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 from south to north; hence, in the south (zone 2 in the map) the rainfall is more abundant, and it goes from 600 to 1,500 millimetres (23.5 to 60 inches) per year, while in the north and centre (zone 1), which is part of the Sahel, the annual rainfall is lower than 600 mm (23.5 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 days.
Along the north coast, at least down to the latitude of Dakar, there's a cool sea current 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 (°C)181818192023252525252321
Max (°C)262525252629273031313028
Min (°F)646464666873777777777370
Max (°F)797777777984818688888682

In Dakar, 515 mm (20 in) of rain per year fall; the only very rainy months, with more than 150 mm (6 in) per month, are August and September. Here is the average rainfall in Dakar.
Average precipitation - Dakar
Prec. (mm)0000010652351753000515

In Dakar the sun shines for a good number of hours all year round, though a little less from July to September, the rainy season.
Sunshine - Dakar
Sun (hours)8991098877888

In the dry season, a dry north-eastern wind, called Harmattan, often blows throughout the country. In the coolest months, December and January, the daytime temperatures range between 30 and 32 °C (86 and 90 °F) in inland areas, while they are much cooler along the coast, around 25 °C (77 °F) in Dakar. Sometimes it can get a bit cool at night across the country, with night temperatures below 15 °C (59 °F), while at other times it can get very hot during the day, with highs around 35 °C (95 °F) in Dakar and 40 °C (104 °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 38 °C (100 °F) in the south and 40 °C (104 °F) in the north, while the coast is definitely more temperate, especially in Dakar, where the average maximum is still around 26 °C (79 °F) in May. The peaks are noticeable: around 45 °C (113 °F) in inland areas, and even 47/48 °C (117/118 °F) in the north-east (see Matam).
Here are the average temperatures of Matam
Average temperatures - Matam
Min (°C)182023262828272626262218
Max (°C)313537404139363435373632
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 and centre, 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 as high as 9 in Matam. In Ziguinchor, in the humid southern region of Casamance, 1,650 mm (65 in) of rain per year fall, including 560 mm (22 in) in August
Here is the average precipitation in Ziguinchor.
Average precipitation - Ziguinchor
Prec. (mm)110111145405560340160811633

And here are the average temperatures.
Average temperatures - Ziguinchor
Min (°C)161718191921232323232218
Max (°C)323437373533313031323331
Min (°F)616364666670737373737264
Max (°F)909399999591888688909188

On the contrary, in the northern city of Matam, precipitation amounts to 570 mm (22.5 in) per year, with a peak of 215 mm (8.5 in) in August, while in the northernmost region of Saint-Louis, where the Djoudj National Bird Sanctuary is located, precipitation drops to 240/350 mm (9.5/13.5 in) per year. In Dakar, in the centre, as mentioned precipitation amounts to 515 mm (20 in) per year. Temperatures are different as well: the monsoon lowers the daytime temperatures to 29/30 °C (84/86 °F) in the south, while in the north they remain higher, around 35 °C (95 °F).
At the end of the rainy season, between September and November, the temperatures start to increase again, reaching 37/38 °C (99/100 °F) in the centre-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 30 °C (86 °F) during the warmest 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 20/21 °C (68/70 °F), while it becomes warm from June to November, from 25 to 28 °C (77 to 82 °F).
Sea temperature - Dakar
Sea (°C)212020212225272828282724
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.

In the ocean to the west of Senegal, there is an area in which hurricanes, the tropical cyclones of the Atlantic, start to form; after they are formed, they move towards the west or the northwest and head to the Caribbean. In general, Senegal is not affected, because hurricanes are formed at a considerable distance, however, occasionally a hurricane can be formed a short distance from the coast and affect Senegal as well, with rainfall, and storm surges which can damage the coast, as happened at the beginning of September 2015 with Hurricane Fred.
Hurricanes are generally formed in this area in August and September.

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 and 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 November and December, when the sea is warmer than in 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 desert wind; in Dakar, you can add a light sweatshirt; in 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.