Extreme thundershowers over parts of Latvia

By Santa Šmite (Latvian Environment, Geology and Meteorology Centre)

In Sigulda the amount of precipitation reached 123mm in six hours (Figure 1). This was an extreme weather phenomenon for Sigulda, where it was the largest amount of precipitation ever recorded, and also for the whole Latvia – it was the sixth largest amount of precipitation in 12 hours on record.

The most precipitation recorded in Latvia was approx. 160 mm in six hours in Ventspils (in the north east) on 9 July 1973.

Warm and moist air prevailed in Latvia for several weeks. This resulted in development of an intensive convective process over Latvia.

Early in the afternoon (around 12:00 UTC) there were only a few storms present, but their further development was very intense and rapid during the following hours.

An area of weak pressure gradient with weak vertical wind shear contributed to slow motion of clouds and resulted in extremely heavy showers over some parts of Latvia.

The Natural Colour RGB animation, 29 July 10:30–15:00 UTC shows storms developing from midday and reaching a peak during the afternoon. Cyan coloured storm clouds can be seen on the RGB images, which denote ice clouds.

In the Convection RGB animation, 29 July 10:30–15:00 UTC a large cloud system can be seen over Latvia. The orange colour indicates deep vertical development and small ice crystals, which are common indicators of thunderstorm severity.

The animated GIFS of the cloud droplet effective radius, cloud top temperature products and precipitation rate product from MSG Cloud Physical Properties clearly show the growth of the cloud during the day and the intensification of showers.

However, due to considerable cloudiness over Latvia it was quite difficult to follow the development of these severe thunderstorms by using satellite observations only.

The radar product, max dBZ with a radius of 250km around the radar location at the Riga Airport, shows a very small, but intense feature over Sigulda, but there were no obvious indicators of an extreme storm.

The storm caused considerable damage in the Sigulda municipality — intense flows of water and mud slides with falling trees swept downhill in the river valley and destroyed roads on their way. Drainage systems overflowed, which resulted in flooded roads, yards, cellars, gardens and cornfields. Children were reported to be swimming in the meadows and tourists who were camping were caught unawares. The damage of these very local and very severe thunder-showers was estimated to be tend of thousands of euros.

ePort analysis

Several NWP parameters from the EUMeTrain ePort confirmed great potential for the development of severe thunderstorms:

• Steep lapse rate (more than 6.5°C/km) indicated atmospheric instability.
• Very high CAPE (more than 3000 J Kg-1 (violet and purple) represented enough energy to produce severe thunderstorms
• The Global instability Index (GII) had high values.

A complex view on these parameters and analysis of the pressure and moisture field revealed significant indicators for the development of hazardous weather.