Theor. Appl. Climatol. 92, 133–140 (2008) DOI 10.1007/s00704-007-0322-4 Printed in The Netherlands Institute of Physical Geography and Environmental Planning, Adam Mickiewicz University, Pozna n, Poland Synoptic reasons for heavy snowfalls in the Polish – German lowlands E. Bednorz With 4 Figures Received January 11, 2007; revised April 4, 2007; accepted April 5, 2007 Published online July 9, 2007 # Springer-Verlag 2007 Summary Daily circulation patterns responsible for heavy snowfalls in the Polish – German lowlands were analysed. Composite maps of sea level pressure (SLP) and 500 hPa geopotential height means and anomalies were constructed for the days with an increase in snow cover depth by 5 cm. Contour maps show negative anomalies of SLP and 500 hPa level over central Europe, indicating a low pressure system. Strong positive anomalies of SLP appear over Scandinavia and the northern Atlantic with the centre of positive anom- alies located over Iceland. Weaker negative anomalies are observed in the Azores region. This confirms the strong negative correlation between snow cover appearance and the North Atlantic Oscillation index in Europe. The days with heavy snowfalls were clustered using the Ward’s method. Three types of circulation patterns were distin- guished, each of them characterised by a low pressure sys- tem over central Europe. Type 3 represents the northern position of the low with its centre over the Baltic Sea, Type 2 shows the southern position of the low with its centre over the Adriatic and the Ionic Sea and Type 1 represents the low location between the two previous patterns with a wide meridional trough over the Atlantic. 1. Introduction In central European lowlands winter is consid- ered to be the most changeable season. The di- versity throughout the season may be described by cold periods alternating with periods of mild whether and by short- or long-lasting snow cover. Heavy snowfalls, which may occur at any time during the cold season, have a large economic impact. They can cause traffic hazards, com- munication problems and even paralyse commu- nity life. Short but strong snow storms occurred in Germany and western Europe at the end of November 2005 and their economic outcomes justify research into the heavy snowfalls that occur in this area. Therefore, it is important to identify the synoptic mechanisms for episodes of high snow accumulation. Snow occurrence is de- termined by air temperature, precipitation and, indirectly, by atmospheric circulation. The large-scale circulation patterns associated with air masses of specific temperature and hu- midity influence winter snow conditions. The relationships between the North Atlantic Oscilla- tion (NAO) index and the annual number of days with snow cover or snow depth are statistically significant in central Europe but this relationship diminishes eastwards (Clark et al., 1999; Falarz, 2002; Bednorz, 2002, 2004). Gutzler and Rosen (1992) found that the NAO-like teleconnection pattern is the only such pattern exhibiting appre- ciable correlations with anomalies of snow cover extent over Europe. Clark et al. (1999) revealed significant decreases in snow cover extending over central Europe during positive NAO extremes. Changes in European snow extent have also been observed during other circulation patterns, such as