Torrential rain battered central Greece, flooding streets, homes and businesses in the port city of Volos and on the island of Evia, just three weeks after devastating Storm Daniel killed 16 people in the wider region.
A private helicopter crashed into the sea in northern Evia, east of the Greek capital Athens, and a search and rescue operation was underway.
It was not immediately clear what caused the accident.
More than 250 people have been evacuated from the area since Storm Elias struck on Wednesday afternoon, the fire brigade said on Thursday, adding that it had so far received 1,200 calls for help.
The storm hit Volos with rain so heavy that water levels in the city and its suburbs rose rapidly in a few hours. A nearby stream overflowed, adding to the flooding.
The mayor of Volos said that by Wednesday night, power outages caused by the storm and flooding had plunged 80% of the city into darkness. Authorities have stopped all vehicles from going onto the roads.
On Thursday morning, the storm had moved towards the island of Evia were some villages in northern Evia have been ordered to evacuate, state ERT TV said.
Storm Elias is the second major storm to hit the region since Daniel, the most intense storm to hit Greece since records began in 1930, battered the region for three days earlier in September.