Kyushu, map

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 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 receive cold air masses in winter, which are able to lower the temperature to some extent.
In winter, the sky is often cloudy, as is common on 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 because of the thermal inertia of the sea and because of the lower frequency of periods of bad weather.
Here are the average temperatures.
Nagasaki - Average temperatures
Min (°C)44712162024252216116
Max (°C)101215202426303229241813
Min (°F)393945546168757772615243
Max (°F)505459687579869084756455

The rains are abundant, in fact they amount to 1,850 millimeters (73 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, especially on the west coast where Nagasaki is located: sometimes the rains can be torrential. In the southwest of Japan, autumn is a decent season, and already in October it typically doesn't rain much.
Here is the average precipitation.
Nagasaki - Average precipitation

From June to October (but especially from August to early October), 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 not 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 amount of sunshine increases a little as well.
Nagasaki - Sunshine


In winter, the sea in Nagasaki 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.
Nagasaki - Sea temperature
Temp (°C)181718192123262827252320
Temp (°F)646364667073798281777368