I never get full overcast. It's eighter no overcast at all, or some clouds. In most cases it's just blue sky with thunders and rain. After restart situation tends to change, but it's random, so blue sky can change into 20% overcast, or 50% overcast.
Examples. All changes were made in intel section of the editor:
100% overcast 100% rain 100% lightning:
50% overcast, 100% rain: