Blog: How Hurricane Forecasts for the Winter

Learn what goes into building Hurricane's long-range winter forecast

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Glenn revealed his Long-Term Winter Forecast and he says it's going to be an average winter. You can see the forecast here, but you can learn about what goes into forecasting an entire season of weather below.

Part 4: Putting It All Together

So far, we’ve seen some conflicting indications about this winter. What do we do now?

One of the methods many long-range forecasters do is look at past years in which atmospheric conditions were similar to this year. That’s called “Analog Forecasting," and it has shown to be helpful in many years.

But I have been unable to find a really great analog year to compare with this one. The closest I could find was 2006. It was a moderate El Nino, a neutral PDO, above normal Eurasian snow in October (a factor closely related to cold and snow here), a negative NAO in the fall and a positive EPO.

In that year, October temperatures were near normal and November was much warmer than normal, all similar to this year. The only big difference (an important one) is that the QBO was positive in 2006 and is negative now.

So what happened during the winter of 2006-2007?

We started with a very warm December and January -- more than 5 degrees above average -- with very little snow. Then came winter. February temperatures were nearly 7 degrees below average!

March temps were close to normal. Nearly all of our snow fell in February and March. We also had a lot of sleet that winter, suggesting a lot of close calls for rain vs. snow.

Taking that into account, and correcting for the positive QBO effect, it fits pretty well into the typical moderate El Nino pattern. Especially one that weakens later in the winter, as most do.

That logic leads me to predict a winter with temperatures a bit above normal, with the warmest part in the first half. February and March should be colder and snowier.

But can two months worth of real winter give us enough snow to get significantly more than an average winter? Probably not.

So, I’m predicting 18-24 inches in Philadelphia, just a bit above the average of 19.3 inches.

Only one winter (2002-2003) of the 14 moderate El Nino’s I found turned out to have more than 26 inches for the winter -- it had 46.3 inches.

But 2002 had a huge coverage of snow in Eurasia in October, and a strongly positive PDO in the fall, so that could account for much of that extra snow.

We’ll soon see how this all turns out.


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Part 3: Other Teleconnections - The PDO and QBO

El Nino and La Nina aren’t the only teleconnections with winter weather in the Philadelphia area. The PDO (Pacific Decadal Oscillation) and QBO (Quasi-Biennial Oscillation) are less known and less dramatic than ENSO, but there are correlations with both.

While ENSO is an oscillation in the Equatorial Pacific, the PDO is found in the North Pacific. Like ENSO, it involves large areas of ocean -- well above or below normal in temperature. A positive (or warm-phase) PDO has the warm ocean water off the West Coast of North America, and cold water in the North Central and Northwest Pacific. A negative PDO (or cold phase) shows the opposite.

The correlation between a positive (warm) PDO and snowy winters here is very strong. EVERY Philly winter that had more than 20 inches of snow since 1980 occurred with a positive PDO. And, the PDO (being “decadal”) is a more stable pattern than ENSO.

In all of those snowy winters, the PDO was positive by October, or else turned positive shortly afterward. This year, the PDO went positive in September, and became a bit less positive in October. Chances for an abrupt switch back to the negative side seemed small.

This correlation may be the single most impressive I’ve seen for snowfall forecasting in this area.

Why would this correlation make sense? With a positive (warm) PDO, the warm water off the West coast helps lead to a ridge of high pressure in that area, leading to a trough in the East. That brings Canadian and even Arctic air down allowing for East Coast storminess. The opposite is true in the negative (cool) phase.

The QBO is not found on the surface, but very high up in the atmosphere at 60,000 to 70,000 feet near the equator. It takes about two years to switch from one mode to another. The east (or negative) phase is associated with milder winters and less snow in our area, with the opposite true of the west (or positive) phase.

The current QBO is negative, so it is highly unlikely to change to positive before winter is over. Negative QBO winters tend to lead to more blocking patterns in the atmosphere, which can support more storms if the block is in the right place.

Part 2: Teleconnections - El Nino/La Nina

It’s not quite this simple, but if you jump on one end of a seesaw, the other end will go up. Because we see the connection with our eyes, we know that one action was directly related to (and caused by) the second. We wish it was that simple in the atmosphere, but the seesaws are invisible, so we have to look for connections through statistics.

We’ve all heard of El Nino, and many people are aware of its connection to storminess in California. That connection is so direct and obvious that there is no doubt about it.

If we can predict El Nino’s accurately, we can predict how stormy the winter will be in California. This is called a “teleconnection,” -- one of many in the atmosphere.

So, how does that help us here? Our connection with El Nino (and it’s opposite, La Nina) is not very obvious, but a close look at climate records shows significant tendencies.

Overall, moderate to strong El Nino’s lead to stormy, but mild winters in the area. They give us plenty of Nor’easters, but we’re often too warm for snow. Moderate to strong La Nina’s are connected to drier, colder winters in the Delaware Valley. It gets plenty cold enough for snow, but we get few big storms.

The “best” conditions for snowy winters in the Philadelphia area are near neutral conditions in the Equatorial Pacific. In researching our book Philadelphia Area Weather Book, Dr. Jon Nese and I found an average of 18.5 inches of snow in a significant El Nino winter, 16.5 inches in a significant La Nina winter and 24.5 inches during neutral winters.

Sometimes statistical correlations are just random, but in this case we have a physical reason to believe the results.

We have recently seen a weak El Nino (very favorable for cold and snow) quickly increase to the moderate range. That cuts down the chance for a very cold winter. But, we should still see a lot of storms. Almost every time an El Nino increases rapidly, it also decreases rapidly. So I’m predicting that to happen this winter. Our best chances for cold and snow are in February and March, a lot like last winter. If the El Nino weakens more quickly, we can expect even more snow than I’m forecasting now.

Part 1: History and Philosophy

In the good old early days of forecasting (the 70s), we thought anyone who attempted a seasonal forecast was an irresponsible idiot. There was no literature on the subject and the person most famous for long-range forecasts was kicked out of the American Meteorology Society.

A lot has changed in meteorology since then, including seasonal forecasting.

Many people ask: “How can you predict what’s going to happen this winter when you sometimes can’t even get tomorrow’s weather right?”

It sounds reasonable, but it’s a different science. We’re not trying to say that it will snow on Jan. 3, or that a cold spell starting Feb. 6 will last for two weeks. We can only give trend information; that is, general ideas of warm or cold, wet or dry, snowy or non-snowy. There are forecasters who go into a lot more detail than we do, but I have yet to see any verification or published reports to prove that anyone actually has that kind of skill.

The Old Farmer’s Almanac is famous for such predictions -- and they make outrageous claims of accuracy. Whenever they manage to get one right (even a broken clock is right twice a day), the headlines spread nationwide. But, none of the regular failures are mentioned.

I did a major verification project on TFA while at PSU in 1972 and proved that their forecasts were WORSE than normal predictions. Once someone with their company has produced such a product, they lose credibility forever.

While other areas of meteorology have improved with computer modeling, seasonal forecasting has improved with recognitions of “teleconnections.” That’s when a pattern in one part of the world leads to (or from) a pattern elsewhere. There were discussions of such principles even before that name was given to it.

With more data and more investigation, more teleconnections were discovered. But, it got even more complicated, since adding all the teleconnections together often leads to conflicting conclusions.

The best-known teleconnection is ENSO (El Nino, Southern Oscillation).

The 1982-3 strong El Nino changed everything. Devastating storminess in California led to strong research projects and more data where the phenomenon occurs in the Pacific. That led to the 1997-8 event and winter, when the strong El Nino was seen developing in the fall. It was the first opportunity (for me at least) to actually make a seasonal forecast with some information and confidence. “A very warm winter with very little snow” was the theme. And, it worked.

Since that winter, I was asked to do winter forecasts EVERY year, even when there was so much conflicting information that I had little confidence. More often than not the forecasts worked out, including last year when I predicted above average snowfall, with nearly all of it falling in February and March. We had 21.5 inches for the winter, with nearly all of it falling in just two storms (Feb. 3 to 4 and March 1 to 2).

Now it’s on to year 13!

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