The weather in St John is basically the same every day.
From Jan to Aug it warms up apps 1 degree on the low and high ends; then down apps 1 through to Dec.
Chance of Rain (every day):
There is usually a 30% chance of rain every day, for 15 minutes, just depends if you are where the patch of rain comes through or not. We also carry a few ziplock bags for cell phones just in case. During dry spells, it goes a week or more without rain. During rainy spells there is a 60% chance of rain, but you never know where.
The St John Weather chart is enclosed. You can also see St Thomas Weather details.