Map from Google - Antwerp

In Antwerp, city located in the region of Flanders, in the north of Belgium, the climate is sub-oceanic, humid and rainy, influenced by the Atlantic Ocean, so it has cold (but not freezing) winters and quite cool summers.
The average temperature in January is 2.5 °C, while in July it's 17.5 °C.
Here are the average temperatures.
Antwerp average temperatures
Antwerp J F M A M J J A S O N D
Min (°C) 0 0 2 4 8 11 13 13 10 7 4 1
Max (°C) 5 6 10 13 18 21 22 22 19 15 9 6

Precipitation is relatively abundant, about 800 millimetres per year, but above all it is common and distributed throughout the year; however the rainiest season (at least in quantity) is summer, the least rainy season is spring. Rainfall often occurs in the form of short showers or drizzle.
Here is the average precipitation.
Average precipitation Antwerp
Antwerp J F M A M J J A S O N D Year
Prec. (mm) 65 50 65 50 60 75 75 65 65 70 75 75 790
Days 13 10 13 10 11 12 10 9 10 11 13 13 135

The wind blows frequently, and can be intense, especially from November to March.

Antwerp

Winter, from December to February, is cold: the daytime temperatures are on average a few degrees above freezing, and at night they stay around freezing; wind and humidity often exacerbate the feeling of cold.
However, different weather situations may occur.
When the westerlies blow, the temperatures can be quite mild, and can reach 10/12 °C.
On the contrary, when the city is affected by cold air masses from Russia, the temperature drops below freezing, for longer or shorter periods, even during the day. The temperature rarely drops below -10 °C, however, it can do it in the coldest periods. Here are some values recorded in the coldest winters: January 1979: -14 °C; January 1985: -17 °C; January 1987: -13 °C; December 1996: -12 °C; December 2010: -12 °C.
During the "Little Ice Age" (1450-1850) the Scheldt, the river that goes through the city and flows into the sea just north of it, used to freeze quite often; today this happens very rarely.
Snowfalls in Antwerp are quite frequent, though usually not abundant, and occur in the form of sleet or light snow, sometimes however snow can accumulate on the ground, and last for many days, if it occurs during cold waves.

View of Antwerp with the frozen Scheldt (1590), by Lucas van Valckenborch

Spring, from March to May, is initially cold, with possible snowfalls still in March, and then it gradually becomes milder in April and May, although some cold days, with highs around 10 °C, can occur also in May, especially in the first half of the month. However, April and May are, albeit slightly, the least rainy and cloudy months.
Typically, in the second half of May the temperatures become permanently acceptable, and there can be the first pleasantly warm days, with highs around 25 °C or even above.

Summer, from June to August, is mild, and the flow of Atlantic fronts does not cease even in this season. The wind blows constantly from the sea, and even rainfall is quite frequent.
However, there are pleasant periods, when the daytime temperatures are between 20 and 25 °C, alternating with cooler periods, in which even the daytime temperatures remain below 20 °C. At night it can be a little cold, and the temperature can drop below 10 °C even in July and August. Sometimes there can be hot days, in which the temperature can reach 28/30 °C, but this may not occur for a whole month. The highest record is around 35 °C, a value reached in August 2003 and 2012, and in July 2015.

Autumn, from September to November, is initially pleasant, ie in the first half of September, then it gradually becomes more cold, dull, wet and windy. The first night frosts usually occur in November, but sometimes also in October.

The amount of sunshine in Antwerp is not high: the sun rarely shines from October to February, while it goes a bit better from May to August, when the weather is variable, and between a disturbance and the other the sun can come out. There are on average 7 hours of sunshine per day from May to August.

When to go

The best time to visit Antwerp runs from mid-May to mid-September, being the mildest of the year. July and August are the warmest months; however, it's better to bring a sweatshirt or sweater, a light jacket and an umbrella, for cool and rainy days.