Just a Few More Days of Unseasonable Warmth for the Northwest

Posted: May 18, 2023 3:16 pm

Temperatures hovering above the seasonable average will hang around in the West through the early part of next week. However, the record-breaking temperatures are likely in the rearview mirror. This weather pattern will provide the necessary ingredients for storms to ignite unusually early in the Southwest, making it feel more like summer for much of the West Coast. Here is the latest on the forecast for the western third of the U.S.

What Has Been Happening in the West This Week

The heat wave that began late last week is still lingering for parts of the Pacific Northwest. This heat brought down several daily temperature records in cities such as Portland and Seattle. While the mercury has let up a bit in recent days, the temperatures will remain well above the historical average for this time of the year until cooler weather moves at the end of the weekend and the early part of the work week.

The Desert Southwest has also been grappling with hot conditions this past week. This heat has ushered in daily thunderstorm risks more reminiscent of the late summer North American monsoon season.

What Caused the Heat to Take Hold in the Northwest?

The heat in the Northwest has been the result of a large mass of high pressure that set up over the region. It will be a few more days before the high pressure system relents, keeping residents along the Interstate 5 corridor toasty through Saturday.

What has been most notable about this current weather pattern is that the heat has held on for a longer than usual stretch. For instance, the month of May in Seattle usually brings only two days with readings that break the 80-degree mark. The Emerald City has already recorded five days at this level with the forecast indicating that the mercury will approach or eclipse 80 degrees through Saturday.

In addition to the heat, the smoke coming from the wildfires in western Canada has filtered down into the Northwest over the last week. This haze has created stunning sunrises and sunsets for the region, harkening to the dog days of summer when wildfire smoke is a common sight.

Although temperatures in the 80s do not seem abnormal for May for most people in the U.S., the historical average high for mid to late May in Seattle hovers in the mid to upper 60s. Portland typically reaches a consistent 70 degrees by this time of the year, however, readings have been well into the 80s and 90s for the past week.

That is all about to change in the coming days when a storm currently located in the North Pacific Ocean will inch closer to the West Coast, sending its energy and moisture into the Northwest on Sunday. This arrival will deliver significantly cooler temperatures for western portions of Washington and Oregon by the time Monday rolls around.

It will take a bit longer for the cooler air mass to reach the interior portions of the region. For instance, the mercury will continue to land well above average into Monday for cities such as Boise and Salt Lake City.

Storms Fire Up in the Southwest

It also feels more like summer in the Southwest thanks to the arrival of daily thunderstorms and rain showers. The typical North American monsoon season does not get going until later in the summer, sending residents checking their calendars.

The thunderstorm activity is happening as a low pressure system anchored over the northwestern corner of Mexico is bringing moisture up into the Southwest and the Four Corners area. The presence of this moisture-rich air is combining with the above average temperatures to create thunderstorms during the afternoon and evening hours, marking a pattern similar to the monsoon season.

The expansive rainfall zone stretches as far west and north as the Sierra Nevada and Wyoming, down into the higher terrains of Southern California, and as far east as western Texas.

Phoenix will likely see a break from the triple digit temperatures as more clouds build later this week. However, the cooler temperatures will come with a higher chance of rain, threatening outdoor plans. Local officials warn that these random thunderstorms can seemingly come out of nowhere with little warning.

Wednesday was a stormy day for the mountain areas of Arizona. For instance, Flagstaff reported hail the size of ping pong balls.

The severity of this moisture is unusual for the Southwest. May is typically the second driest month of the year in Phoenix and throughout the region. The weather pattern during May and June generally features a massive area of high pressure that does not let up until the monsoon season arrives and low pressure reigns supreme.

Despite the prevalent rain and thunderstorms this past week in the Southwest, forecasters are still predicting a slightly later start to the monsoon season. This consistent threat of moisture should arrive in the early parts of July in the southeastern corner of Arizona before expanding elsewhere. The long-range forecast is also calling for a less severe monsoon season thanks to the exit of the La Niña climate phase.

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