UPDATE 1-Hot summer in store for Japan as power usage eyed

* Eastern Japan, including Tokyo, to have hotter to normal weather

* Western Japan likely to see higher chance of hot weather in July

* Air-conditioning usage eyed amid post-quake power supply problems (Adds details, table)

TOKYO, June 23 (Reuters) - Japan will see mostly average to hotter-than-average weather from July to September, the Japan Meteorological Agency said on Thursday, which could push up air-conditioning usage and exacerbate power supply problems in the wake of the earthquake in March.

Eastern Japan, which is partly served by quake-hit Tokyo Electric Power Co , will have a 40 percent chance of average or higher-than-average temperatures for the period, the weather agency said in its latest three-month forecast, which is issued monthly.

Higher air conditioning usage would increase electricity demand, which could strain the power grid would after the quake and tsunami knocked out nuclear and thermal plants operated by Tokyo Electric and Tohoku Electric Power Co .

The agency upgraded chances for hotter weather in July in western Japan to 50 percent from 40 percent last month, which could adversely affect the tight power supply situation at Kansai Electric Power Co .

Western Japan's Kansai, which is more heavily reliant on nuclear power than most of Japan's regional utilities, has asked its customers to cut power use by 15 percent during peak hours this summer to avoid rolling blackouts, with no timetable for the restart of nuclear units amid worries over safety.

The following figures show the percentage probability of temperatures in the coming months being below average, average or above average (previous forecasts in parentheses):

Forecasts for Northern Japan

BELOW AVERAGE ABOVE

July-Sept 30 40 30

July 30 (30) 40 (40) 30 (30)

Aug 30 (40) 40 (40) 30 (20)

Sept 20 40 40

Forecast for Eastern Japan

BELOW AVERAGE ABOVE

July-Sept 20 40 40

July 20 (20) 40 (40) 40 (40)

Aug 30 (30) 40 (40) 30 (30)

Sept 20 40 40

Forecast for Western Japan

BELOW AVERAGE ABOVE

July-Sept 20 40 40

July 20 (20) 30 (40) 50 (40)

Aug 30 (30) 40 (40) 30 (30)

Sept 20 40 40

Forecast for Okinawa, Amami Islands

BELOW AVERAGE ABOVE

July-Sept 30 40 30

July 30 (20) 40 (40) 30 (40)

Aug 30 (20) 40 (40) 30 (40)

Sept 30 40 30 (Reporting by Osamu Tsukimori; Editing by Chris Gallagher)