Western US faces opposite extremes from the weather this week

One part of the region faces cool conditions amid a pattern known as “June gloom,” while heat will build across another area. There will also be some noteworthy precipitation out West.

Just about everything possible in the realm of weather will be happening in the West this week, except perhaps snow.

June gloom will keep it cool in coastal Southern California as drenching thunderstorms erupt from interior California to the Rockies. Meanwhile, early-season heat will build in the Northwest, AccuWeather meteorologists say.

Clammy conditions to persist in coastal Southern California

A flow of moisture from the Pacific Ocean, known as a marine layer, will persist along the Southern California coast, including the Los Angeles and San Diego metro areas.

"This overall pattern is common during June, but this one is a bit more robust than average," AccuWeather Chief On-Air Meteorologist Bernie Rayno said. As the sun heats the interior southwestern U.S., the air rises and creates a vacuum effect. Air then flows in from the Pacific to replace the rising air over the Southwest.

"The process creates a deck of low clouds, fog and drizzle along the coast, which can extend miles inland," Rayno explained. "Hence, the term 'June gloom'." In some cases, people in coastal areas may not see the sun for days. In others, it may take until the afternoon hours for the low clouds to erode enough to allow the sun to shine through.

The persistent breeze coming in from the chilly ocean will suppress high temperatures along the coast. For example, highs in downtown Los Angeles will range only from the upper 60s to the low 70s this week. The historical average high is in the mid- to upper 70s in early June for the Los Angeles area.

Heat to build in Northwest into midweek

Farther north, much of Washington, Oregon and even part of Northern California will not have to deal with a cool, moist flow of air from the Pacific but rather just the opposite into midweek.

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In this zone, a breeze will blow in the opposite direction as that of Southern California. As the air flows from the interior U.S. toward the coast, descending air from the Rockies will warm eastern Washington and Oregon. As the air descends from the Cascades, it will warm coastal areas from Washington to Northern California.

For example, highs will be in the 80s to near 90 in Spokane, Washington, into Thursday, compared to the historical average high in the lower 70s. Similarly, in Portland, Oregon, temperatures will surge into the upper 80s to the lower 90s through at least Wednesday.

As a storm near the jet stream level of the atmosphere drifts northward later this week, thunderstorms with little or no rain may set off lightning-induced wildfires over parts of the Northwest.

"Because of a lack of storms with big rain this spring, coastal areas of Washington and Oregon have trended abnormally dry, and it is getting worse," AccuWeather Meteorologist Brandon Buckingham said. There is also a large area of moderate to severe drought building over interior Oregon.

"Conditions may trend wetter toward mid-June as some storms move in from the Pacific," Buckingham said.

Provided the upper-level storm does not fall apart later this week, more general shower and thunderstorm activity may start to dampen isolated parts of the Northwest.

Thunderstorms to erupt from California to Nevada, Utah and Arizona

As the upper-level storm swings in from the Pacific and sets up over the Southwest states for a time this week, it will bring a significant uptick in thunderstorm activity in the region.

"With the storm centered in Southern California through Wednesday, southeasterly winds will cause thunderstorms over the Sierra Nevada mountains to roll down the foothills into the San Joaquin Valley," Buckingham said.

Rain during June tends to be rather scarce across much of California, with the impacts of winter storms well in the rear-view mirror and impacts from the North American monsoon not usually into full gear.

Since rain is somewhat of a rarity in California during the month of June, there is the potential for some cities to tally up a month’s worth of rain in a matter of hours this week. The historical average June rainfall for Sacramento is 0.23 of an inch, while Lake Tahoe, California, tends to pick up 0.55 of an inch this month.

It's possible that Fresno, California, as well as Salt Lake City, will be hit by one or more thunderstorms this week. A thunderstorm could even wander close to Las Vegas or Phoenix as the pattern sets up. Meanwhile, showers may dampen San Franciso on multiple occasions.

The same storm will help pump more moisture over the interior West with more widespread coverage of showers and thunderstorms in parts of the region compared to recent weeks.

With the spring thaw over the high country of the West well underway, the added potential for torrential downpours throughout the Rockies and Intermountain Region could push elevated stream levels even higher this week. Experts advise extra caution when venturing in canyons and along streams in the region as downpours miles upstream may lead to a rapid rise of water or flash flooding.

Any snow that falls on the West this week will be restricted to the highest peaks in the region, where few people venture.

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