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What happened to the snow?

Posted by David Epstein  January 22, 2013 01:07 PM

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We knew this would be a difficult forecast and it proved to be very true. Last night, as the latest computer models were arriving, I quickly noticed a trend to keep the heavy snow offshore. This meant that the heavy snow that was possible for some of you, wasn't going to happen. Across southern New England the heaviest ended up being the outer parts of Cape Cod and some southern areas of the south shore. For example Welleleet will put 4.5 inches of snow in the record books from this storm. The highest amount I have seen reported was West Tisbury on the Vineyard at 5.5 inches.
Ruc latest2.png
I'll was updating throughout the night on Twitter at @growingwisdom I will be updating Friday's storm on Twitter over the next few days.

Elsewhere, including the Boston area, a dusting to a couple of inches of fluff fell. Even in New Hampshire, where the most snow was forecast, the storm will not materialize this morning. The basic reason for the change is that the data we are getting continues to keep the heaviest snow about 30-50 miles east of where it was forecast yesterday afternoon. Therefore, the western edge of the heaviest snow will now stay over the ocean and not impact the north shore or southern Maine and New Hampshire as forecast Monday.

Models not good enough for small events yet
The computer models we use can "see" weather events that are bigger than about 12 kilometers. This snow event was so small, that it fell in-between the grids that make up the computer models. Tiny meteorological phenomena get smoothed too much and therefore, while we know they will materialize, location is very difficult. It's a bit like taking a picture of your face with a typical camera, you know your eyelashes are there, but the camera isn't quite sharp enough to see each individual lash. Over the next few years, as computer models get even better, we will be able to forecast these types of events with more certainty. Years ago, it was the big events that were not forecast well, now it's the smaller ones.

The cold that was predicted is still coming, and the next two days look to be the coldest of the winter. Wind chill advisories are up for central and western Massachusetts and parts of northern New England. Highs today will be in the lower 20s, but remain in the teens, even in Boston tomorrow and Thursday. The cold will moderate as the snow begins Friday.
windchill3.jpg

Friday's snow
A more general snow is still in the cards for later Friday with several inches likely. Unlike the forecasting nightmare tonight, Friday will be a more typical weather system so we will have higher confidence in that forecast by later tomorrow and tomorrow night. This looks like a Friday night storm and will be winding down by Saturday morning.
Norlun Trough
The recent forecast hinged 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. Most of you have seen the L for low pressure on the weather maps. Norlun trough dave epstein meteorologist.pngThat 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. Norlun trough Maine dave epstein meteorologist.png 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 Tuesday 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 GrowingWisdom.com

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 »
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