Through the start of Saturday, rain showers will continue to move through Western Washington as the upper-level trough responsible for our unsettled weather slowly pinwheels across the Pacific Northwest and southern British Columbia.
The atmosphere will remain somewhat unstable through the day, allowing showers to develop and drift across the region, especially during the morning and early afternoon hours.
By late Saturday, shower activity should gradually taper west of the Cascades as the trough shifts farther east.
A few lingering showers could persist into Saturday evening across portions of north-central Washington, but most of Western Washington should see improving conditions heading into the overnight hours.
Rainfall totals from Friday evening through Saturday will vary widely from one neighborhood to the next. Most locations will receive less than a tenth of an inch of rainfall, and some communities may see little or no measurable rain at all.
However, areas that spend time beneath a Puget Sound Convergence Zone could pick up considerably more. A few isolated spots may receive over a half-inch of rain, though amounts that high are expected to be the exception rather than the rule.
Temperatures will also remain on the cool side for late August. Seattle’s high temperature on Saturday is expected to top out near 70 degrees, several degrees below the seasonal average of 76 degrees. Recent weekends have had a tendency to come in cooler than average, and this one appears poised to continue that trend.
Sunday should bring a modest improvement. Morning clouds will likely give way to increasing sunshine during the afternoon, allowing temperatures to recover a few degrees. While it won’t be a particularly warm day, it should feel more pleasant than Saturday, especially away from the water.
Looking ahead, the start of next week appears mild and dry, with a return to more typical late-summer weather. However, forecast models continue to suggest another Pacific weather system may arrive around Wednesday and Thursday.
That system could tap into some lingering tropical moisture associated with the remnants of former Hurricane Lala, which produced significant flooding in Hawaii nearly two weeks ago.
While it may sound unusual, this is actually a fairly common process in the Pacific. Tropical cyclones occasionally move northward and weaken over cooler waters, but their moisture can survive long enough to become entrained into the Pacific jet stream.
Once absorbed into the larger-scale weather pattern, that moisture can be transported thousands of miles downstream and ultimately contribute to rainfall events across the Pacific Northwest.
As a result, September could begin on a wetter note than many people might expect after what has been a relatively dry summer around Puget Sound. While temperatures have generally averaged near normal during July and August, rainfall has been harder to come by across much of Western Washington.
In fact, the longer-range forecast continues to favor a pattern that is somewhat more active than normal for the opening weeks of the month. Climate guidance points toward an increased likelihood of above-average precipitation through at least early to mid-September.
That doesn’t necessarily mean it will rain every day, but it does suggest the Pacific storm track may become better established as we begin the transition from summer toward autumn.
For those hoping to squeeze in a few more outdoor plans before the season changes, Sunday’s brighter weather and the dry start to next week may provide one of the better opportunities before the next round of rain arrives.
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