Impact of Cyclone-Cyclone Interaction on Lake-Effect Snowbands: A False Alarm

In this study, the impacts of two cyclone interactions with the environmental flow and with each other on a heavy winter snowfall event over the Great Lakes region are examined by applying a recently-developed diagnostic tool to a lake-effect snowstorm. This winter snowfall event was a false alarm by the Canadian operational model, in which the predicted lake-effect snowfall at the lee side of Lake Erie doubles the observed. The false alarm involves the interactions of two cyclones, whose relative locations are poorly predicted, leading to inaccurate prediction of their induced winds and interactions over Lake Erie at both low and upper levels. In particular, at 1,000 hPa, the cyclone-induced winds and their interactions over Lake Erie caused colder and drier air over warm-moist open-water lake surface that is favorable for more latent heat flux deriving from the lake in the model than in the real atmosphere. At 500 hPa, the location errors of the model predicted cyclones reversed the cyclone-induced winds over Lake Erie from mainly southerly to northerly, especially during the intense snowfall period. An analysis of the conditional symmetric instability (CSI) index over Lake Erie reveals that the operational model systematically over-predicts CSI, more significantly during the intense snowfall period. As a result, the operational model prediction makes a false alarm of a lake-effect snowfall event with large errors in precipitation intensity. Future improvements in operational model predictions are also suggested. Plain Language Summary We have examined the impacts of two cyclone interactions on a heavy winter snowfall event over the Great Lakes region by applying a recently-developed diagnostic tool to a lake-effect snowstorm. This winter snowfall event was a false alarm by the Canadian operational model, with a prediction of doubled snowfall amount observed over Lake Erie. The false alarm involves the interactions of two cyclones, whose relative locations are poorly predicted, leading to inaccurate prediction of their induced winds and interactions over Lake Erie at both low and upper levels. At the low level, the cyclone-induced winds and their interactions over Lake Erie caused colder and drier air over warm-moist open-water lake surface that is favorable for more latent heat flux deriving from the lake in the model than in the real atmosphere. At the upper level, the location errors of the model predicted cyclones reversed the cyclone-induced winds over Lake Erie from mainly southerly to northerly, especially during the intense snowfall period. An analysis of the conditional symmetric instability (CSI) index over Lake Erie reveals that the operational model systematically over-predicts CSI, more significantly during the intense snowfall period. Because of the above reasons, the false alarm of lake-effect snowfall results.