As winter technically gave way to spring this week in the Northern Hemisphere, parts of the western United States and northern Mexico were met not with the gentle warmth of March but with conditions more reminiscent of the fiercest days of midsummer. Instead of a seasonal easing, an immense “heat dome” descended over the region, toppling hundreds of records and sending temperatures into levels never before witnessed so early in the year.
Most striking of all was Martinez Lake, Arizona, where the temperature soared to a blistering 110 degrees on Thursday, March 19. That appears to have set a new all-time national mark for the hottest temperature ever recorded in the United States during the month of March, overtaking the old record of 108 degrees, which had stood since 1954. Other desert communities in California, including Thermal and Cathedral City, also reached a punishing 108, while Phoenix not only hit 105 but marked its earliest 100-degree day ever recorded. [scroll for more:]
The oppressive heat was not limited to the desert floor. Across the West, major cities and elevated areas alike saw longstanding records crumble. Las Vegas climbed to 95 degrees, breaking a March record that had endured for nearly four decades. In the Los Angeles basin, temperatures ran 25 to 35 degrees above normal, with inland sections pushing into the upper 90s. Even Flagstaff, Arizona—perched at around 7,000 feet—surged to nearly 80 degrees, surpassing previous March highs by more than five degrees. San Francisco, not generally associated with such extremes, found itself under its first-ever March heat advisory as an unusual heat risk spread across the Bay Area.
A rapid assessment released Friday by the World Weather Attribution group concluded that a heat wave of such force in mid-March would have been virtually impossible without the influence of man-made climate change. Researchers pointed to the extraordinary strength of the atmospheric ridge, with pressure levels reaching nearly four standard deviations above normal. The result has been rapid snowmelt in the Sierra Nevada and Colorado mountains, stirring concerns about premature runoff, strained water patterns, and the possibility of a longer and more dangerous wildfire season.
Though the core of the heat dome is now beginning to shift, relief is expected to come slowly. Forecasters say the warmth will expand eastward into the Mid-Atlantic and Southeast by Sunday, ushering 80-degree temperatures into places that only a week ago were still dealing with snow. And weather watchers are already focusing on another ridge of high pressure, somewhat less intense but still formidable, that could bring yet another round of unusual and record-threatening heat to the Southwest early next week.