Here this should help:
To remove fog or sand storm in any area, follow these steps as your last part of setting up to abuse.
- Save the game in the area you are abusing.
- Change your DS clock's date to one month ahead of what you just saved at in game.
- Enter the game at any time and encounter a wild Pokemon. When the battle ends, the season will change and the fog/sand will disappear.
- Save your game, now you can start abusing on whatever date your seed dictates.