Sunday, October 30, 2011

Nor'Easters versus Northwest Windstorms

During the last day, a strong Nor'easter (also called Northeasters by some) struck the northeast U.S. with strong winds, rain in the coastal areas, and snow inland.

What made this storm so remarkable was the snow.   Rarely do such storms bring this much snow so early.  Usually the first snow of the season hits the coastal northeast U.S. in mid-December.  From this events, some locations are breaking long term records--such as the earliest 1-inch snow for over a century at New York City.  This storm is NOT a record for earliest snow over the region---historically, most northeast snow have had some snow as early as mid-October. Snow on trees that are still leafed is a major issue, resulting in lots of downed trees and power outages.

 The weekend event is an example of a a Nor'easter--a strong, midlatitude cyclone (low pressure center) that moves up the east coast from the southwest to the northeast over the nearshore waters, roughly paralleling the coast. The winds move around such lows in a counterclockwise way, with the strongest winds generally from northeast over the north and western quadrants of the storm.  Here is a National Weather Service surface chart at 10 PM PST on Saturday night as an example (solid lines are isobars--lines of constant pressure....the closer together they are, the generally the stronger the winds).  Winds are shown by wind pennants at various stations.

Nor'easters are cousins to the big northwest windstorms (e.g., the Inauguration Day Windstorm, Chanukah Eve Windstorm, Columbus Day Storm).    Both get most of their energy from horizontal variations in temperature--thus they are midlatitude cyclones.  This is very different from tropical cyclones, such as hurricanes and typhoons, that derive their power from the warm temperatures and moisture of tropical oceans.

Both Northwest major cyclones and Nor'Easters have impressive satellite signatures, with characteristic cloud swirls and frontal cloud bands.   Want to see?  Here is an enhanced infrared image of the December 12, 1995 storm that hit us hard


and here is the latest east coast storm in the visible:


Large, impressive systems. For the Nor'eastern event...do you see that lines of clouds extending NW to SE from the coast---that is caused by cold air going over warm water.   In general, the very strongest Northwest cyclones are stronger than the top Nor'easters, with our 1962 Columbus Day storm probably being the most powerful non-tropical coastal storm to hit the U.S. in a century. (Beware--those wily east coasters like to call their storms "Perfect" or "Storm of the Century" and get books and movies made about them.  Don't believe their propaganda--ours are bigger and more powerful).

Before roughly 1990, few of these big storms--on either coast--were well forecast by our numerical models.  The 1962 Columbus Day storm came as a big surprise, and the President's Day Snowstorm of 1979 brought unforecast snow to D.C.  (very bad place for forecast failures!).   But by the early 1990s something had changed.  Our models now had sufficient resolution and physics, and satellite information was improving our description of the atmosphere to a point that we began to get these storms right.   For example, both the West Coast Inauguration Day Storm (January 1993) and the East Coast "Storm of the Century" (March 1993--I hate that name), were correctly predicted days before.  And we have only gotten better in time.

Lets see how we did for the current storm.   Here is the model analysis at 5 PM on Saturday.

Nice low off the coast, large coast pressure variations.  The dashed lines indicate lower atmosphere temperature, with the first blue one indication conditions cold enough for snow.  The forecast by the National Weather Service GFS model initialized 36 hr before is shown below...as well as the precipitation forecast (light blue is the heaviest).  Not bad!


Or how about 60hr before? Still quite good, but displaced a big too far offshore.


Keep in mind that some of these strong midlatitude cyclone can have hurricane force winds over water and are the equivalent of category 1-2 hurricanes.  And  midlatitude cyclones are much larger than tropical systems and do not weaken as quickly when they pass over land.

If a big midlatitude cyclone hits the Northwest this year we will have a magnificent view of the details using the new coastal radar.  And if our models are wrong about intensity or position, the new radar will enable us to get out timely warnings in time to save property and lives.  We have finally caught up to the east coast, where they have had radars looking offshore for decades.

Announcement:  The Puget Sound Chapter of the American Meteorological Society will be meeting on Wednesday, November 2 at the Northeast Branch of the Seattle Public Library at 6:30 PM.   UW Graduate Student Luke Madaus will describe the new dual-polarization products available from the newly upgraded National Weather Service radars.  All welcome.   Location: 6801 35th Ave. N.E.  More directions at: http://www.spl.org/locations/northeast-branch/net-getting-to-the-branch

6 comments:

Ferdi said...

It is my understanding that the Columbus Day storm was an extra-tropical storm, the reinvigorated remains of typhoon Freda. And it was forecast because I remember the Seattle times warning of a dangerous storm approaching in an article on the front page that day. My brother and I eagerly anticipated the storm and waited outside for it's arrival. It did not disappoint. It was amazing how fast the wind came up and how strong it got that evening. We were living in Kenmore at the time.

seattle said...

Interesting that the strongest winds of east coast storms are to the north of the low pressure center, while our storms seems to have the strongest winds to the south of the pressure center. Wonder why this is?

Cliff Mass Weather Blog said...

Ferdi...I checked the Seattle Times a few months ago for the days before the storm....no warning. Perhaps someone at the Dept. provided the forecast...cliff

Ferdi said...

It was a long time ago Cliff so I could be wrong. But I remember it as a small one or two paragraph article. Joost definitely knew of it because he got us quite excited. I was 12 at the time and we must have waited an hour in the front yard. At it's height it was difficult to stand against the gusts. Great memory.

larchitech said...

Seattle, take a look at the maps, the circulation around the low, and the fetch over the open water. A noreaster is going up the east coast and has a counter clockwise winds so the land that has the longest fetch is on the north side of the low. Contrast that with the low coming in from the west, the longest fetch is on the south side of the low. The long fetch means there's nothing sticking up to break up the wind.

John McBride said...

Nice discussion of these events, Cliff. Thanks very much. Having been through all of the NW major events, beginning with the Columbus Day Storm, I can't honestly rank them subjectively, even if I know that science might rank order them objectively. The Columbus Day event made a huge impression on my 13 year old mind because I'd never been in wind that could dismantle a building (the old Memorial Stadium in Issaquah was unroofed and the old Memorial field dance hall's roof so badly damaged that it wasn't used much after that and eventually was torn down.

But the other two did a memorable amount of tree damage in the neighborhood we've been in since 1987, including putting a fir tree across us that thankfully DIDN'T do much damage.

I'm still perplexed by the science of these storms, even having read about them. Hurricanes seem much more comprehensible. That the mid latitude low centers can gain so much spin almost seems a contradiction, given the lack of extreme heat that powers hurricanes.

But, then in a sense it's an advantage to being an amateur weather afficionado; weather's science, by being just a little more difficult to quickly grasp retains just a bit of mystery.

By the way, aren't the strongest winds in NW events typically on the southeast and east, or am I confusing a few such examples with the physics of most?