textproduct: State College

WHAT HAS CHANGED

* Continued the Cold Weather Advisory north of I-80 and along/west of I-99, with minor changes to wind chill forecast.

KEY MESSAGES

1) Dangerous, sub-zero wind chills tonight into Tuesday morning.

2) Active weather continues through the end of the week with below normal temperatures and multiple chances for snowfall with the best chance coming late Wednesday and Wed night across the Western half of PA.

3) Large range of uncertainty with respect to how far north significant snow gets from an upper level disturbance and wave of low pressure that will most likely move from the Southern Appalachians to the Carolina Coast over the weekend.

DISCUSSION

KEY MESSAGE 1: Dangerous, sub-zero wind chills tonight into Tuesday morning.

Recent guidance continues to show very good agreement with regards to the frigid airmass across Pennsylvania behind the cold frontal passage this evening into Tuesday morning. Recent guidance might have come in slightly colder, with EC/GFS model guidance both indicating 850 mb temperatures below -20C with portions of southern Pennsylvania in the -15C to -20C temperature range at 850 mb. Low temperatures near zero in NW PA continues to be thrown out by most recent NBM guidance which appears to be justified based on most recent GEFS/ENS and hi-res model guidance. Clearing across southeastern and south-central Pennsylvania will likely lead to slightly lower temperatures compared to more recent NBM model guidance, so have trended slightly cooler. Gusty winds (see Key Message #2) overnight are expected to lead towards wind chills near 0F across the Lower Susquehanna Valley to -20F across portions of NW PA. Wind chills will increase Tuesday morning as temperatures slowly rise and gusty winds become less prevalent; however, sub-zero wind chills are expected through Wednesday morning across portions of NW PA. The Cold Weather Advisory remains in good shape based on most recent forecast, so have made no changes to the headlines in this forecast cycle.

KEY MESSAGE 2: Active weather continues through the end of the week with below normal temperatures and multiple chances for snowfall. The best chance of snow will come late Wednesday and Wed night across the Western half of PA.

Temperatures moderate slightly for the end of the week, though they will remain below normal with highs in the 20s and low 30s. After a brief dry period, snow chances return to parts of the region late Wednesday into Thursday as deep layer WAA develops at the nose of a potent 50-60 kt llvl southwesterly jet ahead of the next shortwave rotating through the persistent trough over the eastern US. The typical lake effect and upslope snow regions of the Alleghenies in Western PA will once again be favored as the surface low is expected to pass well to our north. 1-3 inch amounts look probably across the Western Third of the CWA with a coating to one inch over the Central and Eastern portions of the CWA.

Temperatures trend downward as we head into the weekend, though there is a lot of spread in the ensembles as to how low they actually get. The upper-level pattern supports additional chances for snow with additional shortwaves moving through the trough, but the track and timing of these systems remains unclear. Recent guidance continues to favor a southern wave with significant snow possible south of the area this weekend, but it is far too early for anything except continuing to monitor the forecast.

KEY MESSAGE 3) Large range of uncertainty with respect to how far north significant snow gets from an upper level disturbance and wave of low pressure that will most likely move from the Southern Appalachians to the Carolina Coast over the weekend.

Model trend has been slightly north and more amplified, with hints at a dual jet/transverse circulation and steady snow setting up across the Mid Atlantic Piedmont Sat night into Sunday. The EC ECAI are about 12 hours slower than the GFS and has its mid/upper level trough axis further to the west, allowing for milder 850 mb temps (at or above -10C) to advect over at least the far southern part of PA on Sunday. The strongest llvl t-gradient and 6-12 hour period of SERLY llvl flow off the Wrn Atlantic appears to favor the Delmarva region at this point.

All this being said, chcs appear to be slightly increasing for a widespread light snowfall across Central PA and a moderate accum of several inches over the Southern Tier Counties. Stay Tuned!

AVIATION /00Z TUESDAY THROUGH SATURDAY/

The arctic cold front moving through far Eastern PA early this evening will push east to the Atlantic Coast by midnight.

Much drier/colder air has pushed into the region behind this front and clouds have greatly diminished, leaving just some MVFR CIGS and isolated to scattered snow showers/flurries over the Laurel Highlands and NW Mtns under mean, WSW llvl flow.

Elsewhere across the Ridge and Valley Region of the state, few- sct VFR strato cu will prevail.

Very cold, too, with wind chills in the 0 to -20F range.

The wind will continue out of the W with gusts of 20-25KT at all terminals on Tues. Otherwise...MVFR cigs all day NW, and perhaps (30% chc) at JST.

WED...Light snow/IFR NW, MVFR-VFR elsewhere.

THU...CFROPA, gusty. LIFR/IFR cigs/vis in SHSN N & W, MVFR cigs SE.

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

SAT...Widespread SN/IFR possible, esp S.

CTP WATCHES/WARNINGS/ADVISORIES

Cold Weather Advisory until noon EST Tuesday for PAZ004>006- 010>012-018-019-025-034-037-041-042-045. Cold Weather Advisory until 1 PM EST Tuesday for PAZ017-024- 033.