US WEST COAST — An unusually early heatwave is building across the southwestern United States, bringing temperatures more typical of early summer than mid-March. The National Weather Service forecasts that temperatures in Phoenix, Arizona, could exceed 100°F (38°C) as soon as Wednesday.
This would be the earliest arrival of triple-digit heat in Phoenix in nearly four decades. Parts of southern California reached 100°F on Tuesday afternoon, according to the National Weather Service.
A strong, slow-moving high-pressure system, often called a heat dome, is trapping hot air over the region. The high-pressure system is pushing temperatures 20°F to 30°F above normal in parts of California, Nevada, and Arizona.
The National Weather Service has issued heat advisories for parts of California, including the Bay Area and central coast, where temperatures could reach around 90°F. Highs are expected to reach well above 90°F in many areas, including typically cooler coastal regions, while inland and desert locations could exceed 100°F, with some spots approaching or surpassing 110°F.
"Extreme early-season heat coupled with high tourism rates will make this heat very dangerous, particularly for those not acclimated to the heat and/or traveling from cooler climates," the Las Vegas National Weather Service office said in a statement. Residents in the affected areas are being warned to check on the elderly and other at-risk groups as temperatures climb and are being advised to schedule events around peak heat.
The heatwave is expected to persist for several days with limited overnight cooling. Forecasters warn this could be a prolonged event with only gradual relief possible by the weekend or early next week. Long-standing March temperature records may be challenged by these high temperatures.
The heatwave poses a threat to California's below-average snowpack by accelerating melt at a time when it would normally build or decline slowly. With temperatures running far above normal, snow in the Sierra Nevada is likely to melt weeks earlier than usual. Earlier snowmelt will reduce the amount of water that can be gradually released into reservoirs later in the year, as snowpack provides roughly a third of California's water.
Rapid runoff increases the risk of summer water shortages and leaves landscapes drying out sooner, heightening the potential for a longer and more intense wildfire season. Despite the heatwave, wildfire risk remains relatively low across much of the Southwest due to lighter winds expected to limit the rapid spread of fires.