Latest data from the MAK (Interstate Aviation Committee) reports that the aircraft climbed to 12,400 m. Maximum allowed weight at 12,100 m is 85 t. Take-off weight must have been around 93.5 t, given the distance from Anapa to St. Pete and 160 passengers. This would mean a weight of about 88at at the time of the crash.
The thunderstorm was reported as having been a heavy one, reaching up to 12-13 km.
It looks the aircraft stalled and entered a flat spin when it encountered turbulence at low speed as it was too high up for its weight.