Hey luka thanx for your kind words
Yes mountain biking is a great alternative when the weather turns for the worst.
Its a good way to work on your handling skills and see places that tri-bikes never go!
One tip, is to do all your longer rides on the mountain bike outside.
And the shorter faster workouts on the indoor trainer to guaranty good quality!
That way you somewhat get the best of both worlds
And be very ready for next season!