Long Beach has a mild Mediterranean climate with very little rain and moderate temperatures year-round. Summer (June through September) is warm and mostly sunny, though a marine layer of morning fog is common in June and early July — what locals call "June Gloom" — which burns off by midday. Temperatures rarely exceed 30°C at the coast.
The best time to visit for outdoor activity and beach use is late August through October, when the fog has cleared and the water temperature peaks. Spring (March through May) is also excellent.
The Grand Prix of Long Beach in April is the city's major annual event and draws large crowds to the downtown waterfront. It is worth planning around if you are interested in motorsport, but it affects accommodation availability and pricing for that weekend.
Winter in Long Beach is mild (average highs of 18–19°C) and almost entirely dry. It is a perfectly reasonable time to visit if you are not counting on beach weather.