Climatology and extreme cases of sea-effect snowfall on the southern Baltic Sea coast

Heavy snowfall events of possible sea-effect origin, occurring on the southern Baltic Sea coast, are described in this study. The analysed region is not very snowy, with an average of 40 days with snow cover in the winter season and a mean maximum snow depth of 8-12 cm. Twenty-five snowfall events were selected using the threshold of 20 cm/2 days (i.e., 20 cm of snow depth increase during 2 days), and the synoptic climatology, as well as triggering factors of their occurrence, were defined. The bipolar pattern of sea level pressure with a high-pressure over Scandinavia and a low-pressure system southward of it was identified as favourable for sea-effect snowfall on the southern Baltic Sea coast. This pressure field, causing northeastern circulation in the lower troposphere, brings cold air masses over the relatively warm sea basin, and the negative anomalies of air temperature at 850 hPa geopotential level, amounting to -9 degrees C, encompass Scandinavia, as well as central and northeastern Europe. This makes the temperature difference between the air at the 850 hPa geopotential height and the sea surface exceeding 15 degrees C. This large temperature difference establishes favourable conditions for convection in cold air masses, which become unstable while crossing over the warm sea. The formed clouds give snowfall in the coastal areas. In the further part of the study, three sea-effect snowfall cases were chosen for detailed analysis, including atmospheric sounding, radar and satellite images, air parcel backward trajectories, and synoptic maps. They reveal high-pressure wedges associated with low air temperature over Scandinavia and northern airflow over the Baltic Sea. The wind direction and the temperature of air masses are key components in the occurrence of sea-effect snowfall, and global warming conditions seem to reduce the frequency of snow events over the Polish Baltic Sea coast.

Automated summary

This study describes heavy snowfall events possibly caused by sea-effect on the southern Baltic Sea coast. The analysis identifies high-pressure and low-pressure systems as well as temperature differences as key factors in the occurrence of sea-effect snowfall. Three detailed cases are analyzed, revealing the formation mechanisms of sea-effect snowfall.