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Case study of a cold air outbreak over the Baltic Sea, 2-3 November 2006 Galina Bogdanova, Latvian Environment, Geology and Meteorology Agency, Aviation Forecast Division, Riga, Latvia [email protected] Heavy snowfall along the Latvian coast in autumn and winter is very typical during situations with persistent north-westerly flow of a cold air over the ice-free areas of the Baltic Sea and the Gulf of Riga. These snowfalls cause traffic hazards and communication problems. A proper understanding and forecasting of these events are considered essential. The event of 2-3 November 2006 was selected. Synoptic situation The weather situation is illustrated by NOAA IR image 2006-11-02/20:51 UTC and a set of analysis and prognostic maps. Mesoscale convection bands forming within the cold air mass behind a cold front over warm sea water of the Baltic Sea and the Gulf of Riga were particularly strong along trajectories ending over the coast areas. The following significant weather events 2-3 November 2006 were observed in Latvia : snow showers with visibility of 500-1000m and high snowfall accumulations in the central part, gusty winds. Convective precipitation were confirmed by measurements from the radar at Riga. SMHI Sea surface temperature 2006-11-02 2006-11-02/20:01-NOAA IR image NWP forecast problems The main convective precipitation band over the Baltic Sea was very well forecasted and precipitation accumulation near the Latvian west coast were forecasted in agreement with observations. However, precipitation amount was not so good forecasted for the Gulf of Riga and the central part of Latvia. Convective activities along the trajectories of cold air over the Gulf of Riga in comparison to over the Baltic Sea are often forecasted not so well in forecast models used by forecasters in Latvia. The orography, sea-ice data and sea-surface temperature are very important initial data for model systems to be able to forecast quite well mesoscale convective precipitation in these weather situations. Unfortunately, at present high resolution model outputs are not available for forecasters in Latvia. In case of a cold air outbreak over the Baltic Sea for forecasting heavy snowfall the following parameters shell be considered: Relative top.+TA the centre of a thickness trough (H500/1000) over the Baltic Sea and maximum of synoptic scale cold air advection, T difference significant temperature difference between water surface and the air for severe situations not less than 810°C. Radar and satellite data are essential for short-range forecasting and nowcasting.