Mesoscale convective system (MCS) hits Helsinki

By Vesa Nietosvaara (FMI) and Jochen Kerkmann (EUMETSAT)

A remarkable period of heavy rain was experienced in the Baltic countries and Finland during the second week of August 2005. During this period a series of waves formed in moist and warm southeasterly air stream, bringing daily rainfall to large areas of north-east Europe. In Finland the most remarkable events were those connected with the passing of the cold front during the evening of 9 August. The event was associated with intense precipitation, lightning and severe gusts of wind. These phenomena caused big problems affecting the running of the IAAF World Championships of Athletes in Helsinki that evening.

The Meteosat-8 images and animations below show a cut-off cyclone over the Baltic States and the associated cold front approaching Finland from the south. A convective cloud quickly develops south of St Petersburg and acquires a typical oval mesoscale convective system (MCS) appearance as it moves towards the southern Finnish coastline. At 16:00 UTC (see HRV image), the MCS is fully developed and located right over Helsinki. At this stage a pronounced overshooting top is visible in the centre of the MCS.

Further thunderstorms can be seen over south-west Finland (a mini 'squall line') and in form of a line of deep convective clouds in the St Petersburg area. The Airmass RGB composite image (lower left) shows the typical intense red colour of a cut-off low with high Potential Vorticity (PV) values, indicating that the cyclone over the Baltic states will persist for some more days (usually, this red colour is less intense when a cut-off cyclone is in the final, decaying stage). Additionally, the Dust RGB composite image (lower right) shows the airmass difference across the cold front that stretches from the Gulf of Finland to the Black Sea, with relatively moist air ahead of the cold front.