Heavy storms to accompany cooler air in northwestern US into the weekend

By Renee Duff, AccuWeather meteorologist

Heavy to locally severe thunderstorms are forecast to erupt across the northwestern United States as cooler air moves in through the weekend.

The thunderstorms may not only bring lightning dangers and torrential downpours, but also threaten localized damage in some communities.

A dip in the jet stream associated with a chilly pocket of air high in the atmosphere will bring about the stormy pattern through the weekend.

"This time of the year, when the air aloft is chilly, the August sun still heats the ground and creates a volatile atmosphere that can trigger an eruption of thunderstorms," AccuWeather Senior Meteorologist Alex Sosnowski said.

There is the potential for some of these storms to turn severe with damaging winds and large hail possible threats. 

Every thunderstorm, regardless of its severity, can produce potentially dangerous lightning strikes. This is why it is vital to seek shelter indoors or in a metal, hard-top vehicle at the first rumble of thunder.

Into Friday evening, a large portion of interior Oregon, including Medford, Bend and Pendleton, will be at risk for severe weather.

The severe weather dangers are forecast to shift slightly farther east on Saturday, threatening areas from eastern Oregon through Idaho and the southern two-thirds of Montana.

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Residents in Boise, Idaho, and Butte, Missoula and Billings, Montana, should be on the lookout for rapidly changing weather conditions and have the umbrella ready at the start of the weekend.

Hikers are urged to monitor the sky for darkening clouds. If thunder is heard while you are in the high terrain and no vehicle or building is nearby, head to a valley or depression in the landscape away from lone trees.

Rain and thunderstorms will continue to spread northeastward on Sunday, drenching eastern Washington through Montana.

While the wet weather may disrupt travel and outdoor plans for some, the rainfall will be welcome where abnormally dry to moderate drought conditions exist. 

Most of the meaningful rainfall will miss the severe drought areas of western Washington.

The drought has been a breeding ground for wildfires, with around two dozen large fires currently burning in the Northwest, according to InciWeb.

The pattern through this weekend will carry a risk of storms producing abundant lightning in the absence of meaningful rainfall, which can create sparks that ignite new blazes.

In addition, consistently breezy conditions, both inside and outside of thunderstorm activity, can fan ongoing blazes through the weekend.

The cloudier, wetter weather will keep temperatures from reaching the record heights from earlier this week.

In Spokane, Washington, the high came close to 100 degrees Fahrenheit at midweek, but is forecast to stay in the 60s on Sunday. Boise will drop from a seasonable high of 90 on Friday to around 80 on Saturday and Sunday.

"During next week, the jet stream is forecast to take on a relatively straight west-to-east configuration," Sosnowski said.

This will allow warmer, drier weather to return to the Northwest as early as Monday and Tuesday.

Download the free AccuWeather app to keep track of severe thunderstorms and temperature trends in your area. Keep checking back for updates on AccuWeather.com and stay tuned to the AccuWeather Network on DirecTV, Frontier and Verizon Fios.