States across the US west are bracing for a brutal early-season heatwave threatening to cook several cities through the weekend and into next week. Forecasters warned temperatures will spike 20-30F above normal for several days.

Daily records could be shattered in southern California this week, the National Weather Service said, with a possibility that all-time records for March will be broken as well. Following the warmest winter on record across most of the region, the intense conditions are expected to eat into low snowpack levels, deepening drought concerns.

Caused by a large and persistent dome of pressure settling over a large swath of the west, the blasts of heat are expected to come in two waves, first hitting the west coast on Thursday and spreading east through California, Nevada, Arizona and New Mexico. Another, stronger round, is set to start on Monday, reaching into Utah, Colorado, Oregon, Idaho, and Wyoming.

By Tuesday next week, areas home to roughly 26.4 million people will be affected by moderate heat risks, the second level out of four designated by the NWS, affecting sensitive populations and those without access to cooling or hydration the most. Tens of thousands may face extreme heat – the highest level – with no overnight relief.

Extreme heat so early in the year could pose higher public health dangers. “This time of year, the body isn’t used to dealing with this level of heat, so these highs are more impactful now than they would be in July,” NWS meteorologists in the San Francisco Bay Area warned in a Thursday social media post.

“The next 10-14 days look truly exceptional across the western US, and not in a good way,” climate scientist Daniel Swain wrote about the event on Wednesday. He said a ridge of high pressure among the strongest “ever observed in the south-western US in March” was expected to develop by Friday, but its records would not hold for long, as it “reorganizes into an even broader and stronger ridge next week”.

With a high likelihood for historic heat for early spring, some records might be broken for seven to 10 days consecutively, Swain said. Temperatures could climb past 100F (37.8C) in Los Angeles, up to 90F (32.2C) in the San Francisco Bay Area, and possibly up to 110F (43.3C) in the south-eastern deserts, thrusting the region into summer conditions months before they are typical.

The heat wave comes as water managers across the west nervously eye declining snowpacks, which are expected to shrink further. There had been hopes for another cold storm to bolster the depleted snow, relied upon for water supplies throughout the warmer months. Heat also bakes more moisture out of landscapes, amplifying wildfire risks and extending the seasons when ignitions can quickly become infernos.

“During the upcoming period of exceptional and prolonged heat–which will bring t-shirt and shorts weather even into the higher mountains of the American West–snowmelt will accelerate dramatically,” Swain said.

The spring snowpack, measured on 1 April, “may well be the worst on record across many, if not most, western US watersheds”.