Summerlike pattern keeping Northwest unusually warm will fuel frequent thunderstorms in Southwest
A weather pattern more typical of July is sticking around the West, where above-normal warmth and pop-up thunderstorms will continue into next week, AccuWeather meteorologists say.
Relief is on the way for some across the Northwest this weekend, but temperatures will reach unusually high levels elsewhere.
A weather pattern more reminiscent of the middle and end of the summer will keep a large portion of the northwestern United States warm into the weekend while daily, drenching thunderstorms will roam the Southwest into next week, according to AccuWeather forecasters.
The warmth in the Northwest will be a continuation of a heat wave that began last week and has shattered numerous daily temperature records. However, some heat relief is in sight by late in the weekend and into next week as a cooler, more seasonable pattern returns to the region.
The Southwest has also been warmer than average as of late but will now also have to endure near-daily thunderstorm activity that could prove dangerous for outdoor activities. This weather pattern, which will last into next week, typically occurs later in the summer when the North American monsoon is underway.
People sunbathe and paddleboard at Lake Union Park, Saturday, May 13, 2023, in Seattle. Saturday's temperatures reached record-breaking highs for several cities across western Washington, with a heat advisory in effect until Monday evening. (AP Photo/Lindsey Wasson)
Northwest continues to sizzle, but relief is on the way
For the better part of the last week, a ridge of high pressure across the West has helped deliver an unusual May heat wave to the Pacific Northwest and adjacent parts of Canada, and that pattern will persist for a few more days, according to AccuWeather forecasters.
"Temperatures away from the coast along the I-5 corridor in the Northwest will remain well above historical averages through Saturday," said AccuWeather Senior Meteorologist Heather Zehr.
In Portland, there were three consecutive days with record high temperatures from May 12–14, and for the first time on record in the month of May, there were four consecutive days where the mercury reached 90 or higher, breaking the old record of three consecutive days from May 6–8, 1987. The high temperature on Thursday and Friday reached 84 and 83 degrees, respectively, and similar warmth is expected on Saturday.
Meanwhile, in Seattle, a typical May features only two days with high temperatures of 80 degrees or higher. So far this month, there have been five such days, including four consecutive days that set records from May 12–15. The mercury is forecast to come close to the 80s through Saturday before cooler conditions arrive, according to AccuWeather forecasters.
"Cooler air will arrive in the region as a large storm over the North Pacific slides closer to the West Coast," added Zehr. "A piece of that will spin into the Northwest by Sunday, causing a substantial cooldown for western Washington and Oregon that will last into the start of next week."
The historical average high temperature for mid- to late May in Portland is just 70 degrees while Seattle's is just in the mid- to upper 60s.
A cooldown will be delayed for interior parts of the Northwest and into the northern Rockies, where temperatures will challenge records that are 10 to 20 degrees above the historical average and last into Sunday or Monday. This includes the cities of Billings, Boise and Salt Lake City.
Since the Northwest typically stays cooler year-round compared to other parts of the country, fewer homes have air conditioning units. According to the University of California at Berkeley's Haas School of Business, only 53 percent of households in Washington state are equipped with air conditioners, the lowest rate of any state in the lower 48.
Cooler air will make significant progress across the Northwest on Monday and Tuesday. Tempeartures will be slashed by 20-30 F in many locations when compared to the peak of the heat this past week and this weekend.
In addition to the heat in the Northwest, there have been large amounts of smoke in the upper atmosphere from massive wildfires in western Canada. The haze caused by the smoke has been enhancing sunrises and sunsets across the region.
A smoky sunrise over Seattle, Washington. May 18, 2023.
Storms in Southwest will pose a risk for outdoor enthusiasts
While the typical monsoon season is still months away, residents in the Southwest are getting an early taste of it this week, as showers and thunderstorms have been erupting on a daily basis, and will continue to do so into next week.
Heavy rain has already made a stunning appearance in some Southwest cities. While Las Vegas's historical average rainfall is only 0.04 inches in the month of may, roughly a tenth of an inch fell in Sin City on Friday. Nearby in Flagstaff, Arizona, 0.82 inches of rain fell, which is nearly double the historical average for the month.
"A slow-moving area of low pressure hovering over northwestern Mexico will pull moisture into the Southwest, all the way to the Four Corners region," said Zehr. "This is leading to daily shower and thunderstorm activity all across the Southwest."
"While areas from western Texas to eastern Arizona and northward into Wyoming are in the heart of this rainfall, isolated activity can occur as far west as the Sierra Nevada, as well as the mountains of Southern California," added Zehr.
In Phoenix, 100-degree heat arrived almost right on schedule, with the first triple-digit day of the season coming back on April 30, compared to the average first date of May 2. Recently, there has been a string of 100-degree days, but temperatures are expected to drop slightly into the 90s, as cloudiness and the chance of thunderstorms expand late this week.
As outdoor enthusiasts return to the region's parks and monuments in the warmer months, the thunderstorms, which can often pop up quickly and with little warning amid daytime heating, will pose hazards to those who can't quickly seek shelter. Those threats include large hail, sudden wind gusts that can whip up dirt and dust, lightning and flash flooding from sudden, intense downpours.
On Wednesday, there were reports of 1.5-inch-diameter hail (the size of ping pong balls) in the mountains near Flagstaff and Prescott, Arizona.
Historically, May is the second driest month of the year in Phoenix and in general, across the Desert Southwest, trailing only June. This is often due to a sprawling area of high pressure that sets up shop across the Southwest from the spring into the early summer, suppressing cloudiness and rain chances. The reverse is then true from midsummer through early fall, during the monsoon season, when low pressure and storminess dominate.
AccuWeather's team of long-range forecasters, led by Senior Meteorologist Paul Pastelok, accurately predicted this early spell of shower and thunderstorm activity in the region in AccuWeather's spring outlook. Despite the early activity, the traditional monsoon season is forecast to get underway later this summer, according to AccuWeather's summer forecast.
"In the last two years, the monsoon started in mid-June," said Pastelok. "This year, we expect a more typical, later start around the first week or two of July in southeastern Arizona, before it spreads out to the rest of the region."
AccuWeather is also expecting this year's monsoon to be less potent compared to recent years. "With the loss of La Niña conditions, this year's season is expected to feature less frequent and less intense bouts of moisture compared to 2021 and 2022," said AccuWeather Meteorologist Brandon Buckingham.
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