Map from Google - Guinea_Bissau

In Guinea Bissau, the climate is tropical, hot all year round, with a dry season from November to May, and a rainy season from June to October, due to the African monsoon. The hottest period runs from February to May, before the rains, but the rainy season, though less hot, it's muggy. The heat is intense especially in the interior, while along the coast, where the capital Bissau is located, it's a bit cooler because of the breezes.
Here are the average temperatures of Bissau.
Average temperatures - Bissau
Min (°C)192022232424232323232321
Max (°C)313233333231292829313131

In Bissau, precipitation amounts to 2,000 millimetres per year. Here is the average precipitation.
Average precipitation - Bissau
Prec. (mm)1101171754706804351954022017

The monsoon rains are very heavy, especially from July to September, when they can cause widespread flooding. In addition, in this period the sky is often cloudy. August is the rainiest month, and if possible it should be avoided. The rainfall is more abundant in the south, where it exceeds 2,000 millimetres per year, and less abundant in the north, where it amounts to around 1,500 mm per year.
During winter, the weather is sunny, and the air is dry because of the prevailing wind, the Harmattan, which blows from the desert, and can bring dust making the sky whitish. Normally the daytime temperature is about 30 °C, but sometimes it can touch 40 °C. This, however, is the best time of the year. Starting in February, both the temperature and humidity increase gradually, reaching a peak between March and May, when it can easily surpass 40 °C. Starting in June, the rains lower the temperature to about 28/30 °C, but the air becomes humid and muggy.
Here are the average temperatures of Gabù, located in the north-east, where the heat is intense in the months preceding the monsoon.
Average temperatures - Gabù
Min (°C)161921232423222222222016
Max (°C)323537383733302930323331

Guinea Bissau

The sea is always warm enough for swimming, even though it is warmer from June to November, when the water temperature is about 28 °C, while in March and April it's almost cool, since it goes down to 23 °C.
Sea temperature - Bissau
Sea (°C)252423232628282828282827

When to go

The best time to visit Guinea Bissau is winter, in particular the months of December and January, which are the least hot, among those in which it does not rain. It may be useful to bring a scarf, for the dust brought by the wind. For a beach holiday, the dry season, from November to May, is preferable.

What to pack

In winter: in Bissau and the coast, light clothes for the day, a sweatshirt for the evening, a scarf for the wind-borne sand and dust. In the inland areas, you can add a light jacket for the evening.
In summer: light clothing of natural fibres, a light sweatshirt and a light raincoat for the rain.