Some computer models in recent days have shown an expansive hot weather pattern becoming established in late June, continuing through July and possibly longer.
Not every day during this warm period will be unusually hot; transient cold fronts are still likely to pass through some areas, especially in the eastern United States.
But the predicted weather pattern would favor extended periods of elevated temperatures virtually coast to coast, especially by July, with some more intense spurts of heat on more local levels.
Summer 2020 may go down in the books as much warmer than average for the United States or portions of the country — unsurprising considering the past five summers have been the top five hottest on record for the globe.
There are still uncertainties in the forecast, with regard to the magnitude, extent and duration of the heat. But some atmospheric scientists say that any ridges of high pressure, also referred to as heat domes, that become established could be self-reinforcing.
In either case, NOAA’s three-month outlook highlights near- or above-average odds of anomalously hot weather over the entire contiguous United States this summer.
In part, this outlook as well as the hints from computer models reflect the influence of human-caused global warming, which tilts the odds in favor of hotter summers.
Nationally, summer temperatures are increasing at a rate of 0.11 degrees per decade, though more pronounced temperature increases and spikes in the number of 90-degree days are occurring at the regional level.
The climate science and journalism group Climate Central analyzed summer temperature trends in 242 U.S. cities and found that 42 percent showed an increase in average summer temperatures of greater than 2 degrees (3.6 Celsius) since 1970. Summer low temperatures are increasing faster than daytime highs, the group reported.
Domino falls in the Arctic
The chain-reaction process favoring the heat actually is starting in the Arctic. As the stratospheric polar vortex, or a high-altitude whirlpool of frigid Arctic air, weakened during the spring, it helped shift the position of the jet stream.
Judah Cohen, director of seasonal forecasting at Atmospheric and Environmental Research, a Verisk company, says a dome of high pressure north of Alaska may develop in the next two weeks, which would in turn cause a dip in the jet stream to set up south of Alaska, across the Aleutian Islands, with low pressure and chilly air there.
Contrasting that, downstream “ridging,” or a northward shove of the jet stream, should allow for warm air to build across much of the Lower 48.
“This [Aleutian low] will set up a strong southwesterly flow of air across western North America and will act like a heat pump,” Cohen said in a Twitter message. That southwesterly flow could team up with a building ridge of sizzling high pressure in the West, maintaining a westerly component to the winds across much of the western and central United States.
Persistent heat dome sparks wildfire concerns
The forecast placements of weather systems would act to funnel air up and over the Rocky Mountains, with air drying as it sweeps to lower elevations over the Plains and potentially sapping vegetation of moisture. Any drying would exacerbate the heat even more in the Central states.
“One other factor to watch is how dry the soil gets,” Cohen said. “Right now, it is dry in the West and wet in the East. But if the dry soil intensifies and becomes more widespread, it will help to reinforce an atmospheric heat dome over the North American continent.”
The hot, dry temperatures and gusty winds could also have implications for fire weather concerns, especially in California and the Southwest.
“Keeping the pattern dry and hot [is] not good for fire weather!” wrote Michael Ventrice, an atmospheric scientist for the Weather Company, which is owned by IBM. He foresees an uptick in fire weather in the next one to two weeks.
The fire weather outlook for June shows elevated fire risk across much of the West, including parts of the Pacific Northwest and Northern California.
A tougher forecast in the East
In the East, the forecast is more uncertain. Ventrice says that a dip in the jet stream and associated low anchored near Greenland could send cool air masses down into the East. Computer models may not be catching onto these periods of cooler weather.
“If we get any weakness in the Greenland cyclone, I suspect we could get some transitory cool air masses to push down into the eastern/southeastern U.S.,” Ventrice wrote. “Models will have a difficult time resolving these transient cool down periods.”
It’s a microcosm of what transpired during much of April and May, when parts of the Northeast experienced temperatures well below normal at a time when the Central states were seeing unusual warmth. New York’s Central Park, along with a number of other locations in the Northeast, even reported snow May 9.
The low pressure area near Greenland may have even been bolstered by the remnants of Tropical Storm Cristobal, which infused that Greenland cyclone with a little bit of extra energy, allowing it to persist longer and stronger than it otherwise may have, Ventrice said.
Teetering on the brink of La Niña
The tropical Pacific Ocean is currently in so-called neutral conditions, meaning that there is no El Niño or La Niña present. The latest NOAA forecast, published Thursday morning, shows that though sea surface temperatures in the eastern equatorial tropical Pacific have cooled in recent weeks, a La Niña is not expected to develop during the summer.
Instead, such an event may form during the fall and winter. Mike Halpert, the deputy director of NOAA’s Climate Prediction Center in Maryland, says it’s a “toss-up between whether we stay in a neutral category or whether we tip over into a La Niña.”
Halpert says La Niña doesn’t exert much of an influence on summer temperatures in the United States, though it would boost the odds of an unusually active Atlantic hurricane season.
In order for a La Niña to occur, both the atmosphere and the ocean have to interact in ways that reinforce ongoing trends. Ventrice says there are signs that this is beginning to happen.
Halpert urged caution about buying into projections of continental-scale heat during the next two months, noting that computer model projections showing above-average temperatures are not a big surprise, considering they take global warming-related trends into account.
This can give models a bit of a blind spot, he says, when it comes to unusually cool temperatures, pointing to the failure of some to accurately anticipate the cool spring in the East.
The Northern Hemisphere has seen average temperatures during the period from June through August increase by 0.43 degrees (0.24 Celsius) per decade since 1970.
“It’s a really rare forecast that the models actually forecast below average [temperatures],” he said.
Matt Rogers, a long-range forecaster for the Commodity Weather Group, does think the model idea of extensive heat has some credence.
He said that while La Niñas may not have a big effect on summer temperatures overall, summers during which El Niños transition to La Niñas have a history of being hot, pointing to the summer of 2016 as a recent example.
That summer was the fifth warmest on record for the Lower 48, and every state experienced above normal temperatures, according to NOAA.
Jason Samenow contributed to this report.