Northwest pattern change to offer relief, but raise new risks

By Alex Sosnowski, AccuWeather senior meteorologist Updated Aug. 4, 2021 8:38 AM MDT

The weather pattern is set to change multiple times across the Northwest during the first week of August, which could complicate the ongoing wildfire battles.

Thanks to an unfolding shift in the jet stream, heat is forecast to ease and there may be opportunities for sporadic rainfall in the coming days over the parched northwestern United States and British Columbia. However, AccuWeather meteorologists warn the changes in the weather conditions will not all be good news for the battle against raging wildfires.

As of Wednesday, Aug. 4, 2021, there were 97 large wildfires active in the U.S. with most of those fires in the Northwest, according to the National Interagency Fire Center (NIFC). Of all the fires burning, only four were contained.

A southward dip in the jet stream will pivot from the Pacific Ocean to British Columbia into this weekend, ushering cooler and more humid air inland across the Northwest.

Following highs in the 80s in Seattle and the lower 90s in Portland, Oregon, into midweek, temperatures are predicted to trend downward Thursday and Friday. Highs are predicted to bottom out in the lower 70s in Seattle and the lower to middle 70s in PortlandThe change will result in below-average temperatures. Normal highs in early August range from the upper 70s in Seattle to the mid-80s in Portland.

Accompanying the substantially lower temperatures in coastal Washington and northwestern Oregon will be clouds and showers. Both Seattle, Portland and many other coastal towns and cities, as well as the western slopes of the Washington and northern Oregon Cascades, are forecast to pick up their first measurable rainfall in 40-50 days.

While some rain will fall on part of the Northwest coast of the U.S., rain over western British Columbia may be a bit more substantial.

“There is the potential for 0.6 of an inch to 1.0 inch (15-25 millimeters) of rain along the coast, the western slopes of the Rockies and the southern part of the high country in British Columbia from Thursday through Saturday, which will be greatly beneficial in terms of easing the fire risk and a start on easing drought conditions,” AccuWeather Senior Meteorologist Brett Anderson said.

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Some rain, but lesser amounts of 0.2 to 0.6 of an inch (5-15 millimeters), is likely over the interior valleys of British Columbia, but it should be enough to knock down the dust.

Most of the rain that falls in the northwestern U.S. will tend to occur over the mountains in western Montana and perhaps northwestern Wyoming, with some exceptions. Rainfall in this zone will tend to be spotty and on the order of 0.10 to 0.20 of an inch over the high country from Thursday to Saturday.

Lower temperatures are forecast to reach the interior Northwest as well.

Following highs well into the 90s in Spokane, Washington, and Boise, Idaho, and the triple digits in Medford and The Dalles, Oregon, high temperatures by Friday are projected to be in the 70s in Spokane and the 80s in Medford, The Dalles and Boise. Like in the coastal Northwest, highs will be below average compared to the upper 80s to mid-90s that are typical for early August.

Another positive aspect of the unfolding weather pattern is that the increased winds should help disperse the smoke in areas well away from fires late this week, Anderson said.

Winds are forecast to be highly variable from location to location, but gusts between 15-30 mph can occur in general over the ridges and through the passes and canyons with higher speeds possible in the vicinity of thunderstorms. Winds will generally shift from the south and southwest to more of the west and northwest later this week and into this weekend.

“Most of the concentrated areas of surface smoke will likely be within fairly close range and directly downwind of the larger fires,” Anderson stated.

However, while the lower temperatures and smoke dispersal will be a plus for millions tired of the heat, it is not likely to be enough to deter the actual wildfire threat, especially to the east of the Cascades.

As weak storm systems pivot around the jet stream dip and some of them produce much-need rain along the Northwest coast of the U.S. and into interior British Columbia, less moisture farther to the southeast will likely translate to thunderstorms that bring little or no rain and lightning strikes.

“As a weak disturbance rolled in from the Pacific on Tuesday, thunderstorms in northern Washington state alone produced more than 3,200 lightning strikes during the afternoon and evening hours,” according to AccuWeather Senior Meteorologist Paul Walker.

This same weak disturbance may produce thunderstorms and lightning strikes in western Montana and southeastern British Columbia during Wednesday afternoon and evening.

A new and more potent Pacific disturbance will track inland across the Northwest into Thursday.

This system will likely act as a trigger for locally gusty showers and thunderstorms and is forecast to ignite storms from Oregon and Washington to northwestern Wyoming and western Montana, Anderson said.

“While there will likely be numerous lightning strikes with some of the thunderstorms, which can spark new fires, the primary threat from this will be the increase in winds, especially by Thursday, and any nearby isolated showers or storms could produce locally strong wind gusts, which may help accelerate the advancement of existing fires,” Anderson explained.

The gusty winds may also cause pockets of blowing dust, which can suddenly drop visibility to less than a quarter mile at times and pose a danger to motorists. On July 25, a dust storm along Interstate 15 near Konash, Utah, contributed to a multiple-vehicle pileup that took the lives of eight people.

Another dangerous hazard will be that pockets of heavy rain over previously burned, sloped ground may lead to mudslides and other debris flows. Fires in the western U.S. have already scorched more than 3 million acres this year as of Aug. 3, according to the NIFC.

“People traveling through the region should check their route beforehand to avoid having to turn around, as some roads may have been closed due to fires, mudslides and smoke,” Anderson said.

As an extra precaution for smoke and blowing dust, motorists in the region should turn their headlights on.

(Click to Source)

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