Storm Ahti Aspot

Storm Ahti causes damage in northern Finland

21 June 2021, 00:00-23:45 UTC

Storm Ahti Aspot
Storm Ahti Aspot

On 21 June 2021 strong thunderstorms swept over northern Finland causing widespread damage and flooding.

Last Updated

06 October 2021

Published on

06 October 2021

By Petteri Pyykkö (FMI), Natasa Strelec Mahovic (EUMETSAT) and Ivan Smiljanic (CGI)

The thunderstorms swept over the northern regions of North Ostrobothnia and Kainuu, causing widespread damage to trees and localised flash flooding. One person was reported to have died due to a fallen tree.

A frontal system associated with a low over northern Scandinavia moved across the region in the afternoon hours. The storm was later named 'Ahti' in Finland.

The weather at the time of the storm was exceptionally warm and also very moist. The strongest thunderstorms were associated with the cold front moving in from the west. The front led to a formation of a supercell that hit the city of Oulu.

The cell can be seen best in the infrared channel image (Figure 1 right), depicted as the most red, indicating very cold cloud top temperatures. The cell produced a maximum gust of 112km/h at Oulu Vihreäsaari harbour, and a gust of around 85km/h at the Oulu airport.

Figure 1: Suomi-NPP VIIRS, 21 June, 10:09 UTC. Natural Colour RGB (left) and colour enhanced IR 10.7μm channel image (right).

The wind felled many trees and ripped off some roofs. At one point 11,000 households were without electricity. Near the airport 30-35 mm hailstones were reported. In the city of Rovaniemi, around 180km to the north east of Oulu, 65.1mm of precipitation was measured within 24 hours. Most of the precipitation fell in association with the MCS visible in Figure 2 (left and right) and Figure 3 (left and right). The overshooting tops are also clearly visible in Figure 2 (right). In total the storm produced almost 17, 000 lighting strikes in Finland.

Figure 2: NOAA-20 VIIRS, 21 June, 12:38 UTC. Natural Colour RGB (left) and colour enhanced IR 10.7μm channel image (right).
NOAA-20 Day Micro & True Color
Figure 3: NOAA-20 VIIRS, 21 June, 12:38 UTC. Day Microphysics RGB (left) and True Color RGB (right)

The airmass (air temperature at 850hPa, at around 1.5km height in the atmosphere) was very warm for the time and location, up to 16°C, south and east of the fronts. Deep layer shear was 72km/h, locally higher, and 0–1km shear was up to 54km/h; supportive of supercells, large hail and even some isolated tornadoes. MLCAPE was 1000–1800j/kg, 250–400m2/s2 SREH and mixing ratios were 10–13g/kg according to ECMWF model forecasts.

A comparison of the Natural Colour RGBs from Meteosat-11 SEVIRI and NOAA-20 VIIRS in Figure 4, shows the advantage of high spatial resolution images offered by polar-orbiting satellites compared to lower resolution of the geostationary sensors, especially in higher latitudes, as is the case here over Finland.

Met-11 v NOAAA-20 NCOL
Figure 4: Comparison of Meteosat-11 SEVIRI (left) and NOAA-20 VIIRS (right) Natural Colour RGB, 21 June, 12:30 and 12:38 UTC respectively. 

The 1km spatial resolution now offered by the VIIRS sensor in polar orbit, only few times a day, will be available from the FCI (Flexible Combined Imager) on board Meteosat Third Generation geostationary satellites every 10 minutes, starting in 2023. However, the problem of countries in high latitudes being on the edge of the satellite view will still remain.

The insight into MTG capabilities in the visible range is provided through a HRV channel of SEVIRI — with its nominal resolution matching that of VIS channels' resolution of 1km. Naturally, the shape and size of one pixel changes with lat-long position — around the Oulu, Finland, that equals to a pixel size of 4.38 x 1.36km (pixel shape and size in Figure 5).

HRV pixel size
Figure 5: Shape and size of SEVIRI HRV pixel (nominally 1x1km over 0°N and 0°E) at lat/long: 64.86 N/25.41 E (close to Oulu, Finland). Equidistant cylindrical projection was used.

The HRV imagery, even at high latitudes, proves to be useful tool to track the individual cells in the convective line associated with the cold front passage. Also, the high moisture content can be also confirmed through HRV imagery, note the 'boiling' cumulus field over the lower half of Finland, ahead of the cold front (Figure 6).

Figure 6: Met-11 SEVIRI HRV channel 1-day loop, 21 June 00:00-23:45 UTC, 15 min time step. SEVIRI HRV channel as a proxy for VIS channels of MTG FCI, with nominal resolution of 1km.

Persistence and the shape of the storms (and cloud top features as OT) can be seen from looping imagery throughout the day. However, the microphysical signature comes from a different product, and in this case SEVIRI Severe Convection RGB proves that some convective cells had small ice particles on top of the clouds (more yellow shades), signifying intensive processes inside the storms (Figure 7).

Met-11 SCON 21 June 2021
Figure 7: Meteosat-11 Severe Convection RGB on 21 June 2021, 12;00 UTC, showing most intense storms in yellow shades. Source: ePort EUMeTrain.

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