I am a meteorologist from New York who has been studying and forecasting the local weather for years. I especially enjoy tracking winter storms.
By: NYCvort, 11:45 PM GMT on November 30, 2011
Winter Forecast 2011-2012
Delayed but not Denied!
Other than a remarkable October snowstorm for inland areas, the transition to winter has not been a pretty one for snow lovers. This November has featured a persistent ridge north of Hawaii, western North America trough, eastern US ridge, and a trough near Greenland:
This overall +AO/+NAO/-PNA pattern has kept us warm. Just take any GFS ensemble mean run and put it out to the last prog and you see the same thing: a ridge north of Hawaii, and cold anomalies in the high latitudes/Greenland.
The persistent ridge north of Hawaii sets off an evil chain reaction for eastern US snow lovers:
Cold anomalies in the high latitudes/Greenland (+AO/+NAO) keep the pattern very progressive and the cold air locked up. The once valid argument that we were in a similar situation last year is rapidly falsifying with each passing day. While last year at this time we were in the process of entering into a much colder pattern, this time around we can’t even find a favorable pattern on any one of the extended forecasts for the teleconnection indices.
On to the forecast…
A reasonably significant and, more importantly, fairly predictable index of use for the long range is ENSO state. Whereas the AO, NAO, MJO, etc. may be at times more significant to our pattern than ENSO, I can say with reasonable certainty that we will be in a La Nina pattern this winter. I can’t make a statement nearly as confident with regards to the other indices. So let us begin with what this winter’s ENSO state can tell us.
This year will be our second consecutive La Nina winter. As you can see above, all of the guidance supports this idea. Obviously since it is the second straight Nina with only moderation to neutral in between, the globe has cooled relative to average and thus the pattern will be different from last year. So keeping this idea in mind I decided to make composites using first and second year Ninas as analogs (there were five of them since 1948) to compare them. I came up with some interesting results. First, just to show you for comparison, here are the 500mb pattern composites and corresponding surface temperatures for first year Ninas (what we had last year).
1st Year La Nina Composites: (click to enlarge)
Now, here are the 500mb pattern composites and corresponding surface temperatures for second year La Ninas (what we will be in this year).
2nd Year La Nina Composites: (click to enlarge)
As you can see, these second year La Nina winters were quite warm, especially compared to first year Ninas. There are two different things that I want to call your attention to. Of course, the first thing that you probably see is how much warmer it is over the east in second year Ninas. This can be observed on both the 500mb chart (note the anomalous ridging throughout the Dec-Feb period). There is strong ridging over the Pacific, troughing in the west, and subsequent ridging in the eastern US. But second, take a closer look and note that in first year Ninas we generally see Greenland blocking (positive height anomalies in that region). On the other hand, in second year Ninas, note how there are no such positive anomalies near Greenland. That makes me think that a prolonged negative NAO may not be supported this year. Also note how we see a lot more purples (strong negative anomalies) in the high latitudes in second year Ninas as opposed to first year Ninas. This would also argue for a positive AO to continue much of the time as well.
Also against a negative NAO developing are the recent cold SST anomalies that have gradually developed around Greenland:
Here are the AO, NAO, and PNA forecasts for the next two weeks:
We continue to see mainly a positive AO, positive NAO, and negative PNA into the near future.
I covered multiple factors in favor of a positive NAO continuing. Now for more on the AO, let’s take a brief look at the current state of the stratosphere:
Note that for a while now we’re been running close to the 1979-2008 minimum value for this time of year. That is very bad if you want high latitude blocking. Take a look at how we have a strong polar vortex near the pole, keeping all of the cold air locked up:
A cold stratosphere supports a continuation of a positive AO into the near future, and stratospheric forecasts continue to support this. Considering that there is generally a lag between changes in the stratosphere and the AO, in my estimations this would likely keep us locked in at least through most of December, if not longer.
With a La Nina in the Pacific and a cold PDO state, there is little to argue about the PNA. During the winter, it is likely to remain negative for the majority of the time. The only thing that I could see turning the PNA around for the better (positive) would be the MJO. The MJO could temporarily alter the PNA like it did last January, but unfortunately the MJO is very difficult to predict in the long term.
Another thing that I want to touch is upon quickly is the current state of the QBO. The QBO is expected to be falling into the negative phase this winter:
I took composites of other years when the QBO was anomalously positive in the fall and then fell to anomalously negative during the winter (since 1948).
December does show a ridge over eastern North America. It does also show a strong negative anomaly underneath that I’m not really buying based on where the pattern is going right now. I also don’t buy the Greenland blocking in January. I think it will be a bit slower to develop based on the very cold current state of the stratosphere as well as the 2nd year La Nina analogs. But note how we gradually see the negative anomalies transported to the east by February. That I do buy.
And finally, for what it’s worth (and trust me, it ain’t worth much), the CFS keeps us very toasty in December before gradually cooling us off and moistening things up a bit:
But I do show the CFS because interestingly enough it does match up very roughly with my analogs.
In summary, my forecast calls for an anomalously warm December with a +AO/+NAO/-PNA dominating. Based on the QBO composites, we could see some negative anomalies try to sneak in underneath the ridge, so based on that and the 2nd year La Nina analogs I will keep December just slightly above average. Based on my composites, the temperature departures look to peak in January with a +AO/+NAO/-PNA continuing much of the time. Expect a progressive pattern to continue through December and most of January, where temperatures may be slightly below normal one week but then rebound to well above normal the next. This would also keep storms moving along quickly with little blocking. Then, look for an overall cool down as the pattern retrogrades and becomes less clear-cut (variable AO/NAO/PNA) by February. Thus, I think the latter part of the winter looks to be more promising for potential snowstorm(s).
I also expect total snowfall for the season to be slightly below average this year in New York City.
Updated: 12:44 AM GMT on December 01, 2011
Light Rain Mist