Friday December 20, 2013 | 06:46 AM
Posted by David Epstein

A winter weather advisory is up for the morning as a result of some light freezing rain and drizzle.  Some areas where the roads are untreated could be quite slick and your own driveway and walkway could have become a slipping hazard overnight.  The good news is that coastal areas remained mild enough so icing is not an issue this morning.

There is also a winter storm watch in effect for Saturday night into early Monday.  This has been issued for the potential ice storm (and possible heavy snow well north) over the weekend.

Christmas Eve and Christmas Day

Let's get the good news out of the way.  Both Tuesday and Wednesday look cold and dry with plenty of sunshine and no chance of rain for snow.

Battle of the air masses

There is going to a be a frontal system, along a very strong jet stream, dividing deep winter cold from spring-like warmth cutting through New England this weekend.  Because of where we sit in the country, Maine will most likely stay on the cold side of this boundary all weekend.

Warm air is lighter than cold and while it's still winter-like here at the surface, the warm air is going to pour on top of the cold layer all weekend long.  This scenario of warm air over cold air creates a situation known as overrruning.   This is the perfect set-up for sleet and freezing rain, the latter of which can cause tremendous problems.

If the cold air is very thick at the ground, the rain that is falling will have a chance to refreeze into sleet.  A sleet pellet is a little ball of ice that bounces around when it hits the ground.  It's not hail as that type of precipitation forms in thunderstorms.

Sleet or Freezing Rain

You can see in the graphic the difference between freezing rain and sleet.  For precipitation to be all snow, remove the warm layer in the middle, and for all rain, remove the cold layer at the bottom.   This weekend, the immediate coast is most likely to have rain, inland sleet and freezing rain and the far north snow.

While there has been some light freezing rain this morning,  the main thrust of precipitation  holds for  Saturday night and Sunday.   The exact structure of the atmosphere is going to be critical to determine what falls across Maine.

I will repeat what I wrote on Thursday about ice melt.  I advise you to do today is buy an extra bag of the stuff in case this turns out to be a bad ice storm.  I am not forecasting as widespread a situation as we had back in 1998, when a major ice storm shut down much of the central and southern part of the State.  However, freezing rain, even a small amount,  can make walking nearly impossible.  If you don't have ice melt on hand or the stores run out, it can be a real issue.  If you keep it in a dry place it won't collect into a ball of unusable hard mess for a while.

I think inland areas away from the immediate coast and north of Portland run the greatest risk for significant icing this weekend.   Climatologically, Gray, Augusta, Waterville and surrounding towns along that axis are under the biggest threat.  However, even areas outside my oval could see an icing issue.  As you get further north I think there will be less of an icing threat and more in the way of snow and sleet. Amounts could be significant.

What I will be working on through the weekend is how much precipitation will fall, what type and giving you a better idea of when each wave of this stuff will come through.  Remember, cold air is very dense and powerful and unfortunately can get stuck at the ground to create these potential ice storms. I'll be giving lots of updates here and on Twitter @growingwisdom as the situation unfolds. 

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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 GrowingWisdom.com, 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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