"The effect of the storm warning is extended for a day on August 13 and 14, as well as on ... August 15," Izvestia reports, citing the press service of the Sochi administration.
Powerful showers with thunderstorms, hail and squally increase of wind up to 27 m / s will pass over the city.
The water level may rise with the risk of small mudflows.
The authorities also warned of possible tornadoes over the sea in the Magri - Vesyoloye area.
Emergencies Ministry officials urge citizens to give up resting by the water for a while.
All resort services are on high alert.
Earlier, a storm warning was announced in Crimea on August 13 and 14 with a squally wind and heavy rains with hail.