Wednesday, January 16, 2013

Why the Panic about Winter?

I see a lot of people panicking over this winter; many feel this is going to be a repeat of 2011-2012. This is in part due to how forecasters handled the setup leading into last winter..

Because of this, I thought it would be a good time to go back and look at the winter that wasn't. I can't speak for other forecasters winter outlooks last year. But if they did what I did, then they put too much faith in the fact that it was a La Nina year. And as most of us are aware the winter of 2011-2012 didn't behave the way a typical La Nina should. I paid less attention to the NAO,AO and PNA than I should have. Because of that I busted my long range winter outlook .

Last winter had a lot of things going against it.

1. + NAO and + AO

I wrote about the NAO and AO enough, that I think most of us know they have positive and negative phases. When both are negative, The Northeast sees some of our coldest outbreaks of arctic air, So based on this it shouldn't be hard to figure out that last winter; the phases were a lot more positive than negative.  In fact the winter of 2011-2012 saw the largest sustained positive pressure gradient in recorded history.

On the other hand, winter of 2009-2010 saw the exact opposite, the NAO at that time had some of the lowest pressures ever observed. The result was an extremely active winter across the Northeast.

Here’s a link to a article in Scientific American that goes into this a bit more. it can be found here.   

2. The Polar Jet stream.

The polar jet stream divides cold Arctic air to the north from warmer air to the south. The Jet stream stayed north of the boarder for almost the entire winter. Staying that far north allowed warm southern air to move north and into the Northeast; it also prevented cold fronts to move out of Canada. As a result the cold arctic air stay bottled up in Alaska and northern Canada.

The NAO was one of the Primary reasons the polar Jet acted as it did.  2011 was a very extreme year for severe weather, this was also the result of the Jet stream...The Jet stream being stuck north of the border keep the northern half to the U.S. very warm. However, the south saw a colder winter than usual, in fact Texas saw a White Christmas.   

Here’s a video that explains about the Jet stream.

3. A quiet subtropical jet.

The Subtropical jet is normally quite active and with strong winds during the winter and early spring. Last winter this was not the case., During winter, subtropical jets intensify and can be found between 20° and 50° latitude. Their maximum speed approaches 300 knots, although these higher wind speeds are associated with their merger with polar-front jets. During the winter the subtropical jet will drift northward and merge with a polar-front jet. When they phase we can see very powerful winter storms in the Northeast. The subtropical jet last year was very lackluster. So with the polar jet staying in Canada; along with a quiet subtropical jet; our pattern stayed zonal with not much in the way of storms.  

So far this year.

In my outlook this year I said, this winter would be a slow starter, that the first half would be warmer and less stormy than the second part of winter. The arctic outbreaks that are being forecasted bares out the truth of what I was saying. The pattern in the long range is also looking much more active than we've seen.

In fact the southern Mid-Atlantic is going to be hit by its first major snowstorm tomorrow, this winter! A The snow will start in the morning hours in the higher elevations and will work east through the afternoon and evening. Snowfall amounts of 2-4 inches will be common from DC to Richmond with 3-6 inch snowfall amounts in the higher elevations. As for the Northeast many of us have seen more snow this year than we saw during the entire winter of 2011-2012.

The NAO and AO.


Looking at the NAO chart you can see that since November 19th the NAO has been mostly negative. . It is likely to remain predominantly negative for the remainder of the month. Looking back at 2010 we see a notable long-lived block that developed in the 11/20-12/10. The winter of 2009-2010 saw many long lasting blocking regimes as did the winter of 2009-10 and 2010-11. This blocking lead to very high snowfall totals across the Northeast.

This year the Jet streams are acting completely different than they did last winter. Here's a diagram of how the Jets should behave this winter. As you can see the Northeast is where the two Jets will be battling it out.....So The northern Mid Atlantic and New England should be more active than not.


In my outlook I also said, this winter would see swings between warm and cold outbreaks..So far this too has been the case. The long range teleconnections are pointing in the direction of more cold air intrusions and less in the way of warmer air intrusions for the 2nd half of winter.

So don't lose faith I still think this winter will end up slightly below average in the temperature department with at least average snowfall across the region.........    


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