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WHAT HAS CHANGED

* Increasing chances for heavy snow this weekend. More details provided in discussion section. * Extreme Cold Warning issued for Cambria and Somerset Counties.

KEY MESSAGES

1) Frigid temperatures and dangerous wind chills expand from northwest to southeast Friday afternoon into Saturday morning

2) Increasing odds for double digit heavy snow accumulation and widespread moderate to major winter storm impacts Saturday night through Monday morning, heaviest on Sunday.

3) Arctic blast persists with frigid temperatures and subzero wind chills expected next Monday all the way through next weekend.

DISCUSSION

KEY MESSAGE 1: Frigid temperatures and dangerous wind chills expand from northwest to southeast Friday afternoon into Saturday morning.

Arctic air is poised to return to the forecast area - filtering in from northwest to southeast through Friday night. High confidence in -10 to -20F wind chills across all of central PA Friday afternoon into early Saturday morning. The coldest wind chills, potentially as low as -25F, will occur in the Laurel Highlands. As a result, we have issued an Extreme Cold Warning for Cambria and Somerset Counties. Frigid Arctic air will remain in place prior to the onset of the significant winter storm this weekend.

KEY MESSAGE 2: Increasing odds for double digit heavy snow accumulation and widespread moderate to major winter storm impacts Saturday night through Monday morning, heaviest on Sunday.

Snow accumulations will likely begin with a vengeance from south to north in the predawn hours Sunday. Periods with snowfall rates greater than 1 inch per hour are becoming increasingly likely in the isentropic ascent early Sunday and as low and midlevel frontogenesis becomes stronger later in the day and evening. Given the dry, fluffy snow and wind gusts of 15-30 mph, there will likely be some blowing and drifting. There could be a period of sleet especially across the southern tier Sunday evening cutting into snow totals just a bit, but not before double digit accumulations are expected to fall.

While warm advective, front-end snow is becoming increasingly likely, there is considerable model uncertainty with the placement of a comma head area of snowfall as a secondary low pressure system develops offshore (in a Miller B type evolution). If the comma head sits over central PA Sunday night, several additional inches of accumulation are possible. However, if it misses us to the north and we see more of a dry slot, that would result in snow ending sooner Sunday evening with less accumulation overnight.

All in all, the probability of snowfall amounts exceeding 12 inches has increased to 60-80 percent across much of central PA. The probability of more than 18 inches ranges from 15 to 40 percent. Amounts near 24 inches can't be ruled out (10 percent chance) especially if the comma head sits over central PA Sunday night. Always prepare for the worst case scenario and continue to check the forecast as it evolves. Now is the time to prepare. Think snow blowers, re-arranging travel plans to avoid travel during the storm, what to do in case of a power outage, etc. Remember it will be very cold before, during and after the storm. Never run a generator indoors due to carbon monoxide.

Now if you really enjoy meteorology and want to know *why* we are predicting the most significant accumulations in the past 5+ years, feel free to read on through the remainder of this KEY MESSAGE 2 discussion. We've had our fair share of cold snaps this winter season, and tomorrow (Friday) will be no exception after the next Arctic front pushes through. A large factor driving all this cold weather has been the Polar Vortex displaced from the North Pole, with a tendency for the vortex to find itself over Hudson Bay and northern Quebec. With displacement that far south, we've gotten plenty of cold air, but it's also nudged the storm track south of our area for many of the recent events. If you've ever heard people say "it's too cold to snow", it's not that it's physically too cold to snow, it usually just means that the storm track has shifted far enough to the south that we end up under dry high pressure while our temps are cold.

One key difference this time is the influence of a Pacific storm, located off the coast of southern California this evening, and a strengthening subtropical jet to its south. Over the next 24 hours these features will cross the Baja Peninsula of Mexico and then into Texas. Southwesterly flow out ahead of this southern stream feature will pump warmer air into the Gulf Coast and Southeastern US Saturday night into Sunday. At the same time, a northern stream shortwave trough coming down from western Canada into the Great Plains will bring a reinforcing shot of cold air to the back side of the northern longwave trough. The increasing temperature gradients will result in an overall increase in wind speed aloft at jet stream altitude, and an amplification of the northern stream trough and ridge. By Sunday, the right entrance region of the northern jet and the left exit region of the southern jet will align to produce enhanced upward motion through the troposphere. Remaining uncertainty in the forecast largely depends on the degree of phasing of these two jets, the angle at which they align, and how much amplification there is of the ridge out ahead of the deepening trough.

The very cold air in place ahead of this system also plays other crucial roles to increase impacts. At 00Z Sunday, model consensus depicts a 1040+ mb high over New York State, which only slowly moves northeastward across New England over the next 24 hours. The deep, cold blocking high to the northeast of a winter storm is a textbook feature that supplies a stream of cold air down the Alleghenies and provides a cold dome for warmer, less dense air parcels streaming in from the southwest to ascend over. That upward motion is key to production of hydrometeors - in this case snow. And then there is the key role that cold temperatures have on ice crystal habits. The so called "dendritic growth zone" exists where temperatures are between roughly 0F and 10F. Given low temperatures Saturday night in the single digits, some spots might start off with a DGZ that extends from the ground up 10,000 to 15,000 feet which is pretty remarkable for a large synoptic storm in this region. As long as the column saturates quickly enough, any snow that falls late Saturday night and into early Sunday morning could be very high ratio, fluffy, rapidly accumulating snow that consists of many pristine dendrites (SLRs 15-20:1, could be even higher if the column saturates faster but there is dry air at low levels to overcome at onset). So after a few hours of virga, accumulations have the potential to begin with a vengeance in the predawn hours Sunday, especially from I-80 south.

KEY MESSAGE 3: Arctic blast persists with frigid temperatures and subzero wind chills expected next Monday all the way through next weekend.

A prolonged stretch of frigid temperatures continues next week through the end of January (next weekend). After the cold snap Fri night - Sat, the next round of cold wx headlines (-10 to -20F wind chills) is likely Monday night through Tuesday morning. Given the prolonged nature of the bitter cold, we may begin to see more infrastructure impacts (frozen pipes) by next week.

Occasional lake effect/upslope snow showers are likely next week under a very cold cyclonic flow aloft.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

An Arctic front will cross the region today, but limited moisture will keep clouds and precipitation to a minimum. Partly cloudy skies and VFR conditions are expected across the forecast area today outside of BFD. Cloud cover currently across western New York will continue to gradually shift southward and get into BFD with MVFR conditions becoming more likely after 12Z. Breezy winds will pick up in the wake of the front. Gusts will generally be between 20 and 30 mph.

Outlook...

Sat...Dry most of the day, then SN begins to enter south-to- north late spreading across region and intensifying overnight. Widespread IFR likely Sat night.

Sun...Impactful snow storm continues with widespread IFR restrictions, LIFR possible.

Mon...Storm lifts to the NE and cold northwest flow takes over, bringing snow showers and continued restrictions to the NW, and improving conditions to the SE.

Tue...Lake effect snow across NW PA with restrictions possible; otherwise, dry and breezy.

CTP WATCHES/WARNINGS/ADVISORIES

Cold Weather Advisory from 1 PM this afternoon to 10 AM EST Saturday for PAZ004>006-010>012-017-018-037-041-042. Winter Storm Watch from Saturday evening through Monday afternoon for PAZ004>006-010>012-017>019-024>028-033>037-041- 042-045-046-049>053-056>059-063>066. Cold Weather Advisory from 6 PM this evening to 10 AM EST Saturday for PAZ019-025>028-034-035-045-046-049>053. Extreme Cold Warning from 6 PM this evening to 10 AM EST Saturday for PAZ024-033. Cold Weather Advisory from 10 PM this evening to 10 AM EST Saturday for PAZ036-056>059-063>066.