  We estimate, from first-principles, the rate of inter-party avalanche
involvements. The model suggests that the likelihood of inter-party
involvements is quadratic in the density of parties -- twice as many parties
quadruples the likelihood. The model predicts that when the product of the
party-density and the area of a day's potential avalanches approaches one,
inter-party avalanche involvements will become a substantial fraction of all
avalanche involvements. As a corollary, the relative rate of inter-party
involvements is expected to increase with avalanche size. We argue, with
selected North American inter-party incidents from 2001-2019, that inter-party
involvements are a timely concern. To spur conversation, we enumerate a variety
of strategies that may mitigate inter-party hazard.
