The Oregon coast operates on its own climate logic. Even at the height of summer, the coast is 10–15°F cooler than Portland and almost always windier. July and August are the driest months — not reliably sunny, but the marine layer typically burns off by midday and the evenings are mild enough to sit outside. Bring layers regardless of the season; the wind off the Pacific can cut through a t-shirt even when it looks warm.
Spring and fall are the locals' preferred seasons. The crowds thin dramatically after Labor Day and before Memorial Day, prices drop, and the coast takes on a different character — wilder, quieter, the beach returned to the beachcombers and the surfers. Whale migration peaks in March-April (northbound) and December-January (southbound), visible from the beach and headlands with binoculars.
Avoid major summer holiday weekends — Fourth of July and Labor Day in particular — unless you enjoy traffic and booked-solid restaurants. The town gets genuinely crowded: parking fills by mid-morning, the Prom becomes a pedestrian traffic jam, and the drive home can take twice as long as the drive out. Mid-week summer visits are a much better proposition.