Meteorological summer is less than a week away, but the heat has descended on California and the desert Southwest in full force, with near record temperatures and triple-digit readings likely Wednesday and Thursday.

Excessive heat warnings blanket much of California and Arizona. Temperatures could exceed 120 degrees in Death Valley on Thursday.

Among the most sweltering will be California’s Central Valley, where low humidity and building winds could also combine to cause fire weather concerns by Friday and Saturday. Yet thereafter, an abrupt shift to cool temperatures and perhaps some rainfall is possible — an occasion in California even rarer than the heat at this time of year.

Over the weekend, the dome of heat is expected to shift east, bringing elevated temperatures and stagnant weather to the Great Plains and the middle of the country early next week.

Temperatures to soar along much of the West Coast

The heat is surprisingly right on schedule for this time of year, a lobe of toasty weather cushioned beneath a ridge of dry high pressure stretching along the Pacific Coast. The high pressure zone will bring several pleasant days of sunshine and 70s to Seattle, but the heat will be far less tepid to the south.

Even Medford, Ore. may hit 96 degrees Friday, while Portland soars into the mid-80s.

Sacramento could reach a high of 104 degrees Wednesday and 102 on Thursday, relaxing into the lower 90s for Friday. Modesto was set to climb to 105 on Wednesday, while Fresno could have three 100-degree days in a row.

The southern San Joaquin Valley already had an entire day of wicked heat Tuesday, with Fresno remaining above 90 degrees all the way through 10 p.m.

Closer to the coast, an onshore flow will moderate temperatures, with 70s likely for highs in San Francisco and near 80 with fog in Los Angeles.

Heat arrives on schedule

Despite the early-season sizzler, temperatures near the century mark are right on par for this time of year in parts of the Golden State.

“Most of our records in late May in Bakersfield or Fresno … [are around] 105 or 110,” said Colin McKellar, a meteorologist at the National Weather Service in Hanford, Calif. “We warm up pretty quickly here in the valley in May. It’s not too uncommon.”

McKellar noted that Bakersfield’s average first 100-degree day is May 31, with the heat generally arriving a little later in Fresno, with a mean first date of June 4.

“Depending on where you are in the valley, it’s about late May or early June when [you] see your first 100-degree day.”

California is bracing for highs of 113 to 115 degrees, while Death Valley — aptly named for its hostile climate — could flirt with 120. Across the board, temperatures are running some 15 or so degrees above average.

Desert Southwest bakes in the heat

Parts of the desert Southwest are likely to endure a scorcher as well, with Phoenix looking at highs at or above 110 through Sunday. Slightly less harsh temperatures will arrive by early next week as highs return closer to seasonal norms, in the low 100s.

And, despite the temperatures, even more remarkable is the fact that Phoenix is unlikely to set any record high this week. That’s how prone they are to wild heat!

Hot, dry conditions raise fire weather fears in Nevada

Las Vegas could come within a degree or two of record highs Wednesday and Thursday. The National Weather Service is predicting a 106 on Wednesday, with the mercury inching up to near 108 degrees on Thursday.

The combination of brutal temperatures and arid air could lead to fire weather concerns by Friday or Saturday.

“It’s common to have fire weather concerns this time of year [in southern Nevada] during May and June,” said Andrew Gorelow, a meteorologist at the National Weather Service in Las Vegas. “We get these [high pressure] ridges to build in with hot, dry conditions. With time our fuels become drier and drier, and more critical.”

Gorelow said the main ingredient to watch this time of year is a southwesterly wind, most commonly associated with waves of low pressure passing to the north.

“If we see lows with the southwest winds really kicking up, it can combine those winds with hot, dry conditions,” he explained. “In the valleys down here, we have a lot of grass, so that’s the main fuel for the fires. They typically burn pretty easily, and if you get strong winds they can run pretty good.”

Fortunately, the grass fires are typically smaller in size and yield lesser impacts than their California counterparts.

Abrupt cool-down to end the heat

That same wave of low pressure slated to pass through northern California this weekend could also quell the heat with a period of rain for many in central and northern California, something that McKellar said is “more uncommon than the heat.”

Farther to the east in Nevada, the National Weather Service wrote about “a much more pronounced cooling by dropping temperatures below normal” late in the weekend.

Looking ahead, it appears as though this early-season shot of heat is not necessarily a harbinger of a record-breaking summer. Neither El Niño nor La Niña is present, and McKellar noted that other climate signals have been relatively mute this year, offering little predictive insight into how the rest of the summer will pan out. This particular episode of heat is fleeting in nature.

“These blocking patterns move on and break down … they usually stick around for about a week,” he explained. “I don’t see this as a pattern or the summer … that’s a question of will we see [moving weather systems] or [stalled] blocking patterns. I think right now this is just a quick-hitting ridge.”

Eventually that ridge of heat will shuffle east, lounging over the Plains and the Upper Midwest early next week with warm temperatures and dry weather.

It is worth noting, however, that heat events in California and much of the southwestern United States are becoming increasingly common and severe, both in magnitude and duration, thanks to human-induced climate change.

Southern California is one of the fastest-warming regions of the country, catalyzed by rising ocean water temperatures and changing precipitation patterns.