Tropical Storm Debby intensifies as most of Florida under emergency orders

Storm expected to develop into hurricane before reaching Gulf coast as US agency warns of devastating flooding

Tropical Storm Debby strengthened rapidly Sunday and is expected to develop into a hurricane before making landfall on Florida's Gulf coast, the US National Hurricane Center (NHC) said on Sunday, warning of life-threatening ocean surges and devastating flooding.

The hurricane center forecasted life-threatening conditions, including storm surges up to 7ft (2 meters). As it slowly moves north through the week, the storm may bring potentially historic rainfall" of between 10 and 20in (25-50cm) and catastrophic flooding to Georgia and South Carolina, it said.