Winter Weather

Do you remember the last two Decembers? If not, here’s a short description: Foggy, Rainy, October-like weather. While areas of Northern PA did alright in the snow department, it was an absolute nightmare for snow lovers south of i-80. Many locations in Southern PA recorded absolutely no snow the entire month. Back in December 2015, we were wearing shorts on Christmas Eve.

This trend is about to change in a big way, as predicted in our winter outlook a few weeks ago. There will be a short period of above average temperatures to start the month of December. However, by around December 7th a major pattern change is expected. Yes, we are still 9 days out from this change, but literally everything is in favor for this change.

Earlier this month, a few weather outlets (not us) stated that since the NAO and AO were negative, and Greenland Blocking was in place, there was going to be a storm around Thanksgiving. But there is much, much more to getting a winter storm than that (especially in November). We have been stuck in a zonal-flow pattern. The Pacific Jet has been quickly moving and sitting to our north. There has been no amplification to the jet, which is needed for storms to “dig” and gain energy in the Lower 48. As a result, all of the snow cover has stayed north of the jet in Canada.

Moving into December, many factors that strongly impact the jet stream will become much more favorable. The PNA is finally moving positive, and the EPO is heading negative. For the past month, these teleconnections have been in an unfavorable phase. Furthermore, we will be seeing a low AAM phase, and the MJO moving towards phase 7 which typically favors a cold pattern in the Central and Eastern US.

If at this point you think we are talking in another language, that’s fine. Moral of the story is everything that influences our pattern here in Pennsylvania is becoming very conducive for cold air and winter storms. Below is an image showing the negative (unfavorable) PNA that we have had all of November, and the upcoming change to a positive (favorable) PNA phase.

The models have been picking up on this change in a big way. The American, European, and Canadian model has been indicating this change for a while. Even the usual “blow torch” CFSv2 long range model is predicting this change. Here is the GEFS model, comparing December 1st to December 8th.

So what can we expect out of this major change? High temperatures will be held to the 20s in Western and Northern PA starting around December 8th. Even the Lower Susquehanna Valley, and Eastern PA will see daytime highs only in the low to mid 30s. Nighttime lows will be in the teens and low 20s across the state. How about the thing that really matters, snow?

We expect to see multiple snow chances leading up to Christmas. The potential will be there for bigger winter storms, but smaller snow events (below 6″) look more likely. For those that enjoy having snow on the ground as much as possible during the holiday season, even small events will do the job thanks to the freezing daytime highs anticipated.

As winter begins to get cranking, be sure to have our facebook page liked >>> Click Here to Like PA Weather Action on facebook. Also, download our app as we will be putting exclusive region-by-region maps on it ahead of winter weather events >>> Click Here to Download the PA Weather App!

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