A tropical storm is threatening to disrupt travel plans for millions of people in Japan observing a traditional summer holiday.

The weather system is currently moving west-northwest across the Pacific Ocean south of Japan.

Severe tropical storm Krosa is expected to hit western Japan later this week.

Strong winds and rough seas with waves of up to six meters are forecast. Heavy rain is beginning on Monday with up to 150 millimeters of rain expected to fall in Japan's Tokai region by Tuesday morning. During the same period 100 millimeters is predicted for the Kansai region.

Krosa is forecast to bring more rain mid-week when it advances across the western part of the country.

This week is a peak travel time in Japan with many people visiting their hometowns for what is called the Bon Festival.