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Cold pair of days with a mostly snow event this weekend.

Posted by David Epstein  December 12, 2013 09:30 AM

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A couple of arctic fronts will push through the region the next 30 hours keeping temperatures significantly below what we would expect before mid-December. This very cold air continuing to spill into the region, 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. You will need the humidifiers for sure. The record low for December 12th is 2F set back in 1988, interestingly, that's the last time a record low was set in December.

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.

No record cold
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 4F and 0F, both set back in 1898, a long time ago. So yes, it will be cold, but not severe, just early.

Sunshine will abound both today and Friday. Although highs will remain in the upper teens across much of Worcester County and only reach the low to mid 20s in Boston, 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 teens 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.

Weekend storm
A few things have become clearer about the upcoming storm. First, it isn't going to be a blockbuster snowstorm. Second, the models are trending slightly weaker and colder. Trends are important in forecasting a couple of days away as they often foretell the final solution. Third, the storm is mainly a Saturday night event with some light snow possible before dark Saturday and after sunrise Sunday.

Wrong jet stream
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. Let's look t the weekend case in comparison to a classic New England snowstorm. The first image below is from the Blizzard of 1978, 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.
Jet Stream Map.png
split flow.jpg
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.

Early details
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. Along the coast and south to Cape Cod, a mix with rain or a complete change is probable before the system ends Sunday. Systems of this nature can still be significant. Since we are dealing with the southern branch of the jet stream there can enough moisture to work into a storm to give a plowable event. The question is, where does it fall and how much cold air holds during the event to give your specific area frozen not liquid precipitation? The cold part of the jet stream is very intense and will likely help keep much of the area cold enough for frozen precipitation most of the storm.

Sleet or freezing rain can also be a player in these set-ups with the cold air hanging tough at the ground while warm air streams overhead creating messy situation. I won't have a good handle on exactly where this is likely until Friday, when we can use some the higher resolution models. Even with some freezing rain working into the system, this isn't a situation to create an ice storm with power lines down etc.

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 Boston, 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.

This blog is not written or edited by Boston.com or the Boston Globe.
The author is solely responsible for the content.

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About the author

David Epstein has been a professional meteorologist and horticulturalist for three decades. David spent 16 years at WCVB in Boston and currently freelances for WGME in Portland, ME. In 2006, More »
Boston Forecast & Weather Tracker - Storm Center
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