Map from Google - Guinea

In Guinea, the climate is tropical, with a dry season in winter and a rainy season in summer, due to the arrival of the south-west monsoon.
The rainfall is more abundant along the coast, where even 160 inches of rain fall in a year, while it's less abundant in the north and north-east, where precipitation is slightly above 40 inches per year. Although the rains are heavier on the coast, the rainy season lasts longer in the south-east, where the first showers occur as early as the second half of February, and the last ones occur in November, while in Conakry, on the coast, the rains occur from May to November, and in Kankan, in the east, from May to October.
The inland areas of Guinea are almost everywhere occupied by hills and low mountains, reaching at most an altitude of around 5,200 feet in Fouta-Djallon (in the north-west) and Mount Nimba (in the south-east); from the mountains, several important rivers such as Niger, Senegal and Gambia have their source. The altitude temper a bit the climate, so much so that the warmest cities are those found at lower elevations, such as Koundara in the north, which is located in the plain and is very hot from March to May, before the arrival of the monsoon, when highs are normally around 100/105 °F.
Here are the average temperatures of Koundara.
Average temperatures - Koundara
Min (°F)636672757775727272736861
Max (°F)939910010210093888888909193

As mentioned, the north is the least rainy area of country, and in addition the rainy season is shorter. Here is the average precipitation.
Average precipitation - Koundara
Prec. (in)0000.125.39.8128.13.10.4043.3

The warming in spring is less pronounced along the coast, which is mitigated by the ocean breezes, but also in the inland areas of the south-central, both because of the higher altitude, and the earliness of the monsoon. So in N'Zerekoré, at 1,600 feet above sea level, located in the south-east, the average daytime temperatures do not go above 90 °F in February and March, and in Conakry, on the coast, above 90 °F in April.
Average temperatures - NZerekoré
Min (°F)616468707068686868686663
Max (°F)889090888684818182848686

As mentioned, the south-east is the area where the rainy season is longer. Here is the average precipitation in N'Zerekoré.
Average precipitation - NZerekoré
Prec. (in)

Winter in Guinea, from December to February, is hot, dry and sunny, with the prevalence of the north-east wind, called Harmattan, that can carry the dust of the desert. Nights are usually cool, especially in the interior, while during the day the temperatures are high, around 90/93 °F, but with peaks of 105 °F. In February, early showers occur in the south, while in the north-central the heat becomes even more intense.
The capital Conakry, lying on the coast, experiences little variation in temperatures throughout the year: in winter, the daytime temperatures are around 86/90 °F, while in the rainy season they go down to 81/83 °F, although the high humidity makes the heat definitely muggy. Here are the average temperatures.
Average temperatures - Conakry
Min (°F)727273757573737373737573
Max (°F)868688908886828284868686

Conakry is the rainiest city of Guinea: the majority of the 13 feet of rain that fall in a year, occurs from June to mid-October, in particular the rains are torrential in July and August, when it rains almost every day and the sky is almost always overcast. Here is the average precipitation.
Average precipitation - Conakry
Prec. (in)

Fouta Djalon

The sea is warm enough for swimming all year round, as you can see in the following table.
Sea temperature - Conakry
Sea (°F)817979818282828181828482

When to go

The best time to visit Guinea is winter, particularly in December and January. The weather is good, although it can get hot during the day, especially in inland areas and at lower altitudes, and it can be cool at night, especially in the mountains.
Before the arrival of the monsoon, from March to May, it's very hot in the inland regions, especially in the north, while on the coast the temperature is more stable. On the contrary, in the south the rains begin.
For a beach vacation, the best period runs from December to April, being the dry season.

What to pack

In winter: in Conakry and the coast, light clothes for the day, a light sweatshirt for the evening, scarf for the wind-borne sand and dust. In inland areas, it may be useful to bring a sweatshirt and a jacket for the evening.
In summer: light clothing of natural fiber, umbrella, a light sweatshirt and a light raincoat for the long and heavy downpours. In inland areas and mountain ranges, you can add a heavier sweatshirt for the wet and rainy days.