|Rocket Major eruptions almost exclusively occur during Grotto Geyser's short mode eruptions. They usually come 1.5 to 2 hours into the eruption but sometimes can come earlier or later. Often Grotto finishes its eruption soon after a Rocket Major but not always. Rarely, Rocket can have two eruptions during a Grotto short mode eruption.
Other than the time window, there is not much to look for prior to a Rocket Major. A good sign is a dying down of the activity in Grotto Geyser and larger splashing from Rocket. Once a Rocket Major starts, it takes over the entire eruption from Grotto and Grotto falls completely silent. Towards the end of the major eruption, Grotto slowly starts to come back to life.