OK, well, it's not as regionally restricted as I thought.
While contemplating our current raininess, it dawned on me that other parts of the U.S. can have drastically different climates. For instance, take Arizona's dryness and Washington's (state) wetness. Is April particularly wet (or less dry) for them? Is May particularly known for blooming flowers? Would natives of those areas have heard the saying?
And, of course, it probably only rhymes in English, so if other nations even have the same climate patterns, they may not have a saying for it.
Bullgrit