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 3,000 millimetres (118 inches) per year.
In the northern part of the coast, in the capital Monrovia, rainfall reaches even 5.1 metres (200 inches) per year, with a maximum in June and July, when nearly one metre (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. (mm)3055952155159759953757457702351305135
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 2,000 mm (79 in) per year, as you can see from the average precipitation of Zwedru.
Average precipitation - Zwedru
ZwedruJanFebMarAprMayJunJulAugSepOctNovDecYear
ZwedruJFMAMJJASONDY
Prec. (mm)2565125140205280165180335215100501880
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 centre and north, and the amount of sunshine is good, although some showers are still possible. The temperature is high, around 30/32 °C (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 dry wind blows from the Saharan 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, 1,440 metres (4,724 feet) high, sometimes at night it can be cool or a bit cold.
In March and April, temperatures slightly rise, and showers become more frequent. Sometimes it can be very hot, so much so that the temperature can exceed 40 °C (104 °F).
The real monsoon arrives in May, bringing torrential rains, especially along the coast. From June to October, cloudiness is frequent, and the daytime temperatures drop below 30 °C (86 °F). 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, reaching again 30 °C (86 °F) during the day.
Here are the average temperatures of the capital Monrovia, located, as mentioned, in the northern part of the coast.
Average temperatures - Monrovia
MonroviaJanFebMarAprMayJunJulAugSepOctNovDec
MonroviaJFMAMJJASOND
Min (°C)232324252423232323232323
Max (°C)303131313028272627283030
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 (°C)282828292928272727282828
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 significant decrease 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 fibres, umbrella (even though sometimes it is not useful for the most intense 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 wet and rainy days.