesday afternoon. Model guidance does have some disparity between the southern extent of the snowfall footprint; however, recent 12Z HREF guidance has trended upwards on probabilities of measurable snowfall across northern McKean/Warren counties, thus have increased snowfall mentions in these areas. Based on a consensus of model data, accumulations still look likely to top out at ~1", with probabilities generally below 30%. Some signals do continue to exist for an isolated band of heavy snow to setup in the area and produce an isolated swath of locally heavier snowfall that can exceed an inch; however, low confidence on where this where set up at this time. Those in NW PA during the evening commute hours on Tuesday will need to continue to monitor the forecast in this timeframe for near-term updates over the next couple of forecast packages.

Upstream of the mid-level trough, an intense clipper low will be racing through the Great Lake region Tuesday night into Wednesday morning. Warm air will filter into the region ahead of this anomalously deep Alberta Clipper (one of the strongest for this time of year by historical/climatological MSLP standards) and is expected to spread accumulating light to moderate snow across the NW 1/2 of the CWA for the Wednesday morning commute. Snowfall totals overnight will generally range between 1-2" for NW PA with snowfall below 1" expanding further south into the I-80/I-99 corridor. Snowfall continues and by Wednesday afternoon, 24-hour snow totals are projected to reach 2-4" in the NW mtns with C-1" fcst elsewhere in areas along and north/west of I-80/I-99. Recent model guidance does indicate some potential for an extended warm nose across the Lower Susquehanna Valley that does indicate some freezing rain potential and light ice accumulation if precipitation manages a quick enough eastward expansion, mainly in the 7am-1pm timeframe. Recent model soundings do outline some discrepancies with respect to how warm the lower-levels will get at this time, so there is quite a bit of uncertainty with regards to if ice accumulation will occur, so have continued with NBM model guidance and will highlight this potential in the HWO rather than issuing a low confidence Advisory with some time to iron out the forecast uncertainties in future packages. As the precipitation expands eastward into Wednesday afternoon, marginal temperatures/thermal profiles will favor a snow/rain mix across north/western Pennsylvania as temperatures reach into the mid-30s with plain rain from the central ridges eastward into the Susquehanna Valley.

As the storm tracks through the St. Lawrence River Valley Wednesday night, colder northwesterly flow is expected to trigger lake effect snow with minor snow accumuations over the typical northwestern Pennsylvania snow belts and favored upslope areas along the Allegheny Front and Laurels. Total snowfall in this could reach into the 4-6" range across northwestern Pennsylvania; however, given the uncertainty with the fast- moving shortwave, clipper, and lake effect behind the system there remains quite a bit of uncertainty with snowfall totals. Given this remaining uncertainty in this timeframe, have collaborated with neighboring offices and kept WWA decisions for a future cycle given the uncertainty with this system although we will continue to track this threat in the HWO.

LONG TERM /THURSDAY THROUGH MONDAY/

Lake effect snow and breezy northwesterly winds continue on Thursday with additional snow accums expected over the northwestern mountains and Laurel Highlands. A cold frontal passage early Thursday will promote some snow squall threat as outline in more recent Snow Squall Parameter guidance. The main deterrent for snow squalls will be that instability is largely expected to be limited given that the front is passing on early in the day. At this times, no mentions in even the HWO given the low confidence at this juncture and longer time horizon until the event is on our doorstep.

The next winter weather system is progged to arrive later Friday into Friday night, although models are showing some variance with the evolution and track. Lake effect snow/snow showers continue over the weekend with temperatures staying below the historical average heading into mid-December.

AVIATION /04Z TUESDAY THROUGH SATURDAY/

Main change was to edge winds up later on Tuesday afternoon, as now the TAFS go out to 00Z Wed.

Also took out the 3000 ft CIG at JST later tonight, hard to see much support for this.

Some snow into BFD late Tuesday afternoon, but expect the snow to be real light prior to 00Z Wed.

More information below.

High pressure will maintain VFR conds across Central PA overnight, although a wave of low pressure passing to our south will provide high clouds over at least the southern half of the area for the at least the first part of the overnight period.

Skies will clear from northwest to southeast later tonight.

Tuesday will see clouds increase. CIGS could fall to MVFR by late afternoon/evening across NW PA (BFD), where some light snow is also possible, as a clipper system approaches from the Great Lakes.

Much of the remainder of the week looks chilly and unsettled, with occasional snow showers (possibly rain south) on Wednesday, and occasional snow showers Thu-Sat as clipper systems periodically impact the region.

Outlook...

Wed...Restrictions possible with snow north/west and wet snow or rain showers south/east.

Wed night-Thu...Snow showers continue, mainly N/W PA. Westerly wind gusts of 15-30 kts (possibly higher in the Laurels) are expected.

Fri-Fri night...A round of light snow possible later Fri into Fri night, with a clipper system.

Sat...Improving conds expected.

CLIMATE

Astronomical winter (solstice) begins at 10:03 a.m. on Sunday, December 21st

Record lows for 12/8: MDT 12 in 2002 IPT 3 in 1901 AOO 7 in 2005 BFD -2 in 1970

Record lows for 12/9: MDT 7 in 1989 IPT 3 in 2002 AOO -3 in 1989 BFD -12 in 2002

CTP WATCHES/WARNINGS/ADVISORIES

None.