On the main characteristics of synoptic weather conditions associated with thunderstorm activity during the months of July and August in the city of Thessaloniki (Northern Greece)
The evolution of synoptic scale atmospheric circulation at least two days before the occurrence of thunderstorm or shower activity with high precipitation amounts is examined for the months of July and August for the area surrounding the city of Thessaloniki in Northern Greece. In a period of sixteen years (1985–2000), seventeen cases with precipitation amounts greater than 10mm were found in these two months. The most important factor for the occurrence of the above activities was the approach of cold air, usually of mP origin, into Northern Greece. The advection of cold air was examined at the 700 hPa level. This cold air was advected toward the southern Balkans either from Northwestern Europe, or more directly from higher latitudes. In a few cases cold air that had initially been advected into the Central Mediterranean region finally moved eastward affecting Northern Greece. From these seventeen cases, the first four with the highest precipitation amounts (all exceeding 20 mm) were analysed in detail with the help of weather maps and radiosonde data available for the station at Micra. For these four cases, apart from the detailed analysis of the cold air advection, an attempt was made to investigate whether in the lowest part of the troposphere, the area of Northern Greece and the interior of the Balkan Peninsula to the north, was dominated by warm and humid air in the twenty four hour period that preceded each thunderstorm outbreak. For this purpose the evolution of the spatial distribution of the equivalent potential temperature (e) at the 850 hPa level was determined for the two or three preceding days. Finally, the presence of mesoscale disturbances, that could be considered responsible for triggering the ascending motions that are necessary for the initiation of the thunderstorm development, was investigated for each case.