Hi Ben,
Thanks for your post; it's hard to say as it's depending on the weather and your abilities, but mostly the skiing season is only in late winter and spring.
If you can board on not too steep icy snow than you can board down all year, in summer at least from the summit (5642m) to Mir (3500m).
When there is much snow like there is now you can board all the way down the valley (2100m), but september can go either way. Unless you are very skilled and can stand the cold wind we advise against skiing until January.
Temperature: slopes higher than the Barrels (average): Daytime - up to +10C, at night -10C.
There is normally not much snow as winter is coming, the wind blows away snow powder.
Not really snow storms, but cold strong winds.
How many people climb Mt. Elbrus in the second half of September?
Not so many, but climbing season is not finished yet (untill mid October). There will be some weeks of perfect weather in September-October (it is always so) but nobody knows the exact dates of this Indian summer..
Hope this helps,
best regards,
Harry