While the factors linked to the incidence of cyanobacterial blooms are well studied, the specific conditions that facilitate transition of blooms to surface scums are less understood. Three Mile Lake (TML), a mesotrophic, softwater lake in south-central Ontario, Canada, experienced cyanobacterial blooms in 2005, 2006, and 2007, but a surface scum event only occurred in 2005. Our objectives were therefore to (1) determine how phytoplankton change seasonally in relation to water quality and other environmental factors; (2) examine modern and historical water quality and meteorological data to explore possible causative factors of the 2005 surface scum; and (3) examine regional predictions of future climate (temperature and precipitation) from present to 2100 to assess the risk of future cyanobacterial scums. Our evaluation suggests that while high phosphorus concentration is a predisposition for the occurrence of cyanobacterial blooms, meteorological and hydrological conditions play an instrumental role in the actual development of surface scums. The 2005 scum event occurred during the fall period when air temperature was unusually high and wind speed and precipitation were low. Consequently, runoff from the surrounding catchment, and thus flushing, was minimal. We believe all these factors interacted to prolong thermal stratification and increase residence time, thereby promoting the proliferation of cyanobacteria and formation of surface scums. Our modeled climate data indicate that the frequency of scum occurrence will change over time as climatic change paves the way for the meteorological conditions that promote scum development.