Blue-Winged Olives — Baetis to entomologists, BWOs to fly shops — are the most reliable mayfly hatch on Western trout streams. Sizes 16–22. The hatch fires hardest on overcast, drizzly days; bluebird sun flattens it.
If you see trout sipping in the surface film between 2 PM and 4 PM and you're not sure why, it's almost always a BWO emergence. Match with a parachute or sparkle dun in olive, size 18 by default.