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.

Highs are expected to reach well above 90F in many areas, including typically cooler coastal regions, while inland and desert locations could exceed 100F, with some spots approaching or surpassing 110F, 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 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.

"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, Nevada NWS office said.

Forecasters warn this could be a prolonged event, 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.

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 - since snowpack provides roughly a third of the state's water - and leaves landscapes drying out sooner, heightening the potential for a longer and more intense wildfire season.

Wildfire risk remains relatively low across much of the Southwest despite the heatwave, as lighter winds in these regions are expected to limit the rapid spread of fires.

High temperatures and dry conditions alone are insufficient to drive large, fast-moving fires when wind speeds are low, providing a modest buffer for many communities.

Conditions are different further east, however, particularly from areas near the Rockies to the High Plains, where dry air, parched vegetation, and persistent breezes could create a much more favourable environment for wildfires to ignite and spread.

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.