Typhoon Ma-on weakened to a tropical storm as it moved on a course that will take it over central Japan today, passing south of Tokyo before heading out to sea, the U.S. Navy Joint Typhoon Warning Center said.

The storm was about about 340 miles southwest of Tokyo at 12:50 a.m. local time today, the Japan Meteorological Agency said on its website. Ma-on was moving north-northeast at 10 kilometers per hour with winds blowing at 139 kilometers per hour, the agency said.

The storm may reach Wakayama prefecture in central Japan this morning and be west of Hachijo island, 290 kilometers south of Tokyo by midnight before continuing out to sea, the agency said.

Hawaiian Airlines canceled its flight to Osaka Monday, but today’s flight should depart on schedule, a spokesman said.

Japan Airlines Co. canceled 93 domestic and four international flights, while All Nippon Airways Co. halted 218 flights in Japan, the carriers said in faxed statements Monday.

The Japanese weather agency issued warnings for heavy rain and flooding from Okinawa in the southwest to Fukushima prefecture north of Tokyo, where the damaged plant is located.

An earlier forecast indicated the storm may pass over Tokyo Electric Power Co.’s crippled Fukushima nuclear plant by July 21. Japanese forecasters maintained warnings for heavy rains and landslides for Tokyo and Fukushima 220 kilometers to the north, it said on the website.

JX Nippon Oil & Energy Corp. halted the berthing of vessels at five refineries in eastern and western Japan, the company said yesterday. Idemitsu Kosan Co. halted shipments at two refineries because of the storm, chemicals news service ICIS reported.

Tokyo Electric Power Co., the owner of the Fukushima plant, is rushing to install covers to protect damaged reactor buildings. In 2004, eight cyclones passed over or skirted Japan’s Tohoku region, where the Dai-Ichi station is spewing radiation after an earthquake and tsunami on March 11 triggered the worst atomic accident since Chernobyl 25 years ago.

The quake and tsunami wrecked the plant’s cooling systems and caused three reactors to melt down. The roofs of three buildings were damaged in blasts as water inside reactor cores and spent-fuel ponds boiled away. The utility known as Tepco said in April it plans to install temporary covers within nine months and concrete ceilings over the “medium term.”

Work on a cover for the turbine building of the No. 3 reactor started Sunday, Junichi Matsumoto, a general manager at Tepco, said at a briefing in Tokyo. The cover is needed because the building’s basements and nearby trenches are full of highly radiated water that may overflow in heavy rain, Tepco spokesman Satoshi Watanabe said.