Map from Google - Kitakyushu

The climate of Kitakyushu, a city located on the north coast of Kyushu (the southernmost of the major Japanese islands), in Fukuoka Prefecture, 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), alternating with 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.
Average temperatures - Kitakyushu
KitakyushuJanFebMarAprMayJunJulAugSepOctNovDec
KitakyushuJFMAMJJASOND
Min (°C)2351014192324201494
Max (°C)91014192427303228231712
Min (°F)363741505766737568574839
Max (°F)485057667581869082736354

The rains are abundant, as they amount to 1,700 millimetres (67 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.
Average precipitation - Kitakyushu
KitakyushuJanFebMarAprMayJunJulAugSepOctNovDecYear
KitakyushuJFMAMJJASONDY
Prec. (mm)85801251251552703001701857590701730
Prec. (in)3.33.14.94.96.110.611.86.77.333.52.868.1
Days

In the latter part of summer and in early autumn, from August to October (though more likely in August and September), the city can be affected by typhoons, tropical cyclones of Southeast Asia.
The cherry blossoming in Kitakyushu occurs around the 29th of March.
The amount of sunshine is never very good, however, there are two relative maxima, one in spring (April-May), and the other in August, after the Baiu rains. In October, after the summer rains, the sunshine amount increases a little as well. In winter, on the contrary, the sky is often cloudy.
Sunshine - Kitakyushu
KitakyushuJanFebMarAprMayJunJulAugSepOctNovDec
KitakyushuJFMAMJJASOND
Sun (hours)345665565544

Kokura Castle

The sea in Kitakyushu 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.
Sea temperature - Kitakyushu
KitakyushuJanFebMarAprMayJunJulAugSepOctNovDec
KitakyushuJFMAMJJASOND
Sea (°C)171616182022252726242219
Sea (°F)636161646872778179757266