Tree fruit species and cultivars have discrete thresholds of chill and heat, quantified with environmental models that relate temperature accumulation over time to tree fruit development. The accumulation of heat in the spring progressively leads to the deacclimation and development of reproductive buds of Prunus sp. Phenology scales have historically been used to estimate and relate this developmental progression with lethal temperatures to facilitate orchard management practices. For sour cherry, however, ecodormancy release is accompanied by an approximate 20°C loss of hardiness before clearly distinguishable changes in external bud phenology occur. For this reason, we aimed to characterize the physiological changes of ‘Montmorency’ sour cherry floral buds during their transition from endo- and ecodormancy through growth resumption to determine how well phenology relates to shifts in critical survival temperature of buds. We present a developmental timeline of the first preanthesis changes in floral buds as they relate to lethal temperature (LT50) changes. A simple Growing Degree Hour (GDH) model predicted freeze sensitivity explaining 93% of the variation in LT50 where overlapping phenological changes were difficult to separate. Our results can aid producers to manage frost protection and enable a better prediction of freeze susceptibility.