That I could not tell you. The closest to dormant turf I deal with is grass that has not had a full day's sun for over a week and temperatures below 80 in the day. I would be cautious because herbicide tolerance is based on grass growing out of any injury it might receive from an application. What kind of weeds are you targeting that are still around while the St Augustine is trying to go to sleep?.
They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety.
Benjamin Franklin 1775
Sell not virtue to purchase wealth, nor Liberty to purchase power.
Benjamin Franklin Poor Richard's Almanac1738