In recent years we have had to rely on man made snow versus what Mother Nature has produced. Interestingly enough, the first snow making machine was in 1934 and the first machine installed and used in Lake Tahoe was in 1966 at Diamond Peak. Over the years technology has improved and now most mountains have the machines to keep at least a number of runs open. If it wasn’t for snow making, Tahoe Ski Resorts wouldn’t be open until after January in some years.
Here is a great article speaking on the history of snow making.