OK so i went into my game and booted up. I ran around for a moment before i saw them standing tall and unmoving in the background. these huge white masses. I thought they were glitched clouds and that i was screwed but it seems that they are just something i had never seen before. Now i've been playing since april of 2014 and i had never seen these culionimbus clouds. They apparently can show through the entirety of august. So if anyone ever sees them, don;t worry they're supposed to be there and will go away in time.
Like these?
It's totally normal, just a thing that happens occasionally. They're about as common as a rainbow, or the super orange sunsets that occasionally happen
They are called "cumulonimbus clouds" and can randomly appear when the sky is blue in August between 9am and 3pm... Like others have said they are quite rare (and only happen during one month) which would explain why you had not previously noticed them! I love having them in my town as they are quite beautiful!