In Yokohama the climate is temperate, with fairly mild and sunny winters, and hot, humid 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.
Yokohama overlooks the sea in Tokyo Bay, not far from the Japanese capital. Given the relatively low latitude (we are on the 35th parallel), and the protection of the mountains (we are on the east side), in winter the currents of Siberian origin are felt little, so there are several sunny days, and usually it's not too cold; however, cold and windy periods, with highs around 5/7 °C (41/45 °F) or less and possible snowfalls, can not be ruled out. Night frosts are rare and usually light.
In summer, hot and muggy periods, with highs around 35 °C (95 °F) and high humidity, alternate with periods of bad weather, with wind and rain, and sometimes (especially in June and July) a little cool at night. 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 - Yokohama

Yokohama	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Yokohama	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	2	2	5	10	15	19	22	24	21	15	10	5
Max (°C)	10	10	13	18	22	25	29	31	27	22	17	12
Min (°F)	36	36	41	50	59	66	72	75	70	59	50	41
Max (°F)	50	50	55	64	72	77	84	88	81	72	63	54

The rains are abundant, as they amount to 1,700 millimetres (67 inches) per year; winter is undoubtedly the driest season; the rainiest months are September and October, also due to the contribution made by typhoons. In August there is a certain decrease in rain frequency: among the summer months it is on average, as well as the hottest, the driest and sunniest.
Here is the average precipitation.
Average precipitation - Yokohama

Yokohama	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Yokohama	J	F	M	A	M	J	J	A	S	O	N	D	Y
Prec. (mm)	60	65	140	145	150	190	170	165	235	205	105	55	1690
Prec. (in)	2.4	2.6	5.5	5.7	5.9	7.5	6.7	6.5	9.3	8.1	4.1	2.2	66.5
Days	6	7	12	11	12	14	12	9	13	12	8	6	119

As mentioned, from June to October (but especially from August to early October), Yokohama can be affected by typhoons, the tropical cyclones of South-East Asia, which mostly affect the southern part of the country, but can also arrive here. The most intense typhoons can bring strong winds, torrential rains and heavy damage.
The cherry blossoming (Sakura) in Yokohama takes place around the 30th of March: there are dedicated events in streets and parks where you can admire them.
The amount of sunshine in Yokohama is good in winter (there are no less than 6 hours of sunshine per day in December and January), while it is just mediocre or decent in the other seasons, due to the passage of disturbances in spring and fall and to the summer monsoon. However, in summer August is the sunniest month.
Sunshine - Yokohama

Yokohama	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Yokohama	J	F	M	A	M	J	J	A	S	O	N	D
Sun (hours)	6	6	5	6	6	4	5	7	4	5	5	6

The sea is warm enough for swimming, with a temperature of 23 °C (73 °F) or more, from July to October, as you can see in the following table.
Sea temperature - Yokohama

Yokohama	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Yokohama	J	F	M	A	M	J	J	A	S	O	N	D
Sea (°C)	17	16	16	17	20	22	24	26	25	23	21	19
Sea (°F)	63	61	61	63	68	72	75	79	77	73	70	66