Cool, showery end to the week in the Northwest; heat builds across the West early next week

A welcome round of showers is coming to the Pacific Northwest to end the week, and then nearly everyone in the West will get in on a significant warmup early next week.

The potent, slow-moving trough will begin to move into the Northwest on Thursday, although spotty showers will begin to arrive in Washington and Oregon as early as late Wednesday or Wednesday night.

The rounds of showers will be most persistent across western Washington and Oregon from Thursday through Friday, with some lingering showers into Saturday.

Rainfall amounts will mainly be under a half inch across this area, which is welcome news since much of western Washington is in a moderate drought condition.

There already is a sizable wildfire burning in Grant County, which is in central Washington. The fire, known as the “243 Fire,” has burned over 18,000 acres as of late Wednesday morning and prompting evacuations in the town of Royal City.

Unfortunately, while the cool weather may help firefighters, very little rain will make it over the Cascades to where the fire is, with rainfall amounts mainly under 0.10 of an inch. Even so, there could be some thunderstorms that try to develop east of the Cascades on Thursday and especially Friday. While these can bring rain, the lightning can also start new fires.

Humidity levels are not expected to rise much, and strong, gusty winds over 30 mph will continue Thursday and Friday, which may make the fire difficult to contain.

Another aspect of the trough coming through will be the lower temperatures. Temperatures will struggle to get much past 60 degrees in Seattle, which only happened a couple of times in May.

The air mass will be cold enough for snow levels to drop to around 5,000 feet. There can be a small accumulation of snow, mainly above 6,000 feet through Saturday.

As this trough slides eastward, a broad ridge in the jet stream will develop late this weekend into early next week, which will send temperatures above average in most of the West.

Here’s a look at some of the forecast highs in the West from the GFS for next Tuesday. Even after the cool stretch in the near-term, Seattle will see temperatures in the 80s, and Portland may hit 90 for the first time this year Tuesday or Wednesday.

In the Southwest, after a brief cooldown late this week, temperatures will surge back above 100 degrees in the Sacramento and San Joaquin Valleys, as well as the deserts. Phoenix may reach 110 for the first time this year. Temperatures may even get into the 80s in San Francisco. Coastal Southern California, though, shouldn’t stray too far from average.

This is certainly a huge change from what was largely a much cooler-than-average month of May across the Southwest. Many areas were 3 to 6 degrees below average for the month, which is pretty significant.

Once we get past Wednesday of next week, the ridge will flatten and a broad trough is expected to develop through next weekend. This won’t have a huge impact on the weather, although it will likely lead to pop-up thunderstorms in some of the higher terrain. Again, any lightning will have the potential to start new wildfires, especially in the Northwest where lightning is the leading cause of wildfires.

What this will do, though, is send temperatures back closer to average levels by late next week.

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