Don’t expect blockbuster snowfall this winter

Welcome to November. It’s generally a cloudy, stormy time of year, and it’s also when New Englanders start to think about what the upcoming winter may have in store.

Early predictions for the entire winter are notoriously difficult to compile. These forecasts are also not very reliable, and it’s not prudent to use a single forecast to predict the weather for the next five months. The long-range European models, for example, include some snow across all of the northeastern United States by mid-December, but these forecasts can dramatically shift as frequently as every few days.

The European Ensemble forecast for snowfall over the next six weeks.

In my view, long-range models should be used to gauge trends rather than specifics. One or two big storms that hit or miss can completely change the snowfall totals by the end of the season. In any given year, a relatively small shift in the mean storm track will impact whether southern New England receives snow, rain, or nothing at all. In some years, the jet stream doesn’t produce very many big storms, while in others, we get hit with one each week (remember 2015?).

Our perception of the weather can be almost as influential as the actual weather. Two weeks of Arctic air in the middle of January can be so memorable they overshadow an otherwise milder-than-average winter. If several storms hit on Tuesdays or Wednesdays, they affect us more than if those same storms arrive on the weekends. Back in the winter of 2014-15, when so much snow fell within six weeks, most people forgot that December and much of January had very little cold and snow.

With all of that in mind, Boston’s annual snowfall average in the past 30 years is around 45 inches, with areas in the 495 belt averaging 55 to 60 inches. The winter of 2017-18 will be influenced by a variety of global factors, each of which add a degree of complexity to the forecast. Right now, the odds are leaning toward a typical winter where snowfall is within 10 inches of average. I do not expect a blockbuster snowy season for southern New England this winter.

Making a long-range prediction for cold and snow is best done by looking at some of the more powerful variables that exist in the atmosphere. Here are a few that I’m taking into consideration:

1. We’ll be impacted by La Niña.

Perhaps most notably this year, we are looking at a weak or moderate La Niña, which is when the ocean waters off the coast of Peru turn colder than average. As a result, the jet stream tends to take on the following configuration:

The jet stream during a weak or moderate La Niña.

In this pattern, New England often sees average snowfall totals, but, of course, there are always exceptions. During some La Niña years, New England has received higher-than-average snowfall — but just not as frequently as when snowfall is closer to average. The chart below shows the years when La Niña was weak or moderate, along with the snowfall totals for Boston that followed.

Snowfall during weak or moderate La Niña years, according to NOAA.

Not all of the phases of the El Niño Southern Oscillation (ENSO) are created equal. You can’t compare what happened in the past to know exactly what will happen in the future. Looking back is helpful, though, when trying to make a seasonal forecast.

This graph shows the oscillation between El Niño (red) and La Niña (blue). Each of these phenomena range from weak to very strong. When neither is present, we call this neutral.

It’s also worth noting that the National Weather Service predicts a warmer-than-average winter across the eastern United States, largely because of La Niña.

The January through March period is forecast to be milder than average along the East Coast and in the South.

2. We’ve had a warmer-than-average autumn so far.

Another data point to consider is autumn’s weather. A warm or cold fall is an indication of the atmosphere’s state. What happens from Sept. 1 to Nov. 30 can guide the thinking of what type of winter we’ll experience.

September and October 2017 were very warm — in fact, October in Boston came within one-tenth of a degree of tying the warmest on record — so it’s useful to review snowfall after some of the warmest September-October couplings.

The chart below shows Boston has experienced slightly below average snowfall in the years when it’s warm during the first two months of fall.

Snowfall after the top years with a warm September-October period.

3. November could also be warm.

The weather during November is also a good predictor for what will happen from December through March. If a warm November follows a warm October, the odds are high that the winter will be mild.

Snowfall following the top years with a warm October-November period.

But if November turns cold after a warm October, then the rest of the winter is often quite snowy and cold.

Snowfall following the top years with a warm October but a cold November.

If you’re rooting for a winter without a lot of snow, you’ll like the European model. It’s forecasting a warmer-than-average November, which would increase the odds of less snow in southern New England.

The European model predicts the Northeast and South will experience a warmer-than-average November this year.

In addition to the above, forecasters and climatologists are monitoring a slew of other factors, including the snow cover across the Arctic and Canada, the phase of the Arctic Oscillation, the North Atlantic Oscillation, the Pacific/North American pattern, and more. These measurements are based on different pressure setups and, once again, they help to predict the mean storm track as well as how much cold air impacts the weather.

In New England, our winter weather appears to be highly variable with periods of deep cold followed by periods of mild weather. The snow cover may appear and then disappear in a matter of a week or two.

For all of these reasons, I don’t think this winter will be a blockbuster one. Yes, we will get snow and cold — ’tis the season, after all. Stay tuned for additional details and forecasts, which will be more specific and accurate than a long-range forecast is capable of doing.

You can follow David Epstein on Twitter: @growingwisdom.