Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
This specimen (and a few others I collected but didn't photograph) appear to represent the first finding of Paraleptophlebia sculleni outside the Oregon Cascades, although it is not a monumental leap from there to the Washington Cascades. The key characteristics are fairly clear.
This mayfly was collected from Mystery Creek #249 in Washington on July 25th, 2019 and added to Troutnut.com by Troutnut on July 26th, 2019.