Will our winter weather be brutal, mild, or somewhere in between?

A woman braved snow walking across the Evelyn Moakley Bridge on Seaport Boulevard in February.John Tlumacki/Globe Staff

With the holidays now upon us the question of what the upcoming winter will bring often arises. I’ve written many winter outlooks over the years and I always start them with the same caveat: Seasonal forecasting is still relatively new and while monthly forecasts are getting quite good, looking out for 3 months can be a bit precarious.

With that in mind there are some things we do know that will impact the winter season ahead.

I won’t bury the lead. Right off the bat, most indicators point to a milder and less snowy winter than average in Southern New England. This is also the official forecast from the National Weather Service’s climate division. It’s based on the current state of ocean temperatures as well as some longer-term trends. Due to a warming climate it’s also been very difficult to get a colder-than-average winter and we haven’t seen one for seven years. With that in mind, let’s delve into some details and some caveats.

The outlook is for warmer-than-average conditions in the east this winter.NOAA

In the most general sense, weather is about balancing the atmosphere and the more things that slip out of balance, the more extreme the weather tends to be. This year we have La Niña present in the Pacific.

La Nina is indicated by the presence of colder water than average west of Peru, as noted in blue.NOAA

La Nina is the phenomenon of colder-than-average water off the coast of Peru and this will have implications to the jet stream across the globe.

There are some things we know that generally happen in a La Niña year. The image below shows a typical pattern. If this was the only factor in place the forecast would be more reliable.

A La Niña pattern favors less cold in the east during winter.NOAA

In addition to it being a La Niña year, the state of the oceans is markedly different than it was in our last really cold and snowy winter.

Back in the winter of 2014-15 there was a big warm blob off the west coast and this year we have exactly the opposite. It’s another reason forecasters are reasonably confident in a less-severe winter in this part of the country.

There is a big difference in the state of the oceans this year across the globe compared to our cold winter of 2015. Notice the warmer-than-average water off the west coast of North America in 2014 (at left) and the cold water this year (at right). That is a critical difference and player in our winter weather.NOAA

However, as powerful as the La Niña phenomenon is, other atmospheric oscillations can actually overwhelm La Niña, changing what would typically occur.

For example the winds that circulate at the top of the planet at high levels of the atmosphere — known as the polar vortex — can become disrupted during the course of the winter. This disruption can break off pieces of cold air and send them southward.

We saw an example of this last year across Texas and in our area back in 2015. An already-chaotic global wind pattern can become even more pronounced in this situation and a mild forecast can become cold and snowy.

If the polar vortex weakens it can allow cold air to spill south into North America, Europe or Asia.NOAA

The movement of the arctic southward isn’t predicted well very far in advance, so that is a big wild card.

There are times during the winter where the atmosphere can also become blocked. What this can do is leave one area stuck in very mild or very cold conditions for an extended amount of time. Think about March 2018, when successive nor’easters blew through the area after a relatively mild winter.

When the height of the atmosphere is unusually high over Greenland (red), it can promote a block in the atmosphere with more storms along the east coast.NOAA

Another aspect of the atmosphere that can change a forecast is the position of clusters of thunderstorms around the equatorial region. This parameter is known as the Madden-Julian Oscillation. Depending on the position of these thunderstorms, the jet stream in North America can buckle, bringing colder- or warmer-than-average conditions.

Sometimes a fast flow of air coming out of the Pacific can also keep things unusually mild across the entire lower 48 for weeks at a time. This has happened in the past couple of winters.

While overall the winter looks milder and less snowy than average, that doesn’t mean we won’t see cold and snowy weather at times. We know with pretty good confidence that the end of November and very early December is going to bring cold air and perhaps even some snow. This is likely followed by Pacific air pushing out the early cold, leaving us with a month that may be a little above or a little below average overall.

Snowfall is highly variable from year to year as just a couple of big hits or misses change the final tally by a lot. I would expect 2 to 3 feet of total snowfall this year, below the nearly 4 feet that is average.

Remember, meteorologists consider winter to run from December through February, but the snow season starts earlier and ends later than that.

Snowfall can be under a foot or about 9 feet depending on the year.NOAA

If the polar vortex stays in its place across the northern part of the planet and La Niña remains strong, January and February are likely to have variable weather with several mild periods and no prolonged cold and snow. It’s also important to remember there’s still a ton we don’t know about the atmosphere and a few subtle changes can have a big impact on our daily weather.

The most accurate assessment of the upcoming winter will come in the middle of April — until then, I look forward to the forecast challenges ahead.