The surf was starting to rise along the North Shore and Leeward Coast last night as weather forecasters predicted waves as high as 30 feet overnight and into this morning.

Along the outer reefs, the National Weather Service said waves could reach up to 40 feet.

The city's Department of Emergency Management, formerly O'ahu Civil Defense, will monitor conditions overnight and might encourage city parks and recreation officials this morning to keep beach parks such as Waimea Bay closed to the public.

"If we have bad surf coming in, we'll make the call to parks and rec and say, 'It doesn't look like a safe condition,' " said John Cummings, spokesman for the Department of Emergency Management.

Lifeguards and other officials cannot keep people out of dangerous surf.

"But we may not open the park by keeping the gate locked," he said. "And we can highly suggest that you may die and drown. Most folks heed the warning."

Yesterday morning's choppy surf along the North Shore turned into clean waves of 8 to 10 feet, with occasional 12-footers by the afternoon.

North Shore lifeguards used a personal watercraft to rescue one surfer outside Laniakea at a surf break called Himalayas, said Bryan Cheplic, spokesman for the Honolulu Emergency Services Department.

East shores were expected to have surf today of only 3 to 5 feet with a south swell of 3 to 6 feet.

The rising waves were the result of a large west-northwest swell that the weather service said could wrap around O'ahu.

The possibility of even bigger surf for today came amid a strange weather day for O'ahu yesterday, which began with heavy rains that gave way to clear, muggy skies that were expected to transform into heavy nighttime showers.

The weather service issued flash-flood watches for O'ahu, Kaua'i, Lana'i and Ni'ihau yesterday, which had Department of Emergency Management officials on alert for any problems overnight and possibly this morning.

"We did get a heads-up to expect severe weather, flooding and rains," Cummings said yesterday. "The weather looks good right now, but we're on alert for activation overnight. We've alerted the mayor's deputies and our first responders. At this point, we can only see what happens."

Reach Dan Nakaso at dnakaso@honoluluadvertiser.com.