Heat wave forecast to bake Lower 48
A period of punishing heat may envelop much of the Lower 48 states, potentially extending into parts of Canada and Alaska. Soaring temperatures could encompass much of the Lower 48 in late June and linger for weeks.
The above-average temperatures are part of a major pattern change that would bring anomalous warmth to some parts of the nation that have seen a cooler-than-average spring.
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 high temperatures virtually coast to coast, especially by July.
Summer 2020’s temperatures may be above average for the U.S. or portions of the country — unsurprising considering the last 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.
NOAA’s three-month outlook highlights near- or above-average odds of anomalously hot weather over the contiguous U.S. this summer.
In part, this outlook as well as the hints from computer models reflect the influence of humancaused 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 Fahrenheit since 1970. Summer low temperatures are increasing faster than daytime highs, the group reported.
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, 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. 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.
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. Drying would exacerbate the heat more in the Central states.