Extremes, and then more extremes. It’s been one after the other during the past 3 years:
- Our snowiest winter ever recorded
- 2nd & 3rd biggest snowstorms ever recorded
- Hottest summer ever recorded
- Wettest March, August, September ever recorded
- And now Sandy:
- Lowest pressure ever recorded
- Highest storm surge ever recorded
There were many more records since 2009, but you get the idea. The records involve heat and/or moisture. The two monster snowy winters of 2009-10 and 2010-11 were an unprecedented combination. Then, last winter, practically no snow.
What is the common denominator with ALL of these events? Blocking patterns in the upper atmosphere, that’s what. Any discussion about whether this is related to climate change is for another time. The fact is, we are in a period of increasingly extreme weather, and there’s no argument about that-or at least there shouldn’t be. Here’s the graph of extremes from NOAA -- not just around here, but across the U.S. in the past 100 years.
So, what does this have to do with my winter forecast for 2012-13? Since extreme patterns have become so much more common, we have to consider the chance of the same in the coming winter. Notice, none of the big extremes involves extreme cold, so I’m tossing that one out.
The first thing we usually look for when making winter forecasts is what’s going on in the Tropical Pacific. Is it El Nino or La Nina? This year, it’s neither. So much for that.
The thing that correlates best to our winter weather is what is going on in the Arctic and North Atlantic. This is where the AO (Arctic Oscillation) and NAO (North Atlantic Oscillation) come in. [Click here for a great website that goes into detail about how the AO and NAO influence weather.]
Basically, a negative AO (-AO) and a negative NAO (-NAO) are associated with colder and snowier weather patterns in our part of the world. It doesn’t mean that we get snow every time we have a –AO or –NAO, but the odds go way up. Our biggest snowstorms are almost always connected to one and/or the other.
The -AO and –NAO involve “blocking patterns”, where high pressure is farther north, and lower pressure in our region. Sound familiar? That was the pattern with Sandy, and what caused it to make that sharp turn to the left not seen before. These patterns also lead to Nor’easters, like the one a week after Sandy. Our odds of –AO and/or –NAO patterns have increased in recent years. This may be due to the record ice melt in the Arctic. The ice melt this year smashed all previous records (that many thought wouldn’t be broken for decades!). Dr. Jennifer Francis of Rutgers and others have published recent studies connecting massive Arctic ice melt with more blocking patterns.
So why did we have so little snow last winter? We were just in the right (or wrong, depending on your opinion of snow) part of the pattern that favored drier, warmer weather in our part of the world.
Look at the ocean temperatures lately, compared to normal:
The extremely warm water off the New England and East Canada coast is the main feature. This pattern could have been related to the big blocking high pressure area that was so important in the Sandy disaster and the Nor’easter a week later. This should favor more blocking highs in the North Atlantic. It takes a long time for such an extreme ocean pattern to change.
We also have had a –AO and –NAO for a good bit of October and some coastal storms as well. The most recent one stayed well offshore. But the overall pattern is clear: if we get much of this set-up in the winter, we’ll get some BIG snowstorms.
O.K., what else do we have? We’ve talked for years about the connection between October snow in Eurasia and the resulting winter snow here. The more snow there in October, the more snow here in Dec-Jan-Feb-Mar. It’s not a 100% correlation, but it’s way above 50-50.
And this year…..drum roll…..the October snow in Eurasia is well ABOVE average. Here’s the data for this October compared to the past two:
Compare the area this year to last: 11106 vs. 9294 (it doesn’t matter what the mean, it’s just a lot more). There’s even more this year than in 2010 (before one of our super-snowy winters). The ranks show this year to be the 11th highest number of the past 45 years vs. 28th place last year and 14th place in 2010.
BIG NUMBER = BETTER CHANCES FOR A SNOWY WINTER HERE
Here is the snowfall departure from normal for the Northern Hemisphere in October:
WAY above normal snow on both sides of the North Pole
There are other factors that will affect how cold and snowy this winter will be. Other parts of the alphabet soup of seasonal forecasting are the PDO, PNA, and the QBO. When they are combined with the NAO/AO and Eurasian snow factors, it adds up to a rather snowy winter overall, with some big swings in weather patterns.
THE FORECAST (for Philadelphia):
1. A 70% chance of above average snow
2. Most likely range: 30-35”
3. Only a 10% chance of below 20”
4. A 10+% chance of 40”+
5. Snowiest month-February
6. Least snow-March
7. At least one Nor’easter with more than 10”
1. Slightly above average overall (only +1 degree)
2. A couple of cold periods of 2+ weeks with much below normal temps
3. A couple of mild periods of 2+ weeks with much above normal temps
4. Coldest month-February
At least one strong Nor’easter with 50+mph wind gusts and moderate to major coastal flooding