May and June weather can be mischievous and misleading. Many days during these two months of the year will begin with a clear sky in the morning, but then turn cloudy around or shortly after midday, and often as not, produce scattered showers or thundershowers during the afternoon. It isn't that every day works out this way, but many days do, and May and June are the two months most likely to feature this cycle of morning promise and afternoon disappointment.
The cause is afternoon thermals, columns of rising air, which are strong on sunny days this time of year; strong enough to often generate afternoon clouds and showers or storms. As the summer progresses, the fields will become covered in crops and the solar angle will diminish, causing a weakening of the daily thermal activity, and a bit more stability in the weather.