The real rains, that is the "Rainy Season" Can start anywhere from 01 May to well into July. . . . But more typically during the last week of May. This year I am not seeing the developments for the Rainy season, so perhaps it will be a little late this year.
The definition of the "Rainy Season" is :
"One criterion for the onset of the rainy season is two consecutive days of rainfall accumulation of 30 mm or more, and four of the following seven days are rain days of 1 mm rainfall or more. "
Or put simpler, over 31mm ( 1¼") Rain in a Week, and 6 of the 9 days are rain days, > 1mm.
But the present Swell-ups in Mexico and Guatamala, which could bring rain to Belize are not really the sort of conditions that are Rainy Season, even though they can result in some very heavy rain, sometimes.
2011 the rainy season suddenly started on the 18Jun and that was real rainy season start.
2012 did not really have a dry season, the rainy season sort of started on 23 April and sort of just continued through to October.
27May2013 we had 156mm ( 6" ) of rain in less than one hour. . . . But the rainy season did not start until 17 Jun.
The one thing we can predict, is that the Belize weather is unpredictable.
The above figures are for Belmopan.