Map from Google - Nagasaki

The climate of Nagasaki, a Japanese city located on the west 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 will prevail, while in summer they will be replaced by the hot and humid currents of tropical origin.
The city is located at a low latitude (we are at the 32rd parallel), but close enough to the Asian continent to be received in winter by cold air masses, able to lower the temperature.
In the winter the sky is often cloudy, as is common in the Japanese western side; there are fairly mild periods, with highs above 10 °C, alternating with cold periods, with wind and rain, highs around 5/7 °C 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, 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 - Nagasaki
NagasakiJanFebMarAprMayJunJulAugSepOctNovDec
NagasakiJFMAMJJASOND
Min (°C)44712162024252216116
Max (°C)101215202426303229241813

The rains are abundant, as they amount to 1,850 millimetres 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, especially on the west coast where Nagasaki it's located: sometimes the rains can be torrential. 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 - Nagasaki
NagasakiJanFebMarAprMayJunJulAugSepOctNovDecYear
NagasakiJFMAMJJASONDY
Prec. (mm)65851301501803153151951908585601860
Days1110121111141210106910125

In the latter part of summer and in early autumn, from August to October (though more likely in August and September), Nagasaki may be affected by typhoons, the tropical cyclones of Southeast Asia.
The cherry blossoming in Nagasaki occurs around 31 March.
The amount of sunshine in Nagasaki is never very good, however, you can see two peaks, one in spring (April-May), and the other in August, after the Baiu rains, with 6/7 hours of sunshine per day. Also in October, after the summer rains, the sunshine amount increases a little.

Nagasaki

The sea in Nagasaki in winter is cold but not impossible for the more daring; it is warm enough for swimming (with a temperature of 23 degrees or more) from July to October, and it gets pleasantly warm in August.
Sea temperature - Nagasaki
NagasakiJanFebMarAprMayJunJulAugSepOctNovDec
NagasakiJFMAMJJASOND
Sea (°C)181718192123262827252320