I grew up fishing searuns on the north end of Whidbey Island and we started seeing the fish in good numbers on the eastside beaches starting in May and continuing through to the fall. Any baitfish pattern worked well off structure and rip lines. At times they'd hit a fly on the strip other times during a pause. We'd usually concentrate on a high slack tide period in the evenings. Also, check out the area you will fish at low tide and pay attention to the bottom terrain. Any dip, deviation or depression of the beach can be a good place to find cutts when the water advances again. I'm sure the mouth of the Nooksack or any of the protected beaches in your area would be similar.
I live on the SW WA coast now and encounter cutts in good numbers in the streams when swinging flys for steelhead in the spring. In the fall they are still in the rivers (can't remember if they migrate in to spawn in spring or fall). Grey Hackle Yellows, Borden Specials, Spruce Flys, and a host of other attractor patterns fished subsurface will work. Concentate on "frog water". They love to hang in slow water (stuff that most normal trout fisherman would pass up as dead water). Look for downed trees, stumps, old car bodies, Anything that offers cover and creates a slack water spot. Lots of time your fly will get hammered as soon as it hits. If not I usually make 6-8 inch steady strips to entice action. Yellow seems to be a key color when they're in the streams (have great luck with yellow wooley bugger palmered with grizzly hackle). Check out Steve Probasco's Northwest Fly Patterns book for about a dozen good recipies.
Hope I haven't rambled on too much about stuff you probablly allready know about. I love fishing for these critters. However they've become very scarce in the past few years due to ????. Take care with every one to ensure their survival. Good Luck.
Oh yeah, check with the fly shop at the east end of the Riverside bridge in Mt. Vernon. I'm sure he's dialed into the Skagit/Whatcom cutt scene.