How does warm weather affect your activity? If you do less activity in warm weather, then that could lead to rising fasting levels.
In my case, cycling to work, I cycle slower in warm / hot weather, so I don't arrive sweaty, but cycle home fast, as I'm going to change clothes anyway.
But could also be caused by change in diet, an infection brewing, increasing insulin resistance due to type 2 diabetes progression (not everyone, but can and does happen) etc.