Map from Google - Senegal

The climate of Senegal is tropical, with a dry season from November to May, and a rainy season from mid-June to mid-October, due to the African monsoon, which in summer moves northward; hence, the rainfall is more abundant in the south, where it goes from 24 to 60 inches per year, while in the north-central, which is part of the Sahel, the annual rainfall is lower than 24 in.

Climatic zones in Senegal

Consequently, the landscape varies considerably, being semi-desert in the north, while the south is occupied by the savanna, and has also gallery forests along the rivers. In the north-west, a short distance from the sea there is even a small desert, with sand dunes, the Lompoul Desert.

Lompoul Desert

The rainy season is known as hivernage ("wintering"), but despite the name it's a hot and humid period, especially in the south, while in the north, in this season moist and rainy days alternate with hot and dry ones.
Along the north coast, at least down to the latitude of Dakar, a cool current flows into the sea, which lowers the temperature of the coast to some degree compared with inland areas. The capital Dakar is located in a particularly mild area, because it's on the tip of a peninsula, overlooking the ocean, and enjoys by far the mildest climate of Senegal.
Here are the average temperatures of Dakar.
Average temperatures - Dakar
Min (°F)646464666873777777777370
Max (°F)797777777984818688888682

In Dakar, 20 in of rain per year fall; the only very rainy months, with more than 6 inches per month, are August and September. Here the average rainfall in Dakar.
Average precipitation - Dakar
Prec. (in)000000.

In the dry season, a dry north-eastern wind, called Harmattan, often blows throughout the country. In the cooler months, December and January, the daytime temperatures range between 86 and 90 °F in inland areas, while they are much cooler along the coast, around 77 °F in Dakar. Sometimes it can get a bit cooler at night across the country, with night minima below 59 °F, while at other times it can get very hot during the day, with highs around 95 °F in Dakar and 105 °F in inland areas.
Starting in February, the temperature begins to rise, and reaches its highest levels from April to early June, before the arrival of the monsoon. In the interior, highs are normally around 100 °F in the south and 105 °F in the north, while the coast is definitely more temperate, especially in Dakar where the average maximum is still around 79 °F in May. The peaks are noticeable: around 113 °F in inland areas, and even 117/118 °F in the north-east (see Matam).
Average temperatures - Matam
Min (°F)646873798282817979797264
Max (°F)889599104106102979395999790

From June to October, in the monsoon season, the temperature drops by a few degrees, but at the same time moisture increases. The rains occur in the form of showers in the north-central, while they are more persistent in the south, where the sky is often cloudy: in August, the wettest month, there are only 3 hours of sunshine per day in Ziguinchor, and even 9 in Matam. In Ziguinchor, in the humid southern region of Casamance, 64 in of rain per year fall, including 22 inches in August
Here is the average precipitation in Ziguinchor.
Average precipitation - Ziguinchor
Prec. (in)00000.45.715.92213.46.30.3064.3

Here are the average temperatures.
Average temperatures - Ziguinchor
Min (°F)616364666670737373737264
Max (°F)909399999591888688909188

On the contrary, in the northern city of Matam, 22 in of rain per year fall, including 8.5 inches in August, while in the northernmost region of Saint-Louis, where the Djoudj National Bird Sanctuary is located, precipitation drops to 9.5/14 inches per year. In Dakar, in the center, as mentioned precipitation amounts to 20 inches per year. Even the temperatures are different: the monsoon lowers the daytime temperatures to 84/86 °F in the south, while in the north they remain higher, around 95 °F.
At the end of the rainy season, between September and November, the temperatures start to increase again, reaching 99/100 °F in the center-north; this phenomenon is much less evident in the south. In Dakar, which is a world apart compared with the rest of the country, the trend is very different: the temperature rises gradually until it reaches about 86 °F during the warmer months, which are September and October.

The Niokolo-Koba Park, UNESCO World Heritage Site, as well as the Dindefelo waterfalls, are located in the southernmost humid region.

Niokolo-Koba National Park

The sea, at the height of Dakar, is cool from January to April, around 68/70 °F, while it becomes warm from June to November, from 77 to 82 °F.
Sea temperature - Dakar
Sea (°F)706868707277818282828175

Compared with Dakar, the sea is slightly cooler in the northern part of the coast, and slightly warmer in the southern part.

When to go

The best time to visit Senegal is winter, from December to mid-February: it's warm in the coast, even hot in the interior, the sun shines and it practically never rains. Sometimes the heat can be intense in inland areas, but the humidity is low and the nights remain pleasantly cool. Spring is scorchingly hot, and partly autumn, from mid-October to November, but at least it doesn't rain. It's better to avoid the rainy season, especially from July to September in the south.
A special case is represented by Dakar, particularly mild, where the climate is good for a long period, from December to May.
For swimming, you may prefer to November and December, when the sea is warmer than the following months.

What to pack

In winter: light clothes for the day, sweatshirt and jacket for the evening, scarf or turban for the sand.
In summer: in the north, lightweight clothes, long and made of natural fabric (cotton or linen), turban for the wind from the desert; in Dakar, you can add a light sweatshirt; for the south, a light sweatshirt and a light raincoat for the rain. For women, it is best to avoid shorts and miniskirts outside of the beaches.