A typhoon that briefly strengthened into the Northern Hemisphere's strongest storm of the year has lost much of its punch but could still hit Japan by this weekend.

Typhoon Noru was packing maximum sustained winds of 194km/h with gusts of up to 240km/h per hour as of midday on Tuesday (US time), according to the US military's Joint Typhoon Warning Center.

The storm's trajectory was highly uncertain. But the Japan Meteorological Agency was forecasting it might hit the southern island of Kyushu by Friday or the weekend, but as a much weaker system.

This satellite image taken Monday, July 31, 2017, and released by the National Oceanic and Atmospheric Administration shows Typhoon Noru. Photo / AP
This satellite image taken Monday, July 31, 2017, and released by the National Oceanic and Atmospheric Administration shows Typhoon Noru. Photo / AP

The Weather Channel said the storm strengthened rapidly into a Category 5 cyclone at its peak on Monday, as its wind speeds more than doubled to 257km/h.

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By Tuesday afternoon it was the equivalent of a high Category 3 or low Category 4 cyclone, swirling across a wide swath of open ocean.

Japan has already endured several spates of extreme weather this summer, with heavy rains triggering deadly landslides on Kyushu in June that killed 37 people and left six missing.

Torrential rains in northern Japan flooded parts of northern Honshu island in late July.