Precipitation runoff from the peak and meltwater from unnamed glaciers on its slopes drain into tributaries of the Lillooet, Bridge, and Hurley Rivers.

The mountain's name was submitted by this first ascent party to commemorate Thiassi, the god of storms according to Norse mythology.

[5] Most weather fronts originate in the Pacific Ocean, and travel east toward the Coast Mountains where they are forced upward by the range (Orographic lift), causing them to drop their moisture in the form of rain or snowfall.

As a result, the Coast Mountains experience high precipitation, especially during the winter months in the form of snowfall.

The months July through September offer the most favorable weather for climbing Thiassi.