When I first attempted a winter forecast in November 1997, I looked at one major piece of data: the developing very strong El Nino. It seemed like a simple thing to do. A recent study had shown strong El Nino’s were associated with very warm and non-snowy winters in our part of the world.

So I went on air and said that. And that’s just what happened that winter-less than 1 inch of snow FOR THE WHOLE WINTER.

It hasn’t been that easy in any year since. The succeeding winter forecasts became so popular that we could actually see a big “spike” in the ratings on the night we aired it. Of course, our competitors saw those ratings too, so now every station does it -- some on the same night we do ours. What a coincidence!

There is more confidence in some years than others. But I don’t have the luxury of “passing” on a particularly tough forecast. That was the case last year, when there were conflicting signals in the atmosphere. Would the old correlations of El Nino/La Nina and others work out, or was something different going on?

The Past 2 Amazing Winters:

The previous winter, 2009-10 was astonishing: our snowiest winter, by far, with our second and third biggest snowstorms ever recorded in Philadelphia. That wasn’t supposed to happen with the strong El Nino at the time. Not even close! Strong El Nino’s (not quite as strong as the 1997 one) are known to flood the U.S. with warm air, while helping to trigger lots of storms. But those storms would produce rain, since there wouldn’t be enough cold air around for snow.

We had an opposite pattern in the tropical Pacific last winter. It was a pretty strong La Nina: unusually cool water. Those patterns in the past often allowed arctic air masses to come into the U.S., but there wouldn’t be much moisture around. So snowfall for the season would normally be near or below average.

Of course there are plenty of other factors to consider, but a strong El Nino or La Nina should have been the main factor. Not anymore.

The other major factor we try to consider is the NAO (North Atlantic Oscillation). It has a very clear-cut correlation with our weather. A negative NAO leads to cold and snowy weather here, while a positive NAO leads to the opposite.

There are two problems with this: first, the NAO often changes a lot during the winter; and second, scientists haven’t been able to predict it accurately more than a week or two in advance. So it’s hard to consider the NAO while making a winter forecast in November.

During the 2009-10 winter, the NAO, and it’s “cousin”, the AO (Arctic Oscillation) were extremely negative, especially the AO. That brought down enough air to counteract the supposed warmth of the El Nino. Now we had cold AND lots of storms, which added up to the record snows. It wasn’t an especially cold winter, but just cold enough that the big storms were virtually all snow, without the usual mix or change to rain.

During last winter, the NAO and AO were mainly negative again, which made the winter stormier than would be expected with the strong La Nina. We had one giant snowstorm in December and another in January. Even with less snow than average in February, we still ended up with 44 inches for the season.

This Winter:

Will we see another freakish winter this year? First, we want to see if there is a particular reason for the prevailing negative NAO and AO the previous two winters. Is there a new factor to consider? After predicting only slightly above normal snowfall in my November forecast in 2010, which was promptly followed by that December storm, it all started adding up. I did a Winter Forecast update on January 3, 2011 which accounted for my new findings, and went for a 30- to 40-inch snow season, with lots of January snow. That update (something I’ve rarely done in the past) pretty much nailed it. I am now making this winter forecast based on the “new” philosophy.

Here is an animation of ocean temperatures compared to “normal” in degrees Celsius.

This not only shows the La Nina, but a pattern in the North Pacific known as a negative PDO. There’s very warm water over much of the Central North Pacific, with a cold water area off the coasts of Alaska and Canada. This pattern is very similar to last fall. It usually leads to the worst of the cold air coming into the western half of the country, so we only get pieces of arctic air reaching the Mid-Atlantic States.

Next, take a look at the warm colors in the North Atlantic. It’s not as warm as it was last fall, but it’s still warmer than “normal”. This is sometimes called the AMO (Atlantic Multi-decadal Oscillation), where we get into a cycle of lots of years with these warm waters. And new research from NASA shows that this warm ocean pattern is closely related to blocking patterns in the atmosphere (such as negative NAO). Here’s a great graphic that shows North Atlantic Ocean temperature anomalies compared to the number of days of blocking.

(The number of winter blocking events (black and blue lines) correlates strongly with fluctuations in the temperature of surface waters in the North Atlantic Ocean (red line). For their analysis, the researchers removed the effect global warming has on water temperatures.)

We also are in a negative QBO (Quasi-Biennial Oscillation) pattern. This is associated with more blocking patterns in the atmosphere. All of this leads me to believe that there is a good chance (60 to 70 percent) of a prevailing negative NAO/AO this winter. But we are now in a positive NAO pattern, so doesn’t that mean something? Well, we had a positive NAO in October of the past two years and look how they turned out!

The NAO often changes drastically from October to December.

There are few years that have patterns very similar to this one. These are called “analogs,” and the more we have, the greater confidence we have that history will repeat itself. The closest winter I found was 2000-01, when we ended up with an above normal 26.1 inches of snow. Another close analog is just last winter, although the La Nina isn’t as strong, and the QBO was positive last winter.

In a study I did for our Philadelphia Area Weather Book back in 2003, I found that the pattern that led to the least snow here were moderate to strong La Nina’s. “Neutral” winters led to the most snow. So last year was an exception.

If the weaker La Nina makes it more favorable for snow than last winter, and the negative QBO this year makes it more favorable for snow than last winter, why aren’t I predicting more snow? It is possible, but the North Atlantic Ocean temps were warmer last year. And the October snow in Eurasia was higher last year (it seems weird, but it’s an important factor). Still that 44 inches has to be factored in.

So, when we combine the La Nina with the negative PDO, the negative QBO, the warm AMO, and expected negative NAO, I have to go for another winter with above average snowfall. But with less Eurasian snow, I’m also predicting less cold than last winter. In fact, due to expected blocking patterns, I expect periods of very cold weather but also some very warm air -- possibly record warmth for a day or two.

Here are the numbers:

30 to 40 inches of snow for the winter (the average is about 23), with at least one storm of 10 inches or more. But temperatures should end up pretty close to average.

Month	Estimated Snowfall	Estimated Temperatures
December	10 inches	2 degrees below normal
January	14 inches	1 degree below normal
February	12 inches	temps near normal
March	no measurable snow	3 degrees above normal

And why have I put this strange looking guy in my winter forecast on TV? His name is Piers Corbyn, and he’s a British astrophysicist. He’s also the only person I know of in the world who nailed the forecasts for the past two winters, both in Europe and the U.S. He uses forecast techniques that are unproven and unpublished, but I figured it was only fair to share his forecast for this winter. The only quotes I could find on You Tube were that January would be “very cold” and that there will be “massive snow” in our part of the world Jan. 11 to 14.

Let’s see how he does.

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