Monday January 21, 2013 | 10:18 PM
Posted by David Epstein
We knew this would be a tough forecast and it is proving to be true.  The latest computer models I use are keeping the heaviest bands of snow east of the region and therefore I have lowered the expected snow amounts. 
I'll be updating the situation on Twitter at @growingwisdom. This doesn't mean there won't be any snow. I am still looking at a dusting to a few inches along the coast from Brunswick to Portsmouth.  There may be a few areas that stick out into the water that see slightly more, but the trend is for this to be a smaller event, not a larger one.  The snow we get is going to be like a puffed up pillow, it looks like a lot, but will compress to very little.
The snow will fall from about 2 AM to about 11AM Tuesday with the trend for the heaviest to occur along the immediate coast. 
After the storm leaves the area it will turn even colder.  Temperatures Wednesday and Thursday will struggle to the teens along the coast and stay under 10F degrees inland. Skiing and other outdoor winter sports will require your warmest weather gear.
<strong>Norlun Trough</strong>
The forecast hinges on a very complicated and tricky meteorological phenomenon known as the Norlun trough.  The Norlun trough is a unique area of low pressure that is actually not very uncommon.  We often see a situation like this once or twice a winter and southern Maine is the ideal spot for the event. Most of you have seen the L for low pressure on the weather maps.  That L represents the place on the map where the air is rising and often creating snow or rain.  If the low is close enough to our area then we get inclement weather.  Often our bigger storms are the result of big low pressure centers.   What is unique about a Norlun trough is that the low is very far out in the ocean and would normally be too far away to give New England any precipitation.  However, in the Norlun situation a bit of the energy from the low reaches back to the west and connects with another weaker storm often over Ohio or New York.  I drew a yellow line on this map to illustrate the connection between these two storms.  
Eventually, the whole system can end up forming one storm in the Gulf of Maine which will rapidly move east and then pull down cold air in its wake.  One of the reasons why it will be so cold Wednesday and Thursday is that as this Norlun system leaves, colder arctic air can filter in from the north. 
The tricky part of the whole situation is that the exact configuration of the trough is critical to who gets flurries and who gets heavy snow.  Areas close to the trough can see heavy snowfall much like during a lake effect event.   Snow can accumulate many inches in a just a few hours.  There isn’t much wind with these situations and the snow is often very light which help it to pile up rapidly.  I have seen Portland get a foot of snow that you could almost push away with a broom from a Norlun trough storm.
If you are curious about the name of this system , the “Nor” comes from Steve NOguieRa, and the “lun” comes from Weir LUNdstedt.  In 1993 these two meteorologists authored a paper which described the aforementioned situation that has come to be known as the Norln Trough.
Once the snow ends later Tuesday it will be dry and very cold for Wednesday and Thursday. Temperature will remain in the teens for both of those days and at night many places could near zero.  On Friday another storm threatens, but let’s get through Tomorrow before we focus on those details.

Gardening this week All the dry air associated with the cold isn't good for our indoor houseplants. Recently, I took a trip to greenhouse and saw how they care for their plants in winter. Hopefully, some of the tips help to keep your plants healthy until spring. I'd love to hear your thoughts on this blog or any others. Please follow me on Twitter at @growingwisdom and check out my latest videos at



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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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