People on the east coast of England are being warned to be very careful because really bad flooding may be on the way.
The government's environment agency has said there is "extreme danger to life and property" because of huge waves now forming in the North Sea.
Waves three metres higher than normal are expected, and the problem could last for two days.
A weather expert said waves like these - which will be worst on Friday morning - are only seen every 20 years.
The Environment Secretary Hilary Benn told other MPs that police will be on standby in areas where flooding is expected.
He added the flood defences may not be enough in on the coast and in tidal rivers, especially in East Anglia.
"Major" tidal surge
The Norfolk Broads and the coast south of Great Yarmouth could be badly hit, and the problems could reach as far as Kent.
An Environment Agency spokesman said the "major" tidal surge had been caused by gale force winds in Scotland making their way down the coast of England, combined with a high tide.