If we could be 100% certain of weather three weeks in advance, we would be rich. I use this site for averages: http://weather.msn.com/daily_averages.aspx?wealocations=wc:CHXX0116&q=Shanghai%2c+CHN+forecast:averagesd&weai=3. For long range forecasts I use accuweather.com.
Cross-Country Skiing Burns More. Burning about 381 calories in 60 minutes of snowboarding provides a slower caloric burn than some other forms of winter exercise. A 160-pound person burns about 533 calories in an hour of slow-paced cross-country skiing and about 419 calories in 60 minutes of light ice skating. The caloric burn from light snowboarding is equivalent to that of light downhill skiing. Related Reading: How to Estimate the Total Calories You Burned While Running.
Once you are into a good routine, you can build on that and do some extra ski workouts, but of course that depends on how much time you have, and how much time is spent out skiing. You will find that in the beginning 2-3 skiing workouts per week is enough to make a significant difference to your skiing fitness.