purely coincidental. humans do not possess the ability to control the weather. as a kid i use to beleive i could make it snow because every time i sang a specific song it would snow. i tried it in the summer and it would rain. as i got older i became sceptical, so whenever i sang that song i would record what happened. the result [unsurprisingly] was that nothing would happen normally. the times it would the weather was usually calling for snow/rain anyway. your mind is a powerful thing, and you want something to be true, so it blocks out the times you were wrong, focusing on the handful of times it worked.
granted, some people do believe in weather spells, but that's a major grey area in magick. as i said, humans do not have such god-like power. if we could, i doubt it would be as easy as thinking really hard. to control the weather means you must control weather patterns. you cannot create a small rain cloud in your area, magick is a force of nature, and nature moves clouds around to create weather [among a ton of other things like evaporation and pressure systems] so if weather spells do work, you would have to focus a lot of energy and move weather patterns which wouldn't be a good thing as a little rain in your area could cause droughts or floods in another. best leave it to mother nature, she's been doing her job just fine for millennia.