As far as I know there is not a specific threshold RC's are bound to, though I think the informal guidance is to be aligned with AYC's weather guidance. Excerpt below, but ultimately it is the RC's call and then each skipper's decision to race or not race.
Examples of weather conditions that could potentially cause us to abandon racing including the following:
• A forecast of sustained winds above 20 knots between 1700 and 2000
• A forecast warning of severe thunderstorms between 1700 and 2000
• Observed sustained wind above 20 knots after 1700
• Observed thunderstorms conditions after 1700
If any of these conditions are met, the race committee will probably abandon racing for that evening