Montserrat, an island and an overseas territory of the United Kingdom, has a tropical climate, hot and humid all year round, with a relatively cool and dry season from January to mid-April, and a hot, humid and rainy season from mid-June to mid-November. In the former period, the northeast trade winds, constant winds typical of tropical climates, blow steadily and with moderate intensity, while in the latter period the winds are more irregular, and may have some breaks, increasing the feeling of sultriness. Between them there are two transitional periods: from mid November to early January (when the northeast wind starts to blow, the temperature decreases a bit and the weather gets better) and from mid-April to Mid-June (when both the temperature and the frequency of downpours gradually increase).
Here are the average temperatures in the old capital Plymouth; as you can see, there are little variations between the coolest and the warmest period.
Plymouth - Average temperatures
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)212121222324242423232322
Max (°C)282829303131313132312928
Min (°F)707070727375757573737372
Max (°F)828284868888888890888482

The rainfall in Montserrat is abundant, in fact it amounts to 1,600 millimeters (65 inches) per year in the former capital Plymouth, of which more than 100 mm (4 in) fall per month from May to January, and more than 150 mm (6 in) from July to November. The driest month is February, with an average of 85 mm (3.3 in), which means that some showers are possible even in this period. After the end of the hot and wet period, the sea is still warm enough to provide energy and moisture for the formation of thunderclouds, so the rains are still quite frequent in the first months of the north-east trade winds.
Here is the average precipitation in Plymouth.
Plymouth - Average precipitation
MonthJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec.(mm)1208511090951101551851701951801401640
Prec.(in)4.73.34.33.53.74.36.17.36.77.77.15.564.6
Days129981013141613141613147

The rains in Montserrat occur throughout the year especially in the form of downpours or thunderstorms, which could be intense, but generally do not last long, so they do not reduce too much the sunshine hours; sometimes, however, there may be a more intense wave of bad weather.
In fact, the amount of sunshine is good all year round, at least along the coasts, as can be seen in the following table.
Plymouth - Sunshine
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Hours888888887777

The interior of the island is occupied by hills and low mountains and is even more rainy. The highest point is Soufrière Hills, a volcano of 915 meters (3,000 feet), which in 1995 produced a strong eruption and devastated the southern part of the island, including Plymouth, the capital. The southern part has been declared "exclusion zone", while the northern part is still inhabited and is believed to be protected from volcanic eruptions.

Montserrat

The sea is pleasantly warm all year round, as we can see from the following table.
Montserrat - Sea temperature
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Temp (°C)272626272828282829292827
Temp (°F)817979818282828284848281

Hurricanes



The greatest danger comes from tropical storms and cyclones, called hurricanes in the Caribbean area, which may pass over the island from June to November, though they are most likely from August to October. A very violent hurricane that struck Montserrat was Okeechobee in September 1928, and another one was Hugo, which hit the island on September 17, 1989.

Best Time



The best time to visit Montserrat is from February to April, since it's the driest one. In December and January, there are some additional showers, but we are still in the least hot period, so all in all they are acceptable months as well.

What to pack



All year round: bring light clothes of natural fabric, a sun hat, sunscreen, and possibly a light sweatshirt and a light raincoat for thunderstorms. A light sweatshirt for the evening from December to March.
When going to the reef, you can bring snorkeling equipment, including water shoes or rubber-soled shoes.