Hokkaido seasonal overview
Best Surf Season (September - November)
If you time it right, autumn is the crown jewel of the Hokkaido surf calendar. The summer monsoon is retreating, the first Northern Hemisphere frontal systems are organising, and typhoon remnants still drift out of the western Pacific. This is the window where the SSW-facing beach sees its best ratio of S/SSW/SW swell to N/NNW offshore wind. September leads the way with 19% ideal wind and an average wave period near 8.6s; October and November keep that momentum as lows deepen over the Sea of Japan and the North Pacific. Waves are rarely huge, generally 2-4ft (0.6-1.2m), but they are more organised and much cleaner than anything summer has to offer.
Fair Surf Season (December - April)
Once the Siberian High takes control, the northwest monsoon dominates. That means plenty of N/NNW offshore wind for the SSW-facing coast, but often too much of a good thing: strong, gusty flow and short-period waves in the 6.6-7.9s range. Swell from WSW/W and S/SW mixes through the winter, and the average heights hover around 2-3ft (0.6-0.9m). December and January are especially cold, with water temperatures down to 3.5-8.7°C. February through April grow more promising as SSE/S swell begins to fill in, but onshore wind also returns. This is a season for the committed, not the casual.
Low Surf Season (May - August)
The summer monsoon is the enemy of Hokkaido's surf. From May through August the wind rose tips hard into the S/SSE quadrant, which blows directly onshore, and the swell is mostly short-fetch SSE/SE windswell in the 2-3ft (0.5-1m) range. June and July are the toughest, with ideal wind at only 10-11% and onshore winds over half of the time. August offers a glimmer of hope as typhoon season raises the average period to 8.8s, but it is still a marginal, wind-affected month. If you are planning a trip, save it for autumn; if you are already there in summer, chase every early-morning NW wind lull you can find.