Thursday December 12, 2013 | 06:43 AM
Posted by David Epstein

 As very cold air continues to spill into the region, it will play a role in our upcoming weather event this weekend.  This cold air is coming straight from the arctic regions of the planet and will not only bring cold temperatures, but very dry air as well.  The record low for December 12th is -15F set back in that brutal winter of 1977-78.  In recent years record lows in December are not commonplace.

I'll be updating the forecast as it changes on Twitter at @growingwisdom Please follow me there. Feel free to comment or ask questions too.

While you are hearing words like frigid, arctic cold, bitter blast etc.  In the span of decades of records, the upcoming cold doesn't even begin to approach those levels.  The records for the 13th and 14th are minus 5F and minus 9F, so yes, it will be cold this week, but not severe.

Sunshine will abound both today and Friday and although highs will remain in the teens across much of the ski areas and only reach the lower 20s in Portland and along the coast.  There will be a bit of breeze today and conditions become even windier Friday afternoon.  However, it won't be a pair of days where being outside is painfully cold.  There are years in the record books where temperatures didn't break out of the single digits on these same days.  That said, be sure to bundle up if you have to wait outside for a bus or the subway the next couple of mornings.

Wrong Jet Stream

A few things have become clearer about the upcoming storm.  First, it isn't going to be a blockbuster snowstorm.   There are very few hard and fast rules in weather, but one thing we often need for a big snow event is for both the polar and subtropical jet streams to merge.  You probably have heard the term jet stream countless times, but you might not realize there are actually several jet streams that circle the globe. 

In winter, the two primary streams playing into our weather are the subtropical jet, with moisture and warmth and the polar jet, with atmospheric energy and cold.  If you combine these two, you can have the beginnings for a good storm.  Of course the exact position of the jet stream is also critical as well as the amount of cold and moisture.  The first image is from the "Storm of the Century" back in 1993, let's call it the gold standard of big storms.  The next image is the projected jet stream this weekend.  Notice how the flow is split.   This split flow means we can't get all the ingredients together for a big storm and to be honest, that's fine by me right now.

Too many of you need the last couple of weekends to shop for Christmas and also there are lots of social gatherings this weekend. I would be fine if the entire system ended up going out to sea, which isn't likely, but I can hope.

What is likely to happen is enough moisture from the southern stream and the cold from the north combine to bring a period of snow to the area later Saturday and Saturday night.  Because the flow is split, and the northern branch is so strong, much of the precipitation should fall as snow, even at the coast.  For sleet and freezing rain to work into the system, we need warm air above the ground and cold air to remain at the surface.  That scenario isn't likely with the arctic cold so firmly in place heading into the weekend.

Behind this storm, another arctic blast will make it into the area keeping temperatures well below their long term averages.  December is off to a cold and perhaps somewhat snowy start.  Over the past 100 plus years in Portland, December brings about a foot of snow to the area.

I'll give you more specific details in terms of snowfall projections this afternoon.  I want to get a look at the European model around 2PM and compare it to the morning models.   

Outdoor project this weekend In this video I show you a project you can do this weekend and bring the kids along too.

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About the Author

David Epstein has been a meteorologist for more than 25 years. He spent 16 years in Boston and currently freelances at WGME13 in Portland.

In 2006, David founded, a business producing educational and marketing videos for the green industry. He currently is a professor at Framingham State College, teaches Jan Plan at Colby College and owns Bloomscapes Inc., a landscape design business.

David authored "Gardens Of New England" and his work has been published in Grolier's Science Annual for 10 years. He lives in South Natick, Mass., and has a summer home in Harpswell.

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