The article examines the impact of ‘late’ winter and spring synoptic conditions on the mixing regime of a polymictic lake during the open water period. Based on joint analysis of meteorological information and field measurements of the water temperature and dissolved oxygen content, the effect of unusual spring 2016 weather conditions, when the lake was steadily stratified at the ice-off moment, on the formation of the thermal and gas regimes of the lake’s water column and near-bottom layers in the summer was studied. Atmospheric impact evaluation using a modified Monin-Obukhov length scale allows determining the critical values of wind speed and total surface heat flux required for the water column overturn. Series of numerical experiments using one-dimensional FLake model revealed that under certain weather conditions, i.e. warm spring, prolonged under-ice convection, warm and calm weather after ice-off, a lake may lose the spring overturn effect, being stably stratified before ice-off. Model calculations proved that the absence of the spring overturn may negatively affect the lake gas regime during the summer, and in the case of warm, low-wind weather in May and June the duration of bottom anoxia in local depressions may exceed 7-8 months.