A significant and unusually early heatwave is building across the south-western US this week, bringing temperatures more typical of early summer than mid-March. In Phoenix, Arizona, the National Weather Service (NWS) forecasts temperatures could exceed 100F (38C) as soon as Wednesday, the earliest arrival of triple-digit heat to the city in nearly four decades. A strong, slow-moving high-pressure system - often called a heat dome - is trapping hot air over the region, pushing temperatures 20–30F above normal in parts of California, Nevada, and Arizona.
The heat is expected to persist for several days, with limited overnight cooling adding to the strain. Millions are likely to be affected as highs are expected to reach well above 90F in many areas, including typically cooler coastal regions, while inland and desert locations could exceed 100F, challenging long-standing March records.
NWS has issued heat advisories for parts of California, including the Bay Area and central coast, where temperatures could reach around 90F. Residents are being warned to check on the elderly and other at-risk groups as temperatures climb, and to schedule events around peak heat. The Las Vegas, Nevada NWS office has cautioned that 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.
Forecasters warn that this event could be prolonged, with only gradual relief possible by the weekend or early next week, raising concerns about health risks and broader environmental impacts. The heatwave also poses a serious threat to California's already below-average snowpack, accelerating melt at a time when it would normally build or decline slowly. Rapidly melting snow is likely to lead to summer water shortages, as snowpack provides about a third of the state's water, and increases the risk of a longer, more intense wildfire season later in the year.
While wildfire risk remains relatively low across much of the Southwest due to lighter winds, conditions differ further east, particularly near the Rockies to the High Plains. This developing heatwave follows recent blizzards and frigid conditions in the Midwest and Great Lakes, along with heavy thunderstorms and tornado threats towards the East Coast. By the end of the week and into next week, above-average temperatures are expected to spread across much of the United States.
The heat is expected to persist for several days, with limited overnight cooling adding to the strain. Millions are likely to be affected as highs are expected to reach well above 90F in many areas, including typically cooler coastal regions, while inland and desert locations could exceed 100F, challenging long-standing March records.
NWS has issued heat advisories for parts of California, including the Bay Area and central coast, where temperatures could reach around 90F. Residents are being warned to check on the elderly and other at-risk groups as temperatures climb, and to schedule events around peak heat. The Las Vegas, Nevada NWS office has cautioned that 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.
Forecasters warn that this event could be prolonged, with only gradual relief possible by the weekend or early next week, raising concerns about health risks and broader environmental impacts. The heatwave also poses a serious threat to California's already below-average snowpack, accelerating melt at a time when it would normally build or decline slowly. Rapidly melting snow is likely to lead to summer water shortages, as snowpack provides about a third of the state's water, and increases the risk of a longer, more intense wildfire season later in the year.
While wildfire risk remains relatively low across much of the Southwest due to lighter winds, conditions differ further east, particularly near the Rockies to the High Plains. This developing heatwave follows recent blizzards and frigid conditions in the Midwest and Great Lakes, along with heavy thunderstorms and tornado threats towards the East Coast. By the end of the week and into next week, above-average temperatures are expected to spread across much of the United States.
