MIAMI, Florida (CNN) -- A tropical storm watch was issued Friday for Bermuda as Hurricane Bertha moved closer to the Atlantic island, causing swells and high turf on its beaches.
A satellite picture from 5:45 a.m. ET Friday shows Hurricane Bertha over the Atlantic.
As of 8 p.m. ET Friday, the center of Bertha was about 250 miles (402 km) southeast of Bermuda. The Category 1 storm was moving north-northwest at near 5 mph (8 km/h).
The National Hurricane Center said a gradual turn toward the north is expected Friday night and Saturday, followed by a continued slow motion toward the north or north-northeast on Sunday.
"On this track, the center of Bertha is expected to slowly pass to the southeast and east of Bermuda during the next couple of days," the agency said in an advisory.
Bertha's maximum sustained winds are at near 90 mph (145 km/hr), with higher gusts, the hurricane center said. 
See Bertha's path »
The Bermuda Weather Service issued a tropical storm watch for the island around midday, meaning tropical storm conditions are possible in the area, the hurricane center said.
The center advised those on the island, a self-governing British colony, to monitor Bertha's progress closely.
Bertha's intensity has fluctuated. At its peak Monday, it was a major Category 3 hurricane with top winds of 120 mph (193 km/h).
Its wind speed dropped to 75 mph (121 km/h), barely hurricane strength, before picking up once again and reaching Category 2 intensity late Wednesday, with top sustained winds of 105 mph (169 km/h). 
See how hurricanes are classified »
The storm formed July 3 off Africa near the southern Cape Verde Islands. It strengthened into a hurricane Monday.
The first tropical storm of the season, Arthur, formed May 31 near the coast of Belize and dumped heavy rain on Central America and southern Mexico.
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