A methodological study of modeling runoff from rain-on-snow events was conducted using the northern Swedish city of Kiruna as a test case, with respect to physiographic, drainage system, and the current and projected future climate data. Runoff simulations were carried out with the PCSWMM, which is a geographic information system (GIS) supported version of the U.S. EPA Storm Water Management Model (U.S. EPA SWMM5) developed by Computational Hydraulics International (CHI). In total, 177 simulations were run covering four scenario categories: eight rain events, three climates (the current and two projected), three soil infiltration rates, and five snow water equivalent (SWE) values. Simulation results were analyzed with respect to influential rainfall/snowmelt/runoff factors and the noted differences were statistically tested for significance. Result analysis revealed new findings concerning the differences between runoff generated by rain-on-snow and summer thunderstorm events. In particular, it was noted that a relatively frequent rain-on-snow event, with a return period of 1.4 year, caused fewer flooded nodes and surcharged pipes in the catchment sewer system, but almost five times greater runoff volume, when compared to the same drainage system performance indicators corresponding to a 10-year event occurring in the summer. Depending on the physical characteristics of the snow cover, among which the depth appears the most important, rainwater and snowmelt may be retained in, or released from, the snowpack, which acts as a dynamic reservoir controlling the generation and release of runoff. Smaller snow depths produce smaller volumes of melt, smaller storage capacity and less effective insulation of soils, which may freeze to greater depths and become practically impervious, until the process of soil thawing has been completed. The impacts of climate change in the study area, described by increases in precipitation and air temperatures, are likely to cause more frequent runoff problems attributed to the future rain-on-snow events. Even though the runoff tendencies reported here reflect the characteristics of the study area and climate, they suggest the need to consider rain-on-snow events in sewer design and storm water management in regions with seasonal snow covers, certainly with respect to runoff volumes.