The chart below shows the January weather history for the past 7 years in the St. Augustine area.
Please note the high and the low temperatures and the average temperature. This is Florida, be prepared for all kinds of weather.
Waterproof your tent, slicker, hat, and boots even though the chance of rain is slim.. There are several spray water repellents in the market.

On many cold mornings  you may be warm by 12 noon. Dress in layers for comfort.