A significant early-season heat wave roasting the desert southwest will reach its peak on Wednesday as temperatures in Las Vegas threaten to crack the century mark. If the city hits 100°F, it would be the earliest triple-digit reading on record there, beating the old record by about a week. The excessive heat will continue across the region through Thursday.

Wednesday could see weather history in Las Vegas as forecasters expect the high temperature to reach 100°F, which would be the earliest such reading ever recorded at the city’s airport. This would break the old record set just a few years ago on May 5, 2017. Las Vegas typically doesn’t see its first triple-digit temperature until the end of May.

It’s felt more like June than April across much of the desert southwest since late last week. The high temperature in Phoenix, Arizona, has been 97°F or hotter every day since April 23, with highs exceeding 100°F on April 26, 27, and 28. The earliest 100°F reading on record in Phoenix occurred on March 26, 1988—far earlier than Las Vegas’ record—with the first triple-digit high temperature typically showing up around the first week of May.

Under normal conditions, the southwest slowly builds toward its characteristic blazing heat through the late spring as persistent high-pressure systems build over the region and the arid environment bakes under the intense sunshine.

This week’s heat arrived so early that forecasters issued excessive heat warnings for the region. An excessive heat warning means that dangerously hot temperatures could lead to injury among at-risk groups, including people who work or exercise outside for long periods of time, as well as the elderly and individuals with health issues who don’t have reliable access to air conditioning. Even though it routinely gets hotter than this during the summer, this alert serves as a heads-up that it’s unusually warm for this time of the year.

The early heat wave is the result of an intense upper-level ridge of high pressure that’s parked itself over the western United States for the past couple of days. Ridges feature calm weather and sinking air that heats up as it descends toward the surface, leading to warmer-than-normal temperatures. Stronger ridges can foster stronger heat waves.

High temperatures should fall below excessive heat warning criteria by the weekend as the ridge weakens and begins to move off toward the east. However, high pressure should still dominate the region in the short term, allowing high temperatures to continue climbing above normal through the first couple of days of May.