Convective sea‐effect snowfall, in the form of snowbands, is observed over the northern Baltic Sea annually. Quasi‐stationary snowbands may last up to several days over the sea and, depending on the wind direction, move towards the coast. This study provides climatology of spatial and temporal occurrence of snowbands in Finland for a 48‐year period (1973–2020). We used a set of detection criteria together with ERA5 reanalysis at off‐shore areas and FMIClimGrid gridded observational data for on‐shore areas to find the days favouring snowband formation. Only those snowband days (SBD) when snow reached the Finnish coastal mainland were considered. The total annual number of SBDs in Finland varied from 6 to 40 with an average of 16. SBDs were detected most frequently over the Gulf of Bothnia near the western coast of the country. The largest increase in snow depth (SDI) during an SBD (67 cm/day) also took place on the western coast, although the long‐term mean of SDI (3–5 cm/day) was highest over the southern coast. Throughout the country, November and December showed the highest frequency of SBDs. However, between the periods 1973–1996 and 1997–2020, the seasonal cycle of SBDs shifted 1 month forward from late autumn to mid‐winter as the decrease in the number of SBDs during December as well as the increase during January and February were statistically significant in Finland. In northern Baltic Sea, long‐term increases in monthly means of sea surface temperature (SST) and air temperature at the atmospheric level of 850 hPa (T850) were in line with the decadal changes in the occurrence of SBDs. The increasing trend in SST favours the formation of snowbands but in late autumn the probability for snowband formation decreased because even larger increases in T850 resulted in diminishing differences between SST and T850.

<p>Convective sea-effect snowfall (snow band) can develop in the Baltic Sea when cold air masses are advected from the mainland over a relatively warm open sea. Snow bands may last for several days over the Baltic Sea and, depending on the wind direction, move towards the Finnish coast. To investigate the spatial and temporal characteristics of snow bands in Finland and statistics of conditions favoring their formation, we used a set of detection criteria together with ERA5 reanalysis at a spatial grid spacing of 0.25° (~31 km) for a 48-year time period (1973–2020). Daily changes in snow depth over land areas were studied from FMIClimGrid gridded observational data. Only snow band cases when snow fell over the Finnish mainland was considered. Based on the ERA5 and FMIClimGrid data, we found on average 16 snow band days (SBD) per year. On average, the accumulated snow depths during SBD were moderate, daily mean varied between 2 cm/day to 5 cm/day in the studied regions along the coast of Finland. The largest daily mean snow accumulation (3.5–5 cm) during SBD was observed over the southern coast, but the largest daily snow depth increase (67 cm in January 2016) in the gridded data set was detected in the western coast of Finland. Neither the annual number of snow band days nor the daily snow accumulation revealed statistically significant changes due to large variations between years. The months of November and December showed the highest frequency of SBD. However, the seasonal cycle of SBD seemed to be shifting one month forward as the decrease in the number of SBD during December as well as the increase during January and February were statistically significant in Finland. The long-term changes in sea surface temperature (SST) and air temperature at atmospheric level of 850 hPa (T850) were in line with the changes in occurrence of SBDs. SST increased in all months during 1973–2020 in northern Baltic Sea. In December, when the decrease in snow band days was largest, also the T850 increased indicating less cold air masses occurring in Finland. So, even with increased SST the temperature difference favoring snow band formation might not reach the minimum threshold (13 °C) to produce snow bands due to too warm air temperatures. On the contrary, during January and February the increased SST together with no changes in T850 could favor the formation of snow bands.</p>