A flood warning, external means that flooding is expected so you should act now. The Environment Agency suggests:
In Dorset, several flood alerts are in place including Christchurch Harbour due to high tides and also Preston Beach at Weymouth, Swanage Bay, Studland Bay, Sandbanks, Poole Harbour, Bournemouth Beach, Hengistbury Head.
Flood alerts on Dorset’s west coast cover Lyme Regis, Charmouth, Seatown, West Bay, Bothenhampton, Burton Freshwater, Burton Bradstock, Cogden Beach, Chesil Beach, West Fleet, East Fleet.
The Met Office has issued a yellow weather warning for the south coast including Dorset, The Isle of Wight and Hampshire until 17:00 BST.
An amber weather warning for strong winds is in place for Cornwall and parts of Devon.