Japanese weather officials say atmospheric conditions are extremely unstable in the Hokuriku Region on the Sea of Japan, as well as in western Japan.

They are warning that lightning strikes, tornadoes, strong gusts of wind, hail and sudden downpours may hit those areas.

The Meteorological Agency says the unstable atmospheric conditions have been caused by a cold air mass flowing into the upper atmosphere.

An even colder air mass is expected to flow in, prolonging extremely unstable conditions well into Monday.

Officials are advising people to take shelter in sturdy buildings, if the sky suddenly turns dark or cold winds start to blow, as these may be signs that thunderheads are approaching.

Temperatures marked the lowest for this season in northern Japan and other regions on Sunday morning.

The lows through Sunday morning dipped to minus 1.5 degrees Celsius in the city of Asahikawa and minus 0.3 degrees in Sapporo, both in Hokkaido.

The day's high is expected to be five degrees in Sapporo and in the city of Aomori in northern Japan, and 13 degrees in central Tokyo.

The highs in Aomori and Tokyo are close to the cold weather usually seen in early December.

Snowfall is expected in the mountains in northern Japan.

Weather officials are urging caution against heavy snow and icy roads.