The Scottish resorts are small in comparison to other European ski destinations, they more than make this up with charm and in good snow some fantastic skiing. Best advice would be to go when the conditions are good, booking weeks in advance could mean at worst no snow and even with snow, a big storm that closes the mountain if you haven't checked the weather. www.winterhighland.info is a great source of this information.  If You're able to go at short notice, that is your best option. 
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You can have a total whiteout one minute, hammering rain the next and glorious sunshine the next after that; it is variable! 

If you can ski in Scotland, you can ski anywhere.