While this weekend has provided fantastic weather, we know it won’t last and the colder pattern of late fall isn’t too far away. Many of you have heard there is going to be an El Niño developing later this fall and continuing through winter. This meteorological phenomenon is somewhat predictable and will have impact on not only our weather, but weather around the world.

Simply put, El Niño is warmer than average water temperatures off the coast of South America west of Peru. When El Niño isn’t occurring the trade winds blow in a westerly direction along the equator. You might not realize it, but these winds pile up warm surface water in the western Pacific, so the sea surface is as much as 18 inches higher in the western Pacific than in the eastern Pacific.

You can learn more by watching this video from NOAA.

The water temperature and height of the seas are intimately connected to pressure systems across the Pacific. This interlock of pressure and sea temperature and their oscillations are knows as the El Niño-Southern Oscillation. Not only do water temperatures fluctuate, but so too is there a see-saw shift in surface air pressure between the eastern and western halves of the Pacific. It was in the middle of the 20th century when climatologists and meteorologists realized that El Niño and the Southern Oscillation were connected.

There is a current of water called the Humboldt Current which depends on the normal flow of the trade winds to function in a typical manner. The Humboldt Current is a cold ocean current which flows north along the coasts of Chile and Peru, then turns west and warms as it moves out into the Central Pacific. When flowing in a typical non-El Niño manner the water is warmer across the western Pacific, but cooler in the eastern.

As the pressure changes across the Pacific and El Niño becomes established, the equatorial westerly winds diminish. Subsequently, the Humboldt Current weakens, allowing the waters along the coast of Chile and Peru to warm, and creates warmer than usual conditions along the coast of South America.

The Pacific is of course vast and meteorologists have created different regions for El Niño. Depending on which region warms, the effects are different. In addition to the location of the El Niño the strength of the El Niño is enormously important.

Upcoming Winter
In general, and remember there are never 100 percent correlations when it comes to weather, a weaker El Niño brings colder conditions to the east during the winter while a strong El Niño brings warmer conditions. There was a weak El Niño during the winter of 1979-1980 and we had very little snow with a cold February.

There are lots of other variables that affect our weather during the winter. The Pacific-North American Pattern (PNA), the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) are three such teleconnections. These can overwhelm the El Niño push to a colder or warmer winter and bust the forecast.

The chart below shows what our winters have been like during strong and weak El Nino events. This year’s El Nino is expected to be moderate.

Right now most forecasts of the computer models agree on a moderate El Niño during the latter half of the fall and continuing into spring of next year. There are many other such years when El Niño is weak and we have seen a lot of snow or a little snow during that winter.

Maine Snowfall
I like numbers and statistics so I often play around with how the weather now could impact the future. One of the things I did was look at other dry Septembers and what the following winter brought to Portland in terms of snowfall.
For this exercise I considered exceptionally dry to be 1.02 inches of rain or less. There are only 13 such Septembers since the late 1800s.

Since snow is highly variable, this method isn’t perfect to predict snowfall statewide, but I like seeing if there are any correlations with such things. Some winters had much less snow than average and others quite a bit more. In the winter of 1957-58 Portland had over 88 inches of snow. That winter took place with a strong El Niño which likely brought big storms, feeding on warmer ocean air. This year El Niño isn’t expected to be so strong. I averaged the winter snowfall after the 13 driest Septembers since 1871 and found the average was 73.5 inches or about normal. This is one piece of date pointing to a more typical winter in terms of snow totals.

There are some correlations between how October and November play out weather wise and what the following three months brings. The following image shows the National Weather Service is predicting our overall warm pattern to continue the rest of the fall.

I like to wait until the fall is complete before really committing to a winter forecast. That said, based on the upcoming El Nino and the Pacific Decadal Oscillation being in a cold mode, many have predicted a cold and snowy winter in the east since early spring.I have not seen any predictions for a mild winter yet.

Bottom line, I’m not yet convinced of a blockbuster snowy winter ahead and leaning away the idea this is going to be a super harsh winter. Stay tuned.

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