Hello
There are several weather experts and some STT/STJ residents on the forum, so hopefully they will chime in with their knowledge. I thought I would give my 2 cents based on our trips and reading others' trip reports over the years, and noting the weather in the various months.
I've only been to St John and the BVIs in April, May or June. For the most part, those months have been great weather, water is warm and calm. We spend hours every day snorkeling, so calm warm water is a must for us
One year, in April we had a solid week of rain and cooler weather (ugh!). I recall that forumite Pia said there is usually a week or two of rain in either April or May every year. In 2012, we went the middle two weeks of May and had a few rain storms, but fabulous weather the rest of the time. Five years ago, we went the last week of April and first week of May, and had dry, sunny weather with calm water every day. It was great.
In the winter months, there is a weather condition called Christmas winds - you can google it to get more info. It can cause windy conditions and waves, especially on the north shore. People who have been on island during these times have posted pictures of Trunk Bay and other beaches with big waves. I remember one trip report said several people were injured going into the water, due to the intensity of the waves. There are beaches on the other side of the island, (Salt Pond, Little Lameshur) which are protected and still calm for snorkeling, even when the waves are high on the north shore. One year we went to the BVIs in late April and had the big waves. Locals said it was unusual for that time of year, but had happened in the past.
People who go in the summer and fall talk about warm, calm water, but it sounds its a bit warmer and more humid. We've talked about going in October some year. If there are no hurricanes, the weather seems great.
If you have time, I would suggest reading some of the great trip reports on the forum, as people often comment on weather, water temperature and snorkeling.
Happy trip planning
