A blizzard aftermath |
The last time a classic Nor’easter
hit the Northeast Coast of North America was during the Blizzard of 2006. Although this storm was extratropical in
nature it formed an eye just as a hurricane does. The center of a snowstorm like this stays
just off the coast of New England . The heavy snow stays on the Northwest
quadrant of the storm. The limiting
factor as to how much snow depends on the
amount of moisture the storm picks up on its way north from its birthplace in
the Gulf of Mexico .
A Nor’easter becomes a blizzard when certain
conditions are met, the temperature must be colder then 20˚ F. At the same time the wind velocity must
exceed 35 MPH. It appears that the storm
plodding its way up the East Coast of North America on Dec.19, 2009 meets these
criteria, and will be a blizzard. The
weather forecasters have already dubbed it to be one.
A storm
like this has its start when a cold wave comes down along the front range of
the Rocky
Mountains
and collides with warm water coming north from the Gulf of Mexico . The cold air plunges down, and its leading
edge turns to the left behind the warm air.
This produces a counterclockwise spin in the atmosphere that forms a Low Pressure Center , and a new storm is born.
The
storm then comes out of the Gulf of Mexico and across Florida where the northward flowing
warm waters of the Gulf
Stream
propel it northwards. At the same time
the Gulf
Stream
keeps pumping more moisture into the storm causing it to contract in upon
itself until under the right conditions it can form an eye like a moderate
hurricane.
It
often requires the merger of several small storms coming together that were
formed by the same mechanism to cause the formation of a Nor’easter. The storm usually forms completely over the
ocean east of North Carolina . By the time they have reached the Midatlantic Coast they are fully developed and
can ravage the coastal states and provinces as far north as Newfoundland .
The
storms follow the Gulf
Stream
northwards along the northeast coast until they pass Newfoundland and vanish into the North Atlantic where such storms have been
known to linger sometimes for weeks.
It is
when the storm’s center crosses the 40 and 70 degree intersection that we get
the most snow. If the center of the
storm is inside this intersection we will have snow changing to rain, often an
icestorm. If it is further out in the Atlantic Ocean the precipitation is lighter, and
we might just get a dusting. If the
storm passes to our west we will receive just rain with warm southerly winds.
Although
they are most common during the months having an “R” in their names a
Nor’easter can occur at any time of the year that the conditions for their
formation is met. They are capable of
producing prodigious amounts of moisture on land from the moisture they pick up
over the Gulf
Stream ,
and deposit it on land as either rain or ssnow.
These
great storms follow a cycle of about twenty years according to a phenomena
called the “Arctic oscillation>” When
this weather phenomena is on its positive side as it has been recently the
airmass moves quickly enough so the storm is unable to draw down large masses
of cold Arctic air upon which it feeds.
If we are in the negative side of the oscillation the air masses move
more slowly, and the storm is able to feed on the Arctic air and thereby
intensify. It appears that we may be
coming under the influence of the negative side of the Arctic oscillation and
can expect colder and wetter winters.
These
storms differ from a hurricane that feeds on warm tropical air; they feed
rather on cold Arctic air. There is
always a cold mass of air to their west upon which the storm feeds. How intense a storm is apt to develop is a
function of the difference in temperatures the two air masses. The greater this difference is the stronger
the storm.
The
name for these storms is derived from the apparent direction the wind comes
from when it is affecting the northeast coast of North America blowing predominantly from the
northeast. It shifts to the northwest
after the center of the storm has passed you.
The name Nor’easter comes from the way the old compasses were laid out,
and the name Nor’easter goes back to the days of early exploration by the
Europeans of the New
World . The earliest use of the word was apparently
1538.
References:
Nor’easter,
Wikipedia, http://en.wikipedia.org/wiki/Nor'easter
Nor’easters, The Weather Channel, http://www.weather.com/encyclopedia/winter/noreast.html
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