Cases of heatstroke are likely to jump as temperatures surge this summer amid Japan's first La Nina phenomenon in six years, according to the Meteorological Agency.

La Nina, a natural cooling of parts of the Pacific that alters global weather and generally brings hot days to Japan during the summer months, is likely to develop within the next two months.

La Nina is "likely to develop by the end of August and expected to last until autumn," Meteorological Agency forecaster Ikuo Yoshikawa said.