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Weather chaos in the West: High heat and concerns about fires and flood

Residents of the West Coast face a multitude of hazards

As abnormally hot weather lingers in the Pacific Northwest, residents of Portland, Oregon take to the banks of the Sandy River in an attempt to cool off on August 14, 2021. (MICHAEL HANSON/AFP via Getty Images)

Weather alerts blanket the West Coast, even as much of the rest of the country remains relatively quiet weather-wise. All across California and the Pacific Northwest, heat advisories and excessive heat warnings are in place, and temperatures are expected to top 100 degrees in spots. A few dry thunderstorms could bring fire concerns, too, and in the Desert Southwest there’s potential for flooding.

The maelstrom of meteorological chaos is delivering seemingly contrasting hazards to one of the most populous corridors of the Lower 48. As places like Fresno, Bakersfield, Sacramento, Portland and Seattle deal with high temperatures 10 or more degrees above what’s typical, Tucson, Phoenix and Santa Fe are bracing for excessive rainfall. It’s an active summertime pattern delivering some high-impact results — and it may continue through at least the remainder of the workweek.

The culprit? A high pressure heat dome parked just east of the Sierra Nevada. It’s delivering hot, dry, sinking air to much of the West Coast, while clockwise flow around it is helping scoop a tongue of monsoonal moisture into the Desert Southwest. That leads to a feast or famine weather pattern. Some wind up with too much rainfall, while others are drought-stricken and moisture-starved.

Excessive heat in California

More than 17 million Americans are under heat advisories on the West Coast, with another million or so included under excessive heat warnings. Advisories stretch through the entire San Joaquin Valley in California and blanket most of Washington state and the northern half of Oregon. Excessive heat warnings are in effect for central Washington state and include the city of Yakima.

The heat will last through Saturday in the Central Valley, where just about everyone is expected to see highs climb to between 102 and 107 degrees. Redding, Calif., has seen highs at or above 102 degrees every day since Aug. 13, and even made it to 110 degrees Tuesday and Wednesday. That tied a 102-year-old record on Tuesday, and hit a new record Wednesday. The city is expected to hit 108 degrees Thursday, 107 on Friday and 108 Saturday before a cool-down back to the century mark into Sunday.

Temperatures in Sacramento won’t be quite as extreme but could still hit 102 degrees Thursday, 105 on Friday and 103 on Saturday.

“Widespread moderate to high heat risk [is] expected,” wrote the National Weather Service in Sacramento. “Hot temperatures will significantly increase the potential for heat-related illnesses, particularly for those working or participating in outdoor activities.”

One slight silver lining to the heat, if it could be called that, is that dry conditions will allow overnight lows to bottom out in the upper 60s to near 70, offering some nocturnal relief.

By Sunday, a trough, or a dip in the jet stream, will approach the California coastline, turning winds onshore and ushering a cooler, more manageable air mass.

A hot day or two in the Pacific Northwest

The heat is shorter-lived in the Pacific Northwest. In Seattle, for example, Thursday is the only day expected to be anomalously hot. After a projected high of 93 degrees, which would break a daily record of 88 set in 1991, the temperatures should fall back to the lower 80s on Friday and upper 70s to near 80 on Saturday.

Farther east, that incipient onshore flow will be more delayed in its arrival, leading to an extra day of heat for places like Kennewick, Wash., in the Columbia River Basin, where Friday could hit 103 after a day around 106. Both weekend days should flirt with or exceed 100 degrees. The same will be true in Yakima, where readings are projected to hit 104 on Thursday and 101 on Friday.

An excessive heat warning is in effect and the local National Weather Service office is warning of “dangerously hot conditions.”

In Portland, the heat is also fleeting. Temperatures on Thursday could hit 95 degrees, but highs will settle back into the 80s thereafter.

Fire concerns for some

The extreme heat will be of serious concern in Northern California and southern Oregon, where “abundant lightning on dry fuels” could become a major issue for wildfire ignition. The heat will desiccate the landscape, extracting what little moisture remains from already dry vegetation, allowing the landscape to act as a tinder box with dry fuels ready to burn. Add a lightning threat and possible ignition source, and the situation becomes precarious.

There is risk for dry thunderstorms both Thursday and Friday — thunderstorms with rain that evaporates in a layer of near-surface dry air before hitting the ground. That means lightning can spark new fires without the presence of rain to extinguish the quickly kindled flames. That concern is expected in parts of central Oregon and Washington, too, albeit to a slightly lesser extent.

Flooding in the Desert Southwest

Over New Mexico and Arizona, the thunderstorms will be far from dry. In fact, they’ll be robust in their precipitation, with rainfall rates briefly topping 1 inch per hour in a few areas. While most storms will be few and far between, they’ll pepper the landscape and blossom in humid southerly flow at the atmosphere’s mid-levels, wafting north across much of Southern California and Nevada, the majority of Arizona and New Mexico, and even a few zones north of the Four Corners.

The term “monsoon” doesn’t mean a downpour. Instead, it refers to the seasonal wind shift that induces downpours. For most of the year, westerly flow keeps the Southwest bone dry, but this time of year it turns more southerly, introducing moisture.

Flood watches are up for most of New Mexico and Arizona, as well as in southern Utah.

Most of the Desert Southwest records less than 10 inches of precipitation per year. A quick-hitting monsoonal downpour can deliver 10 percent or 20 percent of that in an hour or two. While the rainfall is welcomed, when it falls so rapidly it can quickly overwhelm the sandy soil’s ability to absorb it, leading to runoff and pockets of flooding.

Just last week, Death Valley, Calif., recorded 1.46 inches of rainfall in a single day from the monsoon, missing the all-time daily rainfall record by 0.01 inches. The geographically extreme rainfall rates were regarded as a “thousand-year rain event.”