Honshu, map

In Sendai, a Japanese city in the north-east of Honshu Island, in Miyagi Prefecture, the climate is temperate humid, with cold winters and warm 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 (although to a lesser extent than in central and southern Japan).
In winter, temperatures are low: in January and February, lows are slightly below freezing (0 °C or 32 °F) and highs a few degrees above. Waves of intense cold are rare: usually the temperature does not drop below -5 °C (23 °F), although the cold is fairly constant and the air humid. Despite the fact that the city is located on the east coast, snow is quite common, and sometimes abundant. An average of about 70 centimeters (27 inches) of snow falls per year, distributed in 66 days (in most cases, however, it is sleet or light snow).
In summer, average temperatures are not too high, actually during periods of bad weather there is a cool northeast wind called Yamase. However, in this season, periods of hot and humid weather, though more rare than in Tokyo, are possible, and alternate with periods of bad weather, with wind and rain.
Here are the average temperatures.
Sendai - Average temperatures
Min (°C)-2-21611162021181151
Max (°C)56915192226282419148
Min (°F)282834435261687064524134
Max (°F)414348596672798275665746

Precipitation is quite abundant, in fact it amounts to 1,250 millimeters (49 inches) per year; winter is the driest season (at least in quantity, as drizzle or sleet can often fall), while summer is the rainiest season because of the summer monsoon. Here is the average precipitation.
Sendai - Average precipitation

From June to October (but especially from August to early October), Sendai may be affected by typhoons, though typically with less frequency and intensity than in southern Japan.
The cherry blossoming (Sakura) in Sendai occurs around April 13th, about two or three weeks later than in Tokyo.
The amount of sunshine in Sendai is not exceptional: it's pretty good in winter (considering that the days are shorter), it reaches a relative maximum in spring, and then it declines in summer because of the monsoon.
Sendai - Sunshine


The sea in Sendai, cold in winter, becomes acceptable for swimming in August and September, while in July it is still cool, as you can see in the following table.
Sendai - Sea temperature
Temp (°C)1188101317212423201713
Temp (°F)524646505563707573686355