How does the weather change so fast?
We have been gusting to 50 mph all weekend. Wind is air moving from high to low pressure. On today's weather map we see a 982 millibar (28.99") Low Pressure center north of Montreal. To the east, north, and west of that low is some of the coldest air so far this winter. We had a taste of this chill Friday the 18th when Many spots in New Hampshire and Maine cooled to -20. But the High pressure system that brought the chill raced off the Mid Atlantic coast Saturday.Wind on the backside of the high pressure, from the southwest, reversed the chill. When high pressure goes south of New England the cold does not hold. Especially with such a powerful storm in Quebec. When high pressure goes off to our north, we get cold air damming against the mountains and warm air has a very hard time moving back into New England. That set up occurs late this week with a major storm possible.
But before we get there, an Alberta Clipper arrive from the southwest Monday Night with a period of snow likely. The Clipper will intensify rapidly as it races out sea on Tuesday. Usually that is the end of the story, and inch or two of snow and done. But not this time. This time we have to track a Norlun Instability Trough, sort of a tail on the clipper.
Mike Cullen shared this picture with us from Newport Rhode Island where a hundred year old hardwood succumbed to the warm wind.
Even as southern New England warmed to the 50s, a series of snow squalls, some with thunder, roared across Vermont, New Hampshire, and Maine.
Wildcat Mountain New Hampshire Sunday early afternoon.
Moosehead Lake, Greenville Maine noon Sunday. The only real safe ice this winter is in Northern Maine. (Lake Winnipesauke New Hampshire is still open water)
Acadia Maine Sunday Sunset. Spectacular stratus formations.
Now what about the tail on the clipper?
It's called a Norlun InstabilityTrough.
The Norlun trough is a rare event, that can produce extremely heavy snow, and is (or was) nearly impossible to forecast.
But our friend Weir Lundstedt, originator of the theory says this set up reminds him of the storms that inspired his research 20+ years ago.
Weir was inspired by two heavy surprise snowstorms that seemingly came out of nowhere. Each was very localized and developed, then dissipated very quickly.
The first was near Portland Maine on the first day of Spring 1992, when 18" of snow fell in 8 hours.
Then the next winter it happened again, this time 18" of snow fell in 6 hours on Cape Cod. The map for the Cape Cod event is shown below.
This week we have a similar set up for Monday night January 21, 2013 (and maybe again Friday)
The Tuesday morning forecast map is show below, note the similarities. A storm way out in the ocean appears to have a tail.
That 'tail' is a trough of low pressure. A trough is different than a cyclone. In a trough, wind blows in more of a straight line, from the northeast on one side, and from the west on the other.
In a cyclone, air moves in a circular pattern toward thelow pressure center. Both the trough and cyclone have convergent air flow (wind blowing into itself). Convergence forces the air the flow up into the sky.So why are Norlun Instability Troughs (NIT) so special?
1. They become nearly stationary for a 6 hour period.
2. Snowfall rates exceed 2 inches per hour.
3. Liquid equivalent ratios are often double to triple the normal.
Normal snow to water ratio is 10 to 1.
In the NIT, ratios may be 20 or 30 to 1.
That means one half inch of liquid equivalent, can produce 10"-15" of snow instead of the more often observed 5", or 3" in the case of a wet storm.
Cape Cod and many coastal communities usually see the wet, lower ratio snow.
Another factor is wind, though it may be calm for a short time as snow begins, the wind usually cranks up as the snow intensifies, creating a brief blizzard.
Weir, twitter.com/4castrnh, sent out a Sunday tweet that got all of our attention.
"most sgfnt NORLUN trof since 1993 may impact parts of Srn New Eng spcly CAPE COD Monday nite/early tues if models verify!"
I spoke with him about the up coming event.
"We have unbelievable vertical motion in low levels of the atmosphere, great Omega."
"Dentritic growth, snow ratios of 20 to 1 or higher."
"This happened two winter's ago in Danbury Connecticut"
"No doubt 2"-3" per hour snowfall will show up somewhere near the coast, someone should see a foot of snow"
"What could go wrong is that trough get's absorbed into the storm, and sails out to sea"
There is always a "but it could miss".
As Weir loves to say.. "It should be very interesting"
We also talked about why the shorelines of Cape Cod and (New Hampshire) & Maine are shaped just right for this event. What do both shoreline features have in common?
They are Bay like features, the curve of land north of Cape Ann, and Massachusetts Bay each have water more or less surround by land on almost three sides.
Because water is warm (source of heat and moisture) and air over land stays colder, we get a sort of opposite sea-breeze effect. Air rises over water, sinks over land, that is a meso low pressure center. The meso low pressure over the water draws the air to blowin from all sides, convergence.
In the case of this event we may have two, heavy areas of snow. One near the Cape Cod curve, and another near Maine/New Hampshire. You may have noticed that quantitative precipitation forecasts (QPF) from the weather models have been jumping back and forth, and in some cases showing 2 QPF maxes, both north and south of Boston.
The models have become much better in the last 20 years, when events like this seemed to come out of nowhere. Inquiring minds like Mr. Lundstedt needed to know.. and now NIT's are part of regular forecast discussions from all weather services, including The National Weather Service Taunton. That is why we have Winter Storm Watches both north and south of Boston, but not in Boston.
Weir and I will be chasing the snow and keep you updated on twitter.Weir Lundstedt & Steve Noguera presented the original NIT paper in 1993.
Happy 20th Norlun! The good people at weatheranswer.com scanned the paper and posted on the internet for us all to read about the motivation and hard science behind this very special type of snowstorm.
A great contribution to the field of Meteorology.
Matt Noyes also wrote a great paper explaining NIT in more laymen terms, when he saw it approaching Danbury in January 2005.
While we are here...
I have to send a Shout Out to The Frodigh Family and everyone at Black Mountain New Hampshire.
Too Kind, Thank You! See you at The Shovel Handle Pub.