Kyushu, map

The climate of Fukuoka, a Japanese city located on the north coast of Kyushu, the southernmost of the major Japanese islands, is temperate humid, with quite mild winters, and hot, moist, and rainy summers. Like the rest of Japan, the city is affected by the monsoon circulation: in winter the northwest cold currents prevail, while in summer they are replaced by hot and humid currents of tropical origin.
The city is located at a low latitude (we are at the 33rd parallel), but close enough to the Asian continent to receive cold air masses in winter, able to lower the temperature to some extent.
In winter the sky is often cloudy, as is common in the Japanese western side; there are fairly mild periods, with highs above 10 °C (50 °F), but also cold periods, with wind and rain, highs around 5/7 °C (41/45 °F) or less, and possible falls of sleet or snow. Snowfalls, however, are generally light, as are nocturnal frosts.
Summers are hot and humid; there can be periods of good weather, with maximum temperatures around 35 °C (95 °F), tropical nights and high humidity, but also periods of bad weather, due to the summer monsoon. August is the hottest month, both for the thermal inertia of the sea, and for the lower frequency of periods of bad weather.
Here are the average temperatures.
Fukuoka - Average temperatures
Min (°C)44711162024252115106
Max (°C)101114202427313228231813
Min (°F)393945526168757770595043
Max (°F)505257687581889082736455

The rains are abundant, as they amount to 1,600 millimetres (63 inches) per year; winter is the driest season, but only in quantity; the rainiest months are June and July, ie the first period of the rainy season (called Baiu), which in southern Japan is particularly marked. In August there is a certain decrease in rain frequency: on average, among the summer months, it is the hottest, as well as the driest and sunniest. In the southwest of Japan, autumn is a decent season, and already in October it typically does not rain much.
Here is the average precipitation.
Fukuoka - Average precipitation

From June to October (but especially from August to early October), Fukuoka may be affected by typhoons, the tropical cyclones of Southeast Asia.
The cherry blossoming in Fukuoka occurs around 28-29 March.
The amount of sunshine in Fukuoka is never very good, however, there are two peaks, one in spring (April-May), and the other in August, after the Baiu rains, with 6/6.5 hours of sunshine per day. In October, after the summer rains, the sunshine amount increases a little as well.
Fukuoka - Sunshine


The sea in Fukuoka in winter is cold but not impossible for the more daring; it is warm enough for swimming, with a temperature of 23 °C (73 °F) or more, from July to October, and it becomes pleasantly warm in August.
Fukuoka - Sea temperature
Temp (°C)171616182022252726242219
Temp (°F)636161646872778179757266