Arthur has strengthened to a hurricane in the Atlantic, where it threatens to deliver the Carolinas a glancing blow on Independence Day.
The hurricane's maximum sustained winds early Thursday are near 75 mph with some additional strengthening expected.
Hurricane warnings on the coast of North Carolina have been extended and now cover an area from Surf City to the Virginia border.
The hurricane orgininated in a cluster of thunderstorms to the east of Florida and developed into the first Tropical Storm of the Atlantic Hurricane Season.
The system was upgraded to a Tropical Storm on Tuesday, and named Arthur. Although other storms have formed to the west of Mexico, this is the first one this year to form in the Atlantic.
Tropical Storm Arthur became Hurricane Arthur early Thursday.
Warm seawaters are the energy source of a tropical system. Great amounts of energy are transferred when warm water is evaporated from tropical seas. As humid air rises, energy is released when the water vapor condenses, producing the towering cumulus clouds and rain seen within a tropical storm.
The waters to the east of Florida are currently 1C above average, and this gave extra energy to the system and allowed Arthur to strengthen.
The winds will also help the system become stronger. They are fairly similar throughout the atmosphere, something meteorologists call low 'wind sheer'. This will ensure that the system isn't disrupted as it grows vertically throughout the atmosphere.
Arthur is expected to run up the East Coast of the USA over the next few days.
It has already produced heavy rain in Florida, with Miami reporting 98mm of rain on Sunday and Monday.
Today the heaviest rain it is expected to be in eastern Florida and the Bahamas, where up to 150mm of rain could be seen in some locations.
The system will strengthen further and head towards the coast of North Carolina, reaching its closest point on Friday.
This is an unfortunate time for the storm to affect the East Coast, coinciding with Fourth of July weekend when many hope for clear blue skies to accompany the barbecues. Many of the hotels in the region are fully booked for the period.
Officials in North Carolina's Outer Banks said they would close Cape Lookout National Seashore on Wednesday evening and reopen when it's safe. There is a mandatory evacuation for visitors to the Outer Banks' Hatteras Island as of 5 a.m. Thursday. Residents also are advised to leave the island.