Map from Google - Liberia

In Liberia the climate is tropical, hot and humid all year round, with a rainy season from May to October, due to the African monsoon, and pretty frequent rains in other months, except in the short dry season that runs from December to February, which is more marked in the north. In the southern zone, the rains have a relative break from mid-July to late August.
Along the coast, the rainfall exceeds 120 inches per year. In the northern part of the coast, in the capital Monrovia, rainfall reaches even 16 feet per year, with a maximum in June and July, when 3.3 feet of rain per month fall, and a relative decrease in August. Here the rains are abundant already in April, and still in November. Here is the average rainfall in Monrovia.
Average precipitation - Monrovia
MonroviaJanFebMarAprMayJunJulAugSepOctNovDecYear
MonroviaJFMAMJJASONDY
Prec. (in)1.22.23.78.520.338.439.214.829.330.39.35.1202.2
Days43812222421172422169182

In the interior precipitation is less abundant, and it drops even below 80 inches per year, as you can see from the average precipitation of Zwedru.
Average precipitation - Zwedru
ZwedruJanFebMarAprMayJunJulAugSepOctNovDecYear
ZwedruJFMAMJJASONDY
Prec. (in)12.64.95.58.1116.57.113.28.53.9274
Days24811171918222217105155

In winter, from December to February, there is little rain in the center and north, and the amount of sunshine is good, although some showers are still possible. The temperature is high, around 86/90 °F during the day, and the humidity remains high, especially along the coast and in the forests of the interior. Sometimes, however, a hot dry wind blows from the desert, called Harmattan, which can fill the atmosphere with dust. In the inland hilly regions, and especially in the mountain peaks, the highest of which is Mount Wuteve, 4,724 feet high, sometimes at night it can be cool or a bit cold.
In March and April, the temperatures rise slightly, and the showers become more frequent. Sometimes it can be very hot, so much so that temperatures can exceed 105 °F.
The real monsoon arrives in May, bringing torrential rains, especially along the coast. Cloudiness is frequent, and the daytime temperatures drop below 85 °F from June to October. Between mid-July and August, there is a decrease in rainfall, more evident in the south, because the rainy front moves northward.
November is the last month of rainy weather, and the rains already decrease sharply compared with the previous months. The temperature increases a little, due to the higher amount of sunshine hours, exceeding again 85 °F during the day.
Here are the average temperatures of the capital Monrovia, located in the northern part of the coast.
Average temperatures - Monrovia
MonroviaJanFebMarAprMayJunJulAugSepOctNovDec
MonroviaJFMAMJJASOND
Min (°F)737375777573737373737373
Max (°F)868888888682817981828686

For swimming, the sea is warm enough all year round, as you can see in the following table.
Sea temperature - Monrovia
MonroviaJanFebMarAprMayJunJulAugSepOctNovDec
MonroviaJFMAMJJASOND
Sea (°F)828282848482818181828282

Monrovia

When to go

The best time to visit Liberia is winter, particularly in January and February, which are the sunniest and least rainy months. The worst period is from May to October, although between mid-July and August, in the south there is a fairly sharp break in the rains.

What to pack

In winter: in Monrovia and the northern coast, light clothes for the day, a light sweatshirt for the evening, scarf for the wind-borne sand and dust; in Greenville and the south coast, umbrella or light raincoat. In inland areas, it may be useful to bring a sweatshirt and a jacket for the evening.

In summer: tropics-friendly, light clothing of natural fiber, umbrella (even though maybe it is not useful for the monsoon rains), 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.