The greater Tokyo area will be seeing partly cloudy skies and scattered showers during the week. Daytime temperatures will be humid and mainly in the low 30s°C (upper 80s°F). Evening lows will be warm, in the mid-low 20s°C.

What clothes to pack for Tokyo in September?
・Light clothes for summer; short-sleeved shirts; jeans or light slacks; comfortable sneakers or walking shoes are recommended.

Japan nationwide
Looking at the weather forecast for the rest of Japan, most areas will be seeing partly cloudy skies and hot temperatures.

Northern Japan
Hokkaido and the north country are expected to see partly cloudy skies and rain this week. Daytime highs will be in the mid-upper 20s. Evening temperatures will be in the upper teens.

Central Japan
The greater Osaka and Kyoto area is expected to see cloudy skies with showers toward the end of the week. Temperatures will be hot, with highs in the low 30s°C and lows in the low 20s°C.

Okinawa
As for Okinawa, expect partly cloudy skies for much of the week in Naha. Daily highs are expected to be in the lower 30s°C, with lows in the mid 20s.

Details on storms can be found on the Japan Meteorological Agency website.

Check the links below for more on upcoming fun events and festivals!

・Latest Typhoons/Tropical Cyclone Information: http://www.jma.go.jp/en/typh/
・Latest Earthquake Information: http://www.jma.go.jp/en/quake/

Source for forecast and images: Japan Meteorological Agency website / http://www.jma.go.jp/jma/indexe.html