Big weather changes from eastern Pacific Hilary could send flooding rain to Phoenix, Las Vegas
A significant shift in the weather pattern that is very unusual for August will cause temperatures to tumble and may unleash more than a month’s worth of rain for major cities in the coming days.
A pattern not typical for August will unfold over the western United States in the coming days that will cause temperatures to drop and send potentially flooding rain to part of the region.
Major changes are coming to the western third of the nation starting later this week. However, while some changes such as heat and wildfire relief may be welcomed, other developments related to Hilary just off the coast of Mexico in the eastern Pacific could put some individuals in a life-threatening situation, AccuWeather meteorologists warn.
"A pattern not typically seen during the month of August in the western portion of the United States is expected to bring a major cooldown and a risk for flash flooding beginning late this week," AccuWeather Meteorologist Brandon Buckingham said.
Compared to much of this summer, a massive heat dome will set up farther to the north over the middle of the United States later this week.
As the heat dome develops, and a dip in the jet stream forms along the Pacific coast, the door will open for cooler air to flow across the region. This pattern change will break down the Northwest heat wave and pull in moisture throughout the West at the same time.
Northwest heat wave to be swept away
As the heat dome breaks down and cooler air begins to work in from the Pacific, temperatures will trend downward slightly in the coastal Northwest later this week and then will bottom out this weekend. Many areas will experience high temperatures 15-25 degrees lower this weekend compared to the start of the week.
Following an August high-temperature record of 108 degrees Fahrenheit in Portland, Oregon, on Monday, temperatures will settle into the low to mid-80s by this weekend. Similarly, Seattle's high temperatures will trend downward from the mid-90s early this week to the mid-70s by this weekend.
Heat will hold on a bit longer and may not be slashed as much across the interior Northwest, but highs in the low to mid-100s will eventually be swapped with highs in the 80s to near 90 by this weekend.
Temperatures will trend downward later this week and into the weekend in the Southwest as well.
Following highs in the 110s in Palm Springs, California, temperatures may be no higher than the low 90s by Sunday. High temperatures may trend downward to the 70s in Las Vegas, well below the historical average high temperature of 103 F.
Hilary may trigger flash flooding
A clockwise flow of air around the massive heat dome and high pressure in the central Plains will create a southerly breeze in the West later this week to next week.
As the pattern evolves, localized downpours and thunderstorms in the Southwest may escalate into more widespread and serious flooding as some or all of Hilary's moisture is swept into the region.
"The potential exists for tropical moisture [associated with Hilary] from the eastern Pacific to get entangled within a broad area of low pressure lingering along the California coast late this week and this weekend," Buckingham said. "Saturday and Sunday could feature flooding rain across Southern California’s interior deserts, as well as western Arizona, even into Las Vegas."
AccuWeather meteorologists were anticipating the formation of Hilary at midweek. The system was dubbed a tropical storm on Wednesday morning, local time, and is projected to become a major hurricane this weekend.
Heavy rain can fall in parts of the Southwest due to the influx of moisture even if the core of Hilary remains offshore or the storm itself weakens.
Due to the hard-packed and rugged terrain in the region, the rain will quickly run off through the canyons and ordinarily dry stream beds and can lead lead to life-threatening flash flooding.
Rushing water or ponding could force some roads to close. Rocks and other debris may be washed onto roads in hilly areas.
Depending on exactly where the plume of moisture from Hilary extends, flash flooding could occur in Phoenix and Flagstaff, Arizona, as well as part of the Los Angeles and San Diego metro areas.
The stormier pattern may have a double-edged sword
Moisture from Hilary will bring up humidity levels in the region -- and that alone will help to reduce the wildfire ignition risk in the West. On top of that, any downpours that saturate the brush and forested areas will lower the fire risk in the short term.
But not all storms will bring rain and the scope of abnormally dry and drought conditions have been expanding in recent weeks, due to limited impact from the North American monsoon.
"While the moisture surge should be beneficial for firefighting efforts across the region, the potential exists for thunderstorms that can feature frequent lightning strikes as well," Buckingham cautioned.
The lightning could ignite new fires where rain does not fall during the storms.
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