The weather on Snowdon changes month to month, day to day hour to hour and minute to minute.  If you're planning to walk up, it's best to get an accurate forecast which you'll find on this webpage from the Met Office, or pinned up outside Penceunant Café on the notice board, courtesy of the National Park and the

Mountain Weather Information Service.
Mountain Safety Advice

Remember especially that temperatures at the top can be substantially lower than at village level, and that you need to be prepared for the worst.  Don't venture onto the mountain in poor weather unless you are an experienced walker or climber, and even then think twice about it.