The water snowline in protoplanetary disks, the sublimation/condensation surface at ~0.5-10 au around a solar-luminosity star, is considered to have a fundamental role in the formation of large planetary cores and the delivery of volatile ices to regions where super-Earths and small rocky planets are forming. However, both the location and the ice reservoir at the snowline are expected to evolve with time in ways that could speed-up or slow down planet formation. In particular, the unstable accretion phases during pre-main-sequence evolution of young stars produce variable irradiation and heating of planet-forming regions that can quickly sublimate a large part of the ice reservoir during episodic accretion outbursts. This phenomenon has been observed in both protostellar envelopes and strong early-stage FUor outbursts. Recently, by way of JWST's improved spectral resolving power, we are able to distinguish between water lines and conduct a detailed analysis of the radial distribution of water in inner disks, revealing this phenomenon in Class II phase disks as well. Yet, JWST only covers a few water lines that are sensitive to the snowline reservoir at ~170 K, lines with upper level energies of 900-1500 K. PRIMA will observe tens of lines from energies down to ~100 K, providing an unprecedented view of the cold water vapor and ice reservoirs across the snowline. In this paper, we propose to monitor ice sublimation events triggered by accretion outbursts to study processes that may have fundamental implications on planet-forming bodies near the snowline....