textproduct: State College

WHAT HAS CHANGED

* Updated timing for snow squalls this afternoon and evening. * Increased snowfall totals over the Lower Susquehanna Valley for tonight and tomorrow.

KEY MESSAGES

1) Snow squalls move across northwest PA this afternoon, creating hazardous travel conditions.

2) Light snow across southeast PA tonight and tomorrow as low pressure develops off the east coast.

3) An arctic blast Monday will produce little snowfall but lead to dangerous, sub-zero wind chills Monday night and Tuesday.

4) Colder than normal temperatures continue through the end of the month with storm chances (light to moderate snowfall) every few days.

DISCUSSION

KEY MESSAGE 1: Snow squalls move across northwest PA this afternoon, creating hazardous travel conditions.

Light snow across the Lower Susquehanna Valley has moved off to the east and our concern has shifted to the line of snow squalls that has developed along a cold front moving into northwest PA as of 2 PM. Some breaks in the cloud cover has allowed for weak instability to develop which should help to maintain these squalls as they move farther into Pennsylvania through the rest of the afternoon. The sunshine has also helped to warm road surface temperatures into the upper 30s and low 40s, so in addition to rapidly reduced visibility, there will be the potential for flash freeze conditions as the squalls move through.

Most guidance suggests that the greatest snow squall threat will remain north of Interstate 80, but this will need to be monitored over the next couple of hours. Expect the squalls to weaken later this evening with the loss of daytime heating.

KEY MESSAGE 2: Light snow across southeast PA tonight and tomorrow as low pressure develops off the east coast.

The 12Z model suite has continued to trend towards a more amplified system moving through the northeastern US tonight and tomorrow. Very light snow will initially develop in the typical upslope regions of Cambria and Somerset Counties after midnight as the upper trough arrives before energy transfers to a developing area of low pressure off of the east coast. This system will bring light snow to the Lower Susquehanna Valley tonight and tomorrow, with portions of York and Lancaster County likely seeing up to 3 inches of snow. We will likely be issuing a Winter Weather Advisory for portions of southeast PA to highlight this potential.

KEY MESSAGE 3: An arctic blast Monday will produce little snowfall but lead to dangerous, sub-zero wind chills Monday night and Tuesday.

A cold front tracks through Central PA on Monday. Though moisture will be limited with the front, there may be some light snow that accompanies it. In its wake, surface and steering winds out of the southwest or west- southwest should keep lake effect snow bands directed into New York with mainly dry conditions in Pennsylvania. Min wind chills Monday night/Tuesday morning range from 0F in the LSV to minus 20F over portions of the Alleghenies. The current forecast would support a Cold Weather Advisory in the Laurels and northern mountains (approximately along, west, and north of the I-99/I-80 corridor) and possibly an Extreme Cold Warning in the Laurel Highlands.

Forecast highs in the + single digits to ~20F on Tuesday are 15 to 25 degrees below the historical average, though not quite record setting. Wind chills likely stay below zero west of I-99 and north of I-80 through Wednesday morning. Low- level flow becomes more westerly by Wednesday, which will help to direct lake effect snow bands back into northwest PA.

KEY MESSAGE 4: Colder than normal temperatures continue through the end of the month with storm chances (light to moderate snowfall) every few days.

A persistent west coast ridge and east coast trough pattern in the upper levels will ensure the continuation of below normal temperatures through the end of January. Periodic waves of low pressure traversing the High Plains and Great Lakes mean we'll see frequent opportunities for precipitation. Most of the storm systems appear weak and relatively disorganized at this point, which would favor the highest snowfall totals occurring in the typical upslope and lake effect snow regions of the Laurels and northwest mountains through the end of January.

AVIATION /20Z SATURDAY THROUGH THURSDAY/

Several waves of snow showers and a line of heavier squalls will move across the northwest half of Central PA, dying off this evening. The worst conditions will be at BFD and perhaps FIG. Those locations may drop to near zero visibility (VLIFR) while the wind gusts into the 30s as the squall passes. Some speckles of IFR will be found elsewhere near and at UNV and IPT through sunset, but those terminals would only have very brief dips to IFR (10-20mins). These show showers and squalls will fall apart and weaken early this evening. A brief lull in snowfall will end around midnight in Somerset County and by 3 or 4 AM in Adams/York/Lancaster. That light snow will become more heavy through sunrise, especially in York and Lancaster. The heavier snow and LIFR conditions will be mainly S of MDT and MUI. These area will likely (90%) see LIFR for 3-6hrs. The snow and low cigs/visby will lift to MVFR or better in the early aftn. The winds will become less gusty over the W early tonight. The light wind in the east will be light.

Outlook...

Mon...Cold fropa. West winds gusting over 30kts. Restrictions likely with snow showers especially west.

Tue-Thu...Light snow/IFR possible in the NW at times.

CTP WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory until 7 PM EST this evening for PAZ004>006-010-011-017. Winter Weather Advisory from 3 AM to 3 PM EST Sunday for PAZ065-066.