I don't know that it floods in Cayo any more than in any other sub-tropical area with seasonal rains, and certainly there is usually less rain in Cayo than in southern Belize. Much of Cayo district is rolling hills and in some areas low mountains, and you do not get any flooding there. After heavy rains, as anywhere the rivers may rise and you get some water near the river.
Seasonal rains do usually start in late June or early July, but it varies, especially given recent climatic changes, and I have been in Cayo in the summer when it rained hardly any if at all for several weeks.
Overall I prefer Cayo in the green season, as it's a little cooler, greener and there's not the risk of fires as in the late spring, what locals call the dry season.
There is simply no predicting the exact weather in Belize any more than there is in Birmingham or Boise. It could be rainy for several days, there could be a drought with little rain, or most likely in the summer there will be some rain but plenty of time for outdoor activities.
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