Why has western heat been so intense? It boils down to 5 reasons

·3 min read

No other region in the country is warming faster than the western United States when it comes to increasing daytime highs, a trend that became apparent with the unprecedented and record-shattering heat wave that took over the Pacific Northwest earlier this summer.

Heat has been building all across the west this year. In June, Idaho, Utah, Arizona, Nevada and California all had record heat statewide. Salt Lake City had its warmest June in 74 years of records with an average temperature of 80.2 F, which is 8.6 degrees Fahrenheit above normal.

Seattle finished the month of June more than 5 degrees above average, while Portland, Oregon, closed out the month an astonishing 7 degrees above average, with the heat killing hundreds in the United States and Canada.

As a continent, North America experienced its hottest June in recorded history last month.

AccuWeather meteorologists explained five reasons for the heat across the West this summer.

Much of the Southwest, central Rockies and northern Plains were facing a severe drought, while pockets of drought also existed in the Pacific Northwest. The Southwest, in fact, has been in "decadal, long-term drought," according to AccuWeather Senior Meteorologist Paul Pastelok.

Drought conditions across the United States in mid-April, with severe to exceptional drought encompassing much of the Southwest. (U.S. Drought Monitor)

Drought in the spring helps to enhance the risk of extreme heat and drought in the summer due to a positive feedback mechanism. Since there is little moisture in the ground, more of the sun's energy goes directly to heating the surface, drying out the surface even more.

Monsoonal moisture provides key rainfall for the Southwest, and that moisture was greatly lacking in both 2020 and 2021. In the summer of 2020, the position of the upper high-pressure area over the Southwest was nearly stationary, causing prolonged heat and high evaporation rates that led to expanding drought conditions.

In 2021, precipitation shut off in the late winter and spring, speeding up snowmelt across California and the interior West, which set the stage for the extreme summer heat. Monsoonal rain has come back this summer with deadly force across the American Southwest.

In a typical La Niña pattern, storms during the western wet season, which runs from October to April, tend to shift farther north, allowing points to the south to dry out. These impacts took hold in the latter part of the wet season, with dry conditions causing higher evaporation and creating higher pressure aloft, resulting in a stronger heat dome.

This year, though, the high pressure that typically builds over the Southwest was stronger in the West and helped to send record heat into the Northwest, northern Rockies and western Canada.

Water temperatures may have also helped to play a role in causing intense heat in the West. A large area of above-normal sea-surface temperature anomalies has stretched across the northeastern Pacific near the West Coast this summer.

Map showing unusually warm sea-surface temperature anomalies (temperatures compared to normal in degrees Celsius) as of July 24, 2021. (NOAA)

"What this may have done is pulled the upper high pressure farther west most of the time (during the extreme heat in late June over the Northwest), which led to the intense heat in the western U.S. and western Canada," Pastelok explained, describing the formation of a heat dome which helped to trap heat in the western U.S. and western Canada.

Over the last 30 years, the greatest warming in terms of maximum temperatures in the months of June and July has occurred in the western United States, meaning that the record warmth these past few months is not terribly surprising.

The average summer temperature in Portland, Oregon has increased by 2.5 degrees Fahrenheit in the past 50 years. (Climate Central)

Increased warming in the region will help make a typical heat wave in the Pacific Northwest even worse. One recent study found that June's heat wave would have been "virtually impossible without human-caused climate change."

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