Major cooldown to erase heat across the West

The big drop in temperatures will not come alone, with some areas facing the risk of flooding while others see an uptick in the wildfire threat.

Extreme heat is being replaced with a refreshing taste of fall for many in the western United States. However, the change in the weather pattern won't come without challenges including increased risks of fire danger and flooding.

During the first week of September, the weather across much of the West felt more like a continuation of August rather than the start to meteorological autumn. It was a steamy start to September in the Northwest; temperatures in Seattle, for example, soared 4.5 F above the historical average, peaking at 90 F, the hottest it has been since July 9.

Even the milder climates, like Central California, were battling the heat. High temperatures in Fresno, California, were above 100 every day but one, averaging 6.9 F degrees above the early-September norm.

AccuWeather forecasters say that the pattern change will not only wipe away the heat but also bring the return of some much-needed precipitation.

"A big dip in the jet stream along the Pacific coast brought in much cooler air to the Northwest, while others farther south will experience a significant drop in temperature as well," said AccuWeather Senior Meteorologist Heather Zehr.

High temperatures in places like Sacramento and Fresno, California, have dropped as much as 20 degrees Fahrenheit into midweek, with more downturn coming.

This bulge southward in the jet stream is expected to push inland as the week progresses, bringing the cooler conditions across the interior West and shifting the core of the heat out into the Plains.

"Wednesday will be a very different day than the Northwest has seen in a while, with temperatures that will run 15-20 degrees below historical averages," said Zehr. The last time places like Portland, Oregon, were in the 60s was Aug. 23, and before that mid-June.

Temperatures are expected to rebound some for the remainder of the week, with most locations likely to see temperatures at or slightly below historical averages into the coming weekend.

With this refreshing push of cool air, windy conditions will overspread much of the interior West, renewing the fire risk.

Gusty winds, combined with the low humidity and already dry vegetation will make it easy for any spark to quickly swell into a wildfire into Wednesday. The cool, moist air should bring the wildfire risk back down.

The dip in the jet stream will also open the door for a storm to move inland over the Northwest into midweek, adding a noticeably different weather pattern.

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"Rain will spread into the Northwest coast with rainy weather throughout Wednesday. Some showers linger on Thursday, though the center of the storm moves across the northern Rockies," Zehr said.

Widespread rainfall amounts of 0.10-0.20 of an inch are expected from far-Northern California to coastal British Columbia, on east to central Montana and Wyoming. The heaviest rain is likely to concentrate from the Cascades of Washington and Oregon and then west to the coast, as well as in the highest elevations of Idaho and western Montana. In both locations, rainfall could total more like 1.00-1.50 inches.

"Given how dry it is, and how dry it has been, areas across the Northwest may be more susceptible to flooding issues, even with lower rainfall totals," warned Zehr.

It's normal for parts of the West to experience drier weather during the summer, often referred to as the "dry season." However, this past summer has been even drier than the historical average, leaving many locations with dry conditions.

Every state from the Rockies on westward has more than 50% of the state as abnormally dry, and some with larger pockets of moderate or severe drought, especially in the Northwest.

Temperatures are expected to be above average for most of autumn, which may continue to fuel the ongoing drought, according to AccuWeather long-range experts.

Last of the monsoon coming

As the pattern continues to evolve, a last pulse of the North American monsoon is forecast to occur this weekend to early next week with some boost from tropical moisture. Some of the downpours and thunderstorms over the interior Southwest can be robust to the point of producing flash flooding and strong wind gusts.

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