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No one can predict what it will be but last August St.Petersburgh average was 75 degrees farenheit and the highest was 81 degrees, so if the same holds this year looks good, but remember Michael Fish "What hurricane" .
I'd expect the weather you'd expect in the Uk and prepare accordingly. That meexpect anything, hope for the best (and you may be lucky) but also prepare for chillier or wet days too.
Went on this cruise last year at about same time.Varied between very warm to very hot.you wont need coats.