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 100°F (38°C) as soon as Wednesday, marking the earliest arrival of triple-digit heat to the city in nearly four decades.
A strong, slow-moving high-pressure system, often referred to as a heat dome, is trapping hot air over the region, pushing temperatures 20 to 30°F 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.
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, challenging long-standing March records.
The NWS has issued heat advisories for parts of California, including the Bay Area and central coast, where temperatures could reach around 90°F.
Residents in the affected areas 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 NWS office emphasized, 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 have warned that this could be a prolonged event, with gradual relief expected only by the weekend or early next week, raising concerns about health risks and 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.
With temperatures running far above normal, snow in the Sierra Nevada is likely to melt weeks earlier than usual, reducing the amount of water that can be gradually released into reservoirs later in the year. This rapid runoff increases the risk of summer water shortages, as snowpack provides roughly a third of the state's water, and it may lead to landscapes drying out sooner, heightening potential wildfire risks.
Despite the heatwave, wildfire risk remains relatively low across much of the Southwest due to lighter winds expected to limit rapid fire spread. However, conditions differ further east where dry air and vegetation combined with persistent breezes may create a favorable environment for wildfires.
This developing heatwave follows recent blizzards and frigid conditions in the Midwest and Great Lakes, as well as 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.
}
