October-like chill coming to California, other parts of western US

A dramatic cooldown coming later this week may be a shock to millions of people in the western United States following relentless hot weather this summer.

A big break from the summer heat is coming to a large part of the western United States later this week that will feel like autumn, with some people reaching for long sleeves and jackets, AccuWeather meteorologists say.

The chill will follow a storm that brought some cool air, clouds, showers and thunderstorms to part of the Northwest this past weekend.

"A more notable storm with a dip in the jet stream will barrel into Washington, Oregon and northern California from late week into the upcoming weekend," AccuWeather Meteorologist Alyssa Glenny said.

This new storm will usher in unseasonably cool conditions from Thursday to Saturday in much of California, Oregon, Nevada, and parts of Idaho, Utah and Washington. Temperatures will be more on par with October, running 10-20 degrees below the historical average for late August.

“For example, Redding, California, will experience high temperatures that trend downward from the 90s F early this week to the 70s by Friday," Glenny said. "By Friday morning, temperatures may dip to within a few degrees of their record low of 55, last reached 30 years ago on the date in 1995. The record low for Saturday morning in the mid-50s may also be challenged."

The temperature swing will be dramatic, especially in areas where extreme heat has been the rule for much of the summer. Redding has been experiencing temperatures of 5 degrees above the historical average for the period since June 1. The city also set its all-time record high of 119 on July 6.

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Las Vegas is forecast to have its first sub-100-degree high since mid-June later this week. Should highs be in the 90s for three or more days in a row, it would be the first time since May. The Nevada desert city has experienced a scorching summer with temperatures of 6.5 degrees above the historical average, including an all-time record high of 120 degrees set on July 7.

Along the immediate Pacific coast and portions of the Interstate 5 corridor, the cooldown will be less pronounced but still noticeable, as temperatures will briefly spike ahead of the drop, with many people from Seattle and Portland, Oregon, to San Francisco and Los Angeles noticing the cooling trend later this week.

As cool as the air gets near the ground, it will get even chillier at the level of the atmosphere where jets fly. That setup may allow an eruption of clouds, showers and thunderstorms. Some of the storms could be locally heavy to severe. Where little rain falls, lightning strikes could ignite new wildfires, even with the cool conditions and higher humidity levels.

The cooler and more humid air should help ease the quick spread of wildfires and may assist in firefighting efforts, although gusty winds ahead of the cool storm may help fan the flames beforehand.

"Along with the chilly air will come the risk of frost in the normally colder locations," AccuWeather Senior Meteorologist Brett Anderson said, "That would tend to be in the mountains and particularly the mountain valleys or high-elevation plateaus later this week."

Temperatures are forecast to dip to near the freezing mark at Tahoe Valley, California, Friday night and Saturday night.

As a storm system pushes farther to the east, temperatures over the western third of the nation will trend upward once again later this weekend and next week.

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