During summer in Northern America, Guatemala is having its rainy season. It is not the ideal time to travel to Central America; but as a teacher, it is the easiest time for me to get away.
My two days in Livingston were fortunately not too wet. In general, in both Belize and Guatemala, I found that although it rained about every other day, it only rained in the evenings and for about 10 minutes at a time. Also, the rain is warm and kind of enjoyable sometimes.
The 10 minute rain intervals were interesting for a California girl, like me. It would POUR buckets of water and then suddenly stop! I recommend packing a light rain jacket if you are traveling during rainy season -- the inland areas of Guatemala are chilly at night anyway, so the jacket will come in handy.