Here’s what October has in store for southern New England

The Euro model sees New England having average or cooler-than-average readings for October, but that would be a huge shift from the past three months.

Marlboro VT10/09 /2014  Fall Leaf Peeping in Marlboro Vermont at Hogback Mountain. has a burst of color.  Globe Staff/Photographer Jonathan  Wiggs 
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Fall leaf peeping in Vermont. –Jonathan Wiggs / The Boston Globe

You may not remember, but last October was the warmest one since 1947, meaning that for most of you reading this, it was the warmest October of your lifetime.

Mean average temperatures
Mean average temperatures. —ACIS

With the new month upon us, you might be wondering what the weather will be like in New England this fall. While long-range predictions are notoriously difficult and unreliable, there are a few indications of what the forecast has in store for us this month (and potentially beyond).

What the models predict:

Most of the computer models show a pool of cold air up in Canada and quite a bit of warmth over the East Coast and the western U.S. There are two forecasting challenges, though: Just how warm will it be this month? And will the first half of October be warm before it cools off later in the month?

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The two maps below show the GFS and Euro models for October. There are similarities between the two predictions, but the biggest differences are in the middle of the country and southern New England.

The latest version of the CFSv2 shows a lot of warmth across the lower 48 in October:

GFS model
The latest version of the CFS V2 has a lot of warmth. —WeatherBell

The Euro is colder overall, but mainly in Canada and the inter-mountain West. Note that New England won’t be as warm under this model:

euro model
The European weeklies predict some very cold air in Canada and the northern part of the U.S. —WeatherBell

Other factors to consider:

Monthly forecasts rely on a host of factors such as where thunderstorms are occurring in the Pacific Ocean across the equatorial belt, known as the Madden-Julian Oscillation; how warm the waters are off the coast of South America, known as the El Niño Southern Oscillation; as well as the ocean temperatures in the north Atlantic. As we get deeper into October, the snow cover across northern Canada starts to prominently factor in just how much cold may or may not make it into the United States.

Note the following NOAA model through early November has little-to-no snow in New England, which would be typical:

Snow forecast through November
Snow forecast through early November. —NOAA

Additionally, El Niño is forecast to strengthen in the coming months and peak in early winter. The effects of El Niño are well documented, but they tend to be more pronounced later in the fall and winter than in October. A developing El Niño can mean an average or cooler-than-average start to fall.

El Nino
A weak El Niño is forecast to develop as the fall progresses. —The International Research Institute for Climate and Society at Columbia University
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El Niño conditions can bring warmer or colder-than-average conditions, but in the September to November period, the odds of it being cooler than average are greatest.

The chart below compares climate averages for meteorological fall and whether we have an El Niño, a La Niña, or neither. The odds of a cooler fall are actually highest with the El Niño condition. September was already very warm, and October looks to start that way. It would take a very cold second half of October and November to give us a colder-than-average meteorological fall. It’s not impossible, but it’s not likely.

In the September to November period, the odds of it being cooler than average due to El Niño are greatest. —NOAA

What I’m forecasting:

The fact that the Euro model sees New England as having average or cooler-than-average readings for October would be a huge shift from the past three months. I am holding this forecast suspect as it looks to me like October will start warm, and any cold later in the month might not be able to bring temperatures back to average.

I suspect the odds that central Canada will be colder than average are quite high and the odds that the Intermountain West and the Eastern Seaboard will be warmer than average are also equally as high.

There also has been a strong area of high pressure off the southeast coast for months, and it will be tough to dislodge this for the next few weeks.

Here’s the bottom line: 2018 has been a very warm year. While March and April were much colder than average, and January and June were slightly below the norms, February, May, July, August, and September have all been very warm months. The strength of high pressure in the middle of the Atlantic makes it highly likely October will be warmer than average, too.

october temperatures
The first 10 days of October are likely to be warmer than average. —NOAA
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