Map from Google - Sapporo

In Sapporo, the main city of the island of Hokkaido, the climate is semi-continental, with freezing and snowy winters, and warm and rainy summers.
In winter, Siberian currents prevail, which pick up moisture when passing over the Sea of Japan, so the temperature is almost permanently below freezing (0 °C or 32 °F), and it snows about every day or so (during the mildest days, more frequent in the first half of December or in March, it can also rain). The large amount of snow available allows to organize in February (usually in the first ten days) the Sapporo Snow Festival, which attracts millions of visitors to admire statues and sculptures made of ice. During the most intense cold spells, the temperature can drop to -15 °C (5 °F), or even below; the coldest temperature measured since 1872, recorded in a period when the climate was colder, is -28.5 °C (19.5 °F).
In spring and autumn, disturbances are quite frequent; in summer comes the monsoon, but with less force than in central and southern Japan, however, it is capable of causing some periods of bad weather. In summer the temperature is generally pleasant, but we can not exclude a few hot and humid days, with maximum temperatures around 30 °C (86 °F) or above.
Here are the average temperature of the meteorological observatory of the city.
Average temperatures - Sapporo
SapporoJanFebMarAprMayJunJulAugSepOctNovDec
SapporoJFMAMJJASOND
Min (°C)-7-7-3381317191481-4
Max (°C)-1041217222526221682
Min (°F)191927374655636657463425
Max (°F)303239546372777972614636

In the southern outskirts of the city (therefore outside the city, but also farther from the sea), the temperatures are lower, especially in winter and at night, as can be seen from the averages of the Hokkaido Agricultural Research Center, located in Toyohira.
Average temperatures - Toyohira
ToyohiraJanFebMarAprMayJunJulAugSepOctNovDec
ToyohiraJFMAMJJASOND
Min (°C)-11-11-615101517125-1-7
Max (°C)-2-121016202425211571
Min (°F)121221344150596354413019
Max (°F)283036506168757770594534

In Sapporo, 1,100 millimetres (43 inches) of rain or snow per year fall; precipitation is particularly frequent in the cold half of the year, while the driest period is spring, before the summer monsoon. Here is the average precipitation.
Average precipitation - Sapporo
SapporoJanFebMarAprMayJunJulAugSepOctNovDecYear
SapporoJFMAMJJASONDY
Prec. (mm)1159580555545801251351101051101105
Prec. (in)4.53.73.12.22.21.83.14.95.34.34.14.343.5
Days1816149978910121415140

Sapporo may also be affected by typhoons, tropical cyclones, which mostly affect the southern part of Japan, between summer and early autumn (and especially in August and September), and here in the north they pass more rarely and generally weakened. The fact remains that they can bring strong winds and abundant rains.
The cherry blossoming (Sakura) in Sapporo occurs around the 5th of May, with some variability depending on year (the earliest in the last 10 years has occured on April the 25th, the latest on May the 17th).
The amount of sunshine in Sapporo is never very good: there are on average 3 hours of sunshine per day in December and January, there's a maximum of 6 hours and a half in May, in spring, which as we have seen is the driest season, and a new decrease to about 5 hours and a half in summer, in July and August, due to the monsoon.

Sapporo

The sea in Sapporo is very cold in winter (but on the northeastern coast of Hokkaido, where a cold current flows, pushing the ice to the coast, it is even colder), and remains cool in summer, even though it reaches 21 °C (70 °F) in August.
Sea temperature - Sapporo
SapporoJanFebMarAprMayJunJulAugSepOctNovDec
SapporoJFMAMJJASOND
Sea (°C)645691318212016128
Sea (°F)433941434855647068615446