Fox News Weather Center

Dramatic cooldown to replace stifling heat in northwestern US

Following record-challenging heat in the Pacific Northwest early this week, a significant cooldown is set to follow by late week.

An expansive area of high pressure bulging northward to the Canadian border is responsible for the surge of oppressively hot air through midweek.

Temperatures through Wednesday will surge to levels 10- to 20-degrees Fahrenheit above normal in the interior Northwest. The abnormal heat will linger through Thursday farther inland through the northern Rockies.

Residents living in Salt Lake City can expect little relief from the sweltering heat through Thursday as temperatures soar into the middle and upper 90s, or near-record levels.

In Great Falls, Montana, temperatures will soar well past the normal high of 70 F on Wednesday and Thursday, likely surpassing the 90-degree mark on Thursday.

However, a major pattern shift will first eye the Pacific Northwest on Thursday and the northern Rockies by Friday.

“After a few weeks of generally warm and dry weather, a pattern change will bring much cooler and wetter weather to the west, with rain arriving by tonight in coastal Washington, Oregon and Northern California,” said AccuWeather Meteorologist Ryan Adamson.

Northwest cooldown static

The increase in cloud cover and the arrival of precipitation will signal an abrupt end to the extreme heat.

“Temperatures will be 10 to as much as 25 degrees lower between the highs that occurred on Tuesday and the forecast highs by Friday and Saturday,” Adamson added.

In Seattle and Portland, Oregon, high temperatures in the 80s will be replaced by temperatures some 5-10 degrees below normal from late week into the upcoming weekend. In fact, temperatures may struggle to reach the lower 60s on the coolest days.

Farther inland, temperatures in Great Falls and Spokane, Washington, will also dip into the 60s by the weekend and replace near-record heat with unseasonably chilly air.

As the major storm system plows eastward into the northern Rockies on Friday, severe thunderstorms capable of producing damaging wind gusts and torrential downpours may rattle residents from Cut Bank, Montana, to Missoula, Helena and Great Falls, Montana.

The drastic change in the weather pattern will have residents not only scrambling for jackets and umbrellas, but also on alert for potential power outages and property damage from the storms.

For areas west of the Cascades, a brief break in the rain on Thursday night will quickly be erased as showers resume by Friday afternoon.

Adamson added that an even colder pocket of air will swirl across the Pacific Northwest this weekend, with the greatest effects being felt from Oregon into Northern California.

“The air will be cold enough for snow to be seen above 6,000 feet, so hikers and outdoor enthusiasts will need to be prepared for a brief return to winterlike conditions in the highest elevations,” Adamson said.

Fortunately, snow levels will remain much too high for snow to fly and cause travel issues on Interstate 90 from Missoula to Snoqualmie Pass, Washington.

The large vortex of chilly and unsettled conditions will slowly trek eastward through the northern Rockies by late in the weekend and early next week, allowing drier conditions to return to the Northwest.

However, a major storm system is likely to develop in the northern Plains by Tuesday, resulting in a dose of heavy and potentially flooding rainfall across portions of eastern Montana and northern Wyoming. How intense the storm system becomes and its exact track can determine where and how much rain falls.

For those eager for a return of midsummer heat, temperatures will not be in any hurry to rebound to near-record levels. Although milder air will return to the region by early next week, temperatures will likely run a few degrees below normal through the week.