EUMeTrain: A long heavy rain period in the Baltic States and Finland 9-11 august 2005

Synoptic Situation and satellite overview

The first signs to cyclogenesis within the lower layers of the troposphere could already be observed on the 8th August, when a sharp upper trough extending frmo Slovakia to Rumania at 00 UTC moved towards Northeast during the day, and caught up and merged with a frontal area extending from Ukraine towards the Baltic States.

This development on 8th August, preceding the strongest cyclogenesis later on, is shown in the following series of IR 10.8 images. The system originally consisted of two cloud areas; the northernmost cloud area is connected to the frontal area separating very warm, humid and potentially unstable air in the east (over eastern parts of Ukraine) from slightly cooler air in the west. A minor wave had developed within the front; Warm Front shield can be seen over the Baltic States moving NW. Over Ukraine strong convection (MCS) developes on 8 August afternoon; strong Cb development can be seen around 50 N 40E at 12 UTC.

The southern cloud band associated with the upper level trough resembles a rather large-scale comma cloud, approaches the front in the NE and catches up with the northern cloud band by the evening of 8 August. The location of the upper trough and fronts are shown in the image sequence below.

08 August 2005/00 UTC. Meteosat 8 IR10.8 image with upper troughs and fronts shown (dashed magenta: upper level trough; solid red: warm front; solid blue: cold front). 08 August 2005/06 UTC. Meteosat 8 IR10.8 image with upper troughs and fronts shown (dashed magenta: upper level trough; solid red: warm front; solid blue: cold front).
08 August 2005/12 UTC. Meteosat 8 IR10.8 image with upper troughs and fronts shown (dashed magenta: upper level trough; solid red: warm front; solid blue: cold front). 08 August 2005/18 UTC. Meteosat 8 IR10.8 image with upper troughs and fronts shown (dashed magenta: upper level trough; solid red: warm front; solid blue: cold front).

From 9th to 11th of August the upper level trough triggered a deepening of a lower tropospheric low within a boundary separating moist and warmer tropical airmasses in the east from the cooler airmasses in the west.

09 August 2005/00 UTC -Meteosat 8 IR10.8 image
     

The IR10.8 image loop shows the 3-hourly-development from 9th to 11th August 2005. On the 9th and 10th of August a depression can be seen at first deepening and moving from south to north, later staying almost stationary southwest of Finland on the 10th August. Several convective cells are seen embedded into the frontal areas associated to this depression.

The cloud features show the fronts associated with the low pressure. As the wave occludes, a well-defined spiral occlusion cloud band is formed over the Baltic States. The low pressure matures during during 10-11 August, bringing continuous rain and showers in the Baltic States and Finland.


The following chapters enable you to look at the stages of the development as seen in satellite images and basic meteorological fields:


Satellite overview

The movement of the developing frontal wave and the occlusion in 12 hourly steps are discussed in the relevant satellite channels and RGB color combinations.

Meteosat 8 IR10.8 images

Meteosat 8 WV6.2 images

Meteosat 8 Airmass RGB (WV6.2-WV7.3; IR9.7-IR10.8; WV6.2i) images


Basic synoptic parameters

Particularly intensive cyclone development and the strong release of convective instability occurred within the cold front moving from southeast Lithuania to the north approaching Finland (on August, 9) and developing to an occlusion the day after. The depression rapidly deepened and acquired a rather vertical axis. This caused the movement of the cut-off low to be slow, and it further enabled a prolonged rainfall episode to take place.

The development of the waves into a closed circulation and further to a cut-off low can be seen in the basic synoptic fields of the height of 500 hPa and Mean Sea Level Pressure fields.

Geopotential Height and Equivalent Potential Temperature at 500 hPa

Mean Sea Level Pressure


Summary

These basic satellite images as well as meteorological fields show that a deepening low was moving from south-southeast towards the area of interest. While it approached, the movement of the low became slower and the low started to fill gradually. The cyclogenesis and the developing fronts can be seen well in satellite images; the most striking features are the well developed cold front on August, 9, passing the Baltic States and Finland and the development of the occluded cloud spiral, which remains over the area for days. Even on August, 11 the Occluded front is still recognisable. both in meteorological fields and on satellite images.

Often the occluded fronts are dealt as "dying" weather systems. This case clearly shows that even if the deepening of the low pressure would cease and the system would become mature, it doesn't mean that the ascending motion would stop and clouds would dissolve. Occluded fronts may give surprisingly long-lasting precipitation and particularly on slow-moving systems occluded fronts can cause remarkable damage in form of flooding, as was in this case.

The next chapter deals in more detail the meteorological parameters to help analysing the event.