2023 was the planet’s warmest year on record, coming in 2.12 degrees Fahrenheit above the 20th-century average. But California bucked the trend. The state overall was just 0.8 degrees above the 1991-2020 average; some places had near- to below-average temperatures.

There’s a 55% chance that 2024 will be even warmer than 2023, according to the National Oceanic and Atmospheric Administration. And for now, California is expected to be in line with this projection.

Seasonal outlooks show that the United States will be warmer than average this summer, though pinpointing exactly how hot is a challenge. Rising temperatures in California in late summer and into fall could prime conditions for potential wildfires.

“This summer looks like a hot one across continental North America,” said UCLA climate scientist Daniel Swain, during an online briefing. “Not only hotter than the recent average, but potentially record hot in some places.”

Warm outlook

This outlook from NOAA’s Climate Prediction Center shows that much of the continental United States could be warmer than average this summer. The colors show the chances for temperatures above normal, not whether there will be record warmth.

California has lower probabilities than states to the east. One reason is the potential for chilly Pacific waters to affect temperatures, said Climate Prediction Center meteorologist Johnna Infanti. 

Bay Area residents are familiar with the cooling potential of the marine layer, which can blanket San Francisco with fog, resulting in May Gray and June Gloom.

“Even though we are expecting above-normal temperatures, there can be a little bit of uncertainty there just due to the normal climatology of the region, where you can see some … cooler temperatures, related to that June Gloom,” Infanti said.

The precipitation outlook calls for equal chances of below-normal, normal and above-normal precipitation.

“We don’t really have a very confident forecast for precipitation over California,” Infanti said. “For this particular forecast, we're in the transition season between El Niño and La Niña, as well as it being summer, where we don't see quite as large of an influence from El Niño or La Niña than we do in winter months.”

Since California typically doesn’t log much summer rain, hard-to-predict individual events like tropical storms monsoonal thunderstorms can have outsized impacts

Summer temperatures

Seasonal forecasts from the Copernicus Climate Change Service point to California facing temperatures a degree or two above the 1993-2016 average in late summer and fall. The anomalies are based on averages from multiple weather models.

Such a difference is generally on par with hot summers during the past decade. During the summer of 2021 when the state was gripped in drought, California temperatures were 3.3 degrees above the 1991-2020 average.

California’s expected summer temperatures are less intense than what’s forecast for the middle of the United States and Canada, which could experience temperatures 3 or 4 degrees above normal.

Though a few degrees may not sound like much, over months on end, that “likely involves individual, rather extreme heat waves that are either very intense and or very persistent,” Swain said.

Making predictions this far in advance comes with caveats. Climate models “get a little squirrely beyond three months,” said Brent Wachter, a fire meteorologist with the Northern Operations Predictive Services unit.

California wildfires

Northern California’s season for significant wildfires is expected to start off slowly, based on the Northern California Geographic Area Coordination Center’s latest four-month outlook. A big reason is “timely cool-moist intrusions” over the coming months.

“That doesn't necessarily mean it has to be liquid water coming out of the sky,” Wachter explained. “That's the deeper onshore flow that kind of cools the fuels down through higher humidity and a little cooler temperatures, periodically.”

Melting snow also provides moisture to soils, Wachter said, keeping plants green and delaying their transformation into dried-out wildfire fuel. And the absence of drought keeps vegetation healthy. 

“We've had a couple of years of decent precipitation so at least in our forests we’re not expecting it to be an excessively bad year for wildfire,” said Lara Kueppers, a UC Berkeley energy and resources professor and faculty scientist at Lawrence Berkeley National Laboratory.

Heat waves are also expected to be relatively short in the coming three months, in the range of two or three days rather than five days or longer, Wachter said.

But things could change by late summer and in the fall.

Swain is wary that warmer-than-average conditions during those months would be bad news given the vegetation that’s accumulated in California. The past two wet winters fueled rampant growth of nonnative grasses. And areas burned during historic fires in the late 2010s and early 2020s have experienced regrowth during less active fire seasons.

“These footprints are now filling in with a significant amount of vegetation,” Swain said. “The conditions the past couple of years have been particularly conducive to the growth of grass and brush.”

Warm, dry conditions in August and September and potentially beyond, could make for an active second half of California’s wildfire season, Swain said.

“Grass fire is always a risk,” Kueppers said. “The grass fire season is going to be longer in a warmer year just because the grasses will dry out faster.”

Still, other factors beyond temperature complicate predictions for how wildfire season will play out this year, and in the future.

“If you could forecast the lightning; the wind; those extended, critically dry periods; both live and dead (fuels); and heat wave events … you're going to nail the season every year,” Wachter said. “But we don’t because there’s always something … that we can’t predict.”

Reach Jack Lee: jack.lee@sfchronicle.com