Weather – June 2006, Vol. 61, No. 6 159 Reijo Solantie 1 Kim Frisk 2 Adina-Eliza Croitoru 3 1 Finnish Meteorological Institute, Helsinki, Finland 2 Finnish Meteorological Institute, Tampere, Finland 3 Babes-Bolyai University, Cluj-Napoca, Romania It is useful for weather forecasters to be able to predict the risk of exceptional weather events and to consider the processes involved, not only from the point of view of the upper air dynamics but also the role of the underlying surface. In this study, three cases of severe cloudbursts in Finland are analysed. The aim is to identify synoptic situations that represent the most danger- ous occasions to be expected in Finland and adjacent countries around the northern Baltic. The first case is characteristic of summer when cloudbursts are most likely. The second case broke records for rainfall at the beginning of the summer. The third case was particularly interesting because extremely high 1-, 5- and 14-day rainfall records were set unusually early in the summer season. Such intense rainfall is only possible following an unusually warm and prolonged heatwave because only then are the water bodies surrounding and within the Finnish mainland capable of feeding appreciable moisture into the convective process. In each case the underlying surfaces fed sensible heat and moisture into the air masses, causing heat-low development, and to some extent steered the movements of the fronts in the lowest part of the atmos- phere. Bodily lifting of air due to topo- graphic features may also be a crucial factor in the triggering of convection through deep layers. The surface and upper analyses over Europe were mainly prepared from those of the Deutscher Wetterdienst (the German weather service). Other analyses were pre- pared using mainly observations of the Finnish Meteorological Institute. Locations referred to in the text are shown in Fig. 1. Case 1 — two consecutive frontal waves in summer On 3 August 1980 a 10-day period with maximum temperatures above 25 °C and clear skies had just ended. A rather warm and moist air mass was still over Finland, but an anticyclone was being replaced by a narrow warm sector into which moisture was fed by a southerly wind. Cloudy skies prevented further high daily maxima. During the afternoon of 3 August, an upper trough moved from the west over the Finnish mainland, causing the lapse rate to steepen. The sounding at Jyvaskyla (62.4 °N, 25.7 °E) at 12 UTC on 3 August (Fig. 2) shows that convection at the saturated adiabatic lapse rate extended to heights up to 10 km. Additional ascent of the warm air was caused by the progress of two cold fronts orientated south-east to north-west, moving in opposite directions (one north- eastwards, the other south-westwards), and thus merging (Figs. 3 and 4). A depression formed in central Finland where these fronts merged and was intensified by heat-low effects over the Finnish mainland. Additional moisture was loaded into the system by the adjacent water-bodies. Evaporation was particularly intense because the surface water temperature was extremely high, typically 22–24 °C in the large lakes (National Board of Waters 1983, p.148). A cloudburst occurred close to the cold front during the afternoon. The highest precipitation on 3 August was 91 mm at Kannonkoski (63.0 °N, 25.2 °E) in central Finland. Four days later, an occluding warm sector, opening to the north-east, was situated over Major summer cloudbursts in Finland: synoptic origins and impact Fig. 1 Map of the study area. The Finnish archipelago and Lake Finland are shown by broken lines. Solid lines indicate regions having the highest frequencies of heavy 1-, 5- and 14-day precipitation totals 1959–2004 (A = coastal regions and B = the heartland of Finland). Sounding stations are denoted by open circles: SOA = 02836 Sodankylä; JYA = 02935 Jyvaskyla; JOK = 02963 Jokioinen; SOR = 22802 Sortavala. Stations with precipitation records referred to are denoted by crosses: 1 Kannonkoski; 2 Perho, Salamajärvi; 3 Lehtimäki, Lippo; 4 Kuopio; 5 Kerimäki, Ylä-Kuona; 6 Lieto, Tammentaka, and 7 Kankaanpää, Niinisalo Fig. 2 Sounding at Jyvaskyla (see Fig. 3) on 3 August 1980 at 1200 UTC showing dry-bulb temperature, dew point, the level of free condensation, moist-adiabats, dry-adiabats, and wind vectors