* WHAT...Increased threat of sneaker waves expected. * WHERE...Beaches in Del Norte, Humboldt, and Mendocino counties that are exposed to the northwest. * WHEN...From Wednesday afternoon through Thursday evening. . * IMPACTS...Large, unexpected waves can sweep across the beach without warning, sweeping people into the sea from rocks, jetties, and beaches. These sneaker waves can also move large objects such as logs, crushing anyone caught underneath. * ADDITIONAL DETAILS...An unusually long period swell for May will begin building Wednesday afternoon at 23 seconds and peak Thursday around 7 feet at 16 seconds. Sneaker wave risk will be greatest early Thursday on beaches with mostly calm winds and exposed to the northwest.