“Judging from the latest seasonal forecasts and the computer models, summer 2024 is likely to be a top 10 hottest summer on record for the Northeast US, but probably not a top 5 hottest summer,” said Jeff Masters, a meteorologist with Yale Climate Connections.
Advertisem*nt
Even with sophisticated weather models and historical data, most meteorologists caution that summer forecasting still remains a bit more nuanced and a little less precise than for wintertime, mainly because the temperature variation during the summer is significantly less than in the winter. Just think about it: In January, we can experience a day where the temperature can reach into the 60s versus waking up to readings below zero.
We don’t have those kind of extreme fluctuations in the warmer months.
Also, “because global warming is changing the overall system, we now have a system on steroids and we don’t know how the steroids are affecting the system and that makes it tricky to know what’s coming down the road,” said Jennifer Marlon, senior research scientist at the Yale School of the Environment.
What we do know is that, overall, summers in New England have been trending warmer. The average summer temperatures across New England have increased dramatically in the past 30 years, with both the high and low values increasing. Take Massachusetts, for example: The average summer low temperature has increased nearly 2 degrees since 1990 — from around 66 to 68 degrees. Average maximum temps began hitting 70 degrees in 1999 and have surpassed the 70-degree mark nine summers since. Similar trends are evident for the other five New England states.
Advertisem*nt
Even more striking is the fact that the planet just registered its hottest May ever, continuing a yearlong streak of unprecedented global heat — and scientists say this summer will likely extend that streak of excessive warmth as a result of continued human-caused global warming.
“The temperature outlook for summer across the Northeast is trending warmer due to climate change, as long-term temperatures have pointed to and it’s a primary factor in the summer outlook,” Dan Collins, a meteorologist with NOAA’s Climate Prediction Center, said.
For summer 2024, several dynamic weather forces are at play, atmospheric conditions that will drive our summer weather and could tilt the odds more in favor of 90-degree heat and humidity or cooler, wetter summer days. Let’s drill down on the elements behind it all.
The jet stream
Our summer pattern here in New England is generally impacted by the state of the jet stream. This narrow band of strong wind in the upper levels of the atmosphere circles the Northern Hemisphere acting as a conveyor belt directing various air masses around the planet and in the summer it’s much less pronounced. How the jet stream ultimately positions itself over the next month or so will determine whether we have a more typical summer (relatively dry and around 80 average daily highs for Boston) or a summer filled with more extreme heat and humidity.
If positioned just right, the jet stream can also unleash a summer with lots of rain and fewer 90-degree days than usual, as happened last summer, when we saw a mere five days of extreme heat.
Advertisem*nt
La Niña’s summer influence
El Niño and La Niña are the warm and cool phases of a naturally occurring climate pattern across the tropical eastern Pacific Ocean that oscillate back and forth every two to seven years, greatly influencing and disrupting the weather across the US and worldwide. Each pattern typically lasts about a year.
With the end of an El Niño now setting the stage for a La Niña to develop and take hold this summer in the US and New England, we could see a weather pattern more skewed toward a hotter summer in our region. That’s because a La Niña can have a major influence on where the jet stream ultimately sets up shop. The cooler-than-normal waters associated with a La Niña generally help push the jet stream farther north, inviting warmer temperatures into the Northeast and generally keeping the East Coast more dry. Although La Niña’s biggest impacts are usually felt during wintertime, data show a faster transition to La Niña during June-August has led to hotter summers.
Forecasters said there’s a 50/50 chance we will see a neutral or La Niña pattern by the end of the month and into July “and we expect La Niña to be in place by the end of the summer,” Anthony Artusa, a meteorologist with the Climate Prediction Center, said.
June here in New England is beginning to show some signs of 90-degree weather headed our way later this week and potentially into next week.
Warming oceans
The health of the world’s oceans is another big player in the strength and positioning of the jet stream each year. For example, as the oceans continue to heat up at record levels, those elevated readings reduce the amount of sea ice and weaken the jet stream, causing more fluctuations to the south in the jet stream by releasing more moisture into the atmosphere.
Advertisem*nt
“We consider sea surface temperatures when it comes to seasonal and monthly outlooks, mainly along the coasts as surface temps are 2.5 degrees Celsius above average along the East Coast, and may become a factor this summer,” Collins said.
Also, a 2023 study published in Nature found the diminished ice sheet is resulting in a jet stream that’s growing slower as well as “wavier,” allowing warm air from the tropics to flow north and become trapped over North America, Europe, and Asia, fueling severe heat and intense wildfires.
What about a wet summer?
With a La Niña forecast to move in, it’s less likely that we will see the soggy stretches like we did last summer, even though summer rainfall is still predicted to be above normal, Collins said.
Summer 2023 rainfall
• Vermont, New Hampshire and Maine — Wettest summer on record.
• Mass. — 2nd rainiest summer, 19.21 inches (7.07 inches above average); 1938 holds the record, with 19.66 inches.
• Boston — 2nd rainiest summer, 20.33 inches; 1955 holds the record, with nearly 25 inches.
• Connecticut — 11th wettest.
• Rhode Island — 13th wettest.
• Providence, Worcester, and Hartford — Past six to 12 months, wettest on record.
The caveat, of course, is the “hyperactive” hurricane season that’s forecast for the Atlantic, which can make for a very rainy summer. NOAA last month issued an unprecedented forecast of 17-25 named tropical storms and 8-13 hurricanes for the June to November hurricane season.
Advertisem*nt
“Once we start moving closer to August, we’re expected to have an active tropical season. You can get enhanced tropical moisture and end up with a great chance for above-average precipitation to align with the East Coast and into New England,” Collins said.
That’s where we may see La Niña’s greatest impact, according to CPC’s Artusa.
New England hasn’t had a hurricane hit its shores since Hurricane Bob in 1991. But we have seen five actively named storms make landfall since then, including Tropical Storm Henri in 2021. Superstorm Sandy in 2012 impacted Southern New England, killing four people in Connecticut and, in addition to 2011′s downgraded Hurricane Irene, caused extensive flood damage; our region saw minimal impacts from a downgraded Hurricane Lee in mid-September.
La Niña summers: 2010 and 2016
The last time we experienced a similar transition from an El Niño to a “La Niña summer” was in 2016 and 2010. During the latter, a strong La Niña was present and summer temperatures across New England ran nearly 3 degrees above average.
Six years later, we saw a transition to a relatively weak to moderate summertime La Niña, with warmer temperatures and less rainfall.
It’s interesting to note that these were also years in which we saw increased hurricane activity in the Atlantic. The year 2016 saw 15 named tropical storms, seven hurricanes and four major hurricanes develop. Years earlier in 2010, those numbers were dramatically higher: 19 named storms, 12 hurricanes and five hurricanes. The average for the Atlantic is 14 named storms, seven hurricanes and three major.
La Niña, our warming oceans and other factors will determine our weather pattern this summer.
But one thing’s for certain: Summers are getting warmer, particularly the nights in our part of the world where it’s getting harder to keep cool. Whereas there used to be summers where you almost didn’t need air conditioning except for a week or two, that is definitely no longer the case. Will the summer of 2024 bring a whopping 30 days of 90-degree weather like New England saw back in 1993? Time will tell, but based on all of the weather outlooks and our overall warming world, you should keep the air conditioners tuned up.
Ken Mahan can be reached at ken.mahan@globe.com. Follow him on Instagram @kenmahantheweatherman.
