The severe thunderstorm outbreak in Finland on 5 July 2002

On 5 July 2002 a fast propagating severe bow echo formed over Eastern Finland causing wind damage in exceptionally vast areas. Near 400 thunderstorm related reports from the Emergency Response Centre at different locations were received during a 6-hour period. The major axis length of the wind damage area was 450 km meeting the criteria of a derecho. As far as the authors know, the 5 July 2002 case is the highest latitude derecho that has ever documented.

The bow echo developed in front of northward moving 500 hPa trough inside of the warm sector of a secondary low. The system moved from south-southeast to north-northwest on the eastern side of the quasi-stationary front away from the secondary low. The leading edge of the bow echo was oriented perpendicular to the low level southerly wind shear. All proximity soundings revealed strong vertical wind shear in the lowest 2.5 or 3 km and generally displayed constant unidirectional shear above that. The bow echo propagated along 850 hPa èe ridge with a speed that was close to the maximum wind throughout the troposphere.

The mesoscale convective system started as a cluster of thunderstorms and became a bow echo a few hours later. The leading edge of the squall line had a strong reflectivity gradient and the region of stratiform precipitation was behind the strongest echoes. Also the development of a line-end vortex in the western end of the bow echo was evident.

At the most intense stage, the rear inflow notch was visible both in radar and satellite pictures. It was in good accordance with the location of an area of the most severe damage. Moreover, the storm relative winds derived from proximity soundings showed the existence of rear-to-front flow. On the surface charts, a cold pool mesohigh was evident. The cold pool progression speed was calculated to be 23 m/s near the area of the worst damage in Central Finland. The downdraft air appeared to originate from 4 km AGL, where the relative humidity was less than 50 %. This probably led to enhanced evaporative cooling and the intense, fast propagating cold pool.