Destructive western US monsoon to trigger new concerns

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Already the 2021 North American monsoon season has gotten off to a tumultuous start. Not only has it been the busiest start to the season in years, the monsoon thunderstorms and their resultant flash flooding have turned deadly across the American Southwest.

AccuWeather forecasters say that although the engine for the monsoon will continue to churn out thunderstorms in the coming days, the activity will shift around in parts of the West. A pattern change could also influence the coverage of storms over the region into the week -- and raise dangers of a different sort.

California, Nevada, Oregon and Washington could all get storm action at times into the week.

Typically during the summer months, a broad area of high pressure develops over the interior West and extends through multiple levels of the atmosphere. This high-pressure area becomes the engine for the monsoon.

The clockwise circulation around this high eventually pulls moisture up from the Gulf of Mexico and sometimes the Gulf of California as well. As that happens, humidity levels tick up, and the extra moisture goes on to fuel daily thunderstorms in the Southwest.

AccuWeather Chief On-Air Meteorologist Bernie Rayno described this season as the most robust since 2015. The surplus of rain this month has helped to knock down the magnitude of the highest levels of drought, especially in New Mexico and Arizona. However, in areas farther to the north and west in California, Nevada, Utah and Oregon, where moisture has been much more limited, the drought has gotten worse.

In some areas, such as in Tucson, Arizona, rainfall this past July ranks high compared to other Julys, or even any month in history. Tucson picked up 8.06 inches of rain for the month, blowing away the wettest month on record, which unleashed 7.93 inches in August 1955.

This year's monsoon has already proven to be dangerous and deadly. There have been multiple incidents of flash floods that have raced through city streets, arroyos and open areas of the Southwest. Dust storms on the leading edge of the monsoon thunderstorms have caused multiple vehicle pileups as well.

Thunderstorm winds flipped over two large rigs near Chiriaco Summit, California, causing one injury, according to the California Highway Patrol.

There are often fluctuations in the areal coverage of moisture from one day and one week to the next. These variances may seem subtle but can lead to stretches of days with thunderstorms every day and just the opposite when thunderstorms can be absent for a long time.

On Friday, satellite images revealed that a surge of moisture was present from western Arizona to much of Utah, much of Nevada, the deserts and Sierra Nevada of California, the Oregon Cascades and south-central Washington.

This image, taken on Friday morning, July 30, 2021, shows a ring of moisture and fuel for thunderstorms was present from western Arizona and southeastern California to parts of Oregon, Idaho and Wyoming. (CIRA at Colorado State/GOES-West)

A pocket of dry air in the atmosphere limited the number of storms in New Mexico, eastern Arizona and southern Colorado at the start of the weekend.

"As some extra energy in the upper atmosphere moves up from the south and southeast, it will trigger storms over a wide area," Zehr explained. The storms could trigger torrential downpours, some of which may be slow-moving, and flash flooding as a result.

At the same time, some of the storms that fire over the Sierra Nevada, Oregon Cascades and parts of south-central and southeastern Washington may bring so little rain that lightning strikes could start new fires -- and gusty winds kicked up could threaten to fan the flames.

Early this week, the upper-level area of high pressure is forecast to shift westward, which will switch up the pattern across the American Southwest.

"The westward shift of the big high-pressure area will tend to weaken the flow of moisture into the West and cause a downturn or pause in thunderstorm activity across California, Nevada and Oregon on Sunday," Zehr said.

Storms are likely to keep firing across the swath from New Mexico and parts of Arizona to Colorado and portions of Utah into the week, but perhaps with less coverage when compared to much of the summer thus far.

The number of storms erupting on a daily basis may continue to dwindle as this week progresses.

However, AccuWeather meteorologists have concerns about one area in particular related to thunderstorm activity coming in from the Pacific this week.

A southward dip in the jet stream is forecast to strengthen over the northeastern Pacific this week. As this dip develops, storm systems with limited moisture are likely to travel into the northwestern U.S. One such storm system may roll ashore in Northern California and Oregon at midweek.

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Even though the jet stream dip and storm system are likely to bring lower temperatures, some cloud cover and perhaps a break in the dry spell for western Washington and coastal Oregon, the features may stir winds and bring just enough moisture to cause thunderstorms to erupt farther inland. In this scenario, the pattern could lead to a new surge in wildfires from both lightning strikes and winds that may further spread ongoing blazes and lead to erratic fire behavior.

Even in the wake of this particular disturbance, a pattern of tricky winds may continue to cause trouble for firefighting efforts later this week throughout the interior Northwest.

As of Sunday morning, August 1, 2021, 87 active large fires had burned 1,771,933 acres across 13 states, mostly in the West, according to the National Interagency Fire Center.

Keep checking back on AccuWeather.com and stay tuned to the AccuWeather Network on DirecTV, Frontier, Spectrum, FuboTV, Philo, and Verizon Fios.

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