Storm cumulus clouds. Credit: Ivan Kurmyshov

Cold ring flash flood in Thessaloniki

10 May 2018 06:00–18:00 UTC

Storm cumulus clouds. Credit: Ivan Kurmyshov
Storm cumulus clouds. Credit: Ivan Kurmyshov

Two short-lived and small, but very rapidly developing, convective cells joined together to bring severe flash floods to Thessaloniki, Greece, for around an hour on 10 May 2018.

Last Updated

05 December 2022

Published on

10 May 2018

By Ivan Smiljanic (SCISYS)

The morning's blue skies in Thessaloniki on 10 May turned into grey, rainy and thundery skies around midday, pouring vast amounts of rain in only a couple of hours.

The short but intense life cycle of convective cells around the second largest city in Greece was directly responsible for torrential rain and flash floods.

In particular, two convective cells that grew into full-size cold ring shaped thunderstorms, merged almost above the city and brought heavy rain, lightning and hail episodes. The hilly terrain around the city contributed to the strength of associated flash floods.

They grew very quickly to reach maturity in less than an hour. Not long after merging the heavy downpours destroyed the merged system and it dissipated in less than two hours after genesis took place. In this case the lack of vertical shear in the wind field led to no separation between updraft and downdraft in the same convective cell.

 Stages of convective system development annotated over infrared 10.8 µm channel, supported by HRV channel
Figure 1: Stages of convective system development annotated over infrared 10.8µm channel, supported by HRV channel

This lifecycle is annotated in Figure 1 where in just a few steps the growth of the system is explained. Some 30 minutes after the last time step in Figure 1 the system started to disintegrate.

The full convective episode is captured on the two animated loops of SEVIRI HRV and the IR 10.8µm channel (with enhanced colour scheme).

Figure 2: SEVIRI HRV, 10 May 06:00-18:00 UTC
Figure 3: IR 10.8µm, 10 May 06:00-18:00 UTC

Confirmation for the existence of multiple updrafts (hinting two) can also be found in the cloud-top signature, namely radial waves seen in the High Resolution Visible channel at the last step in Figure 1 (10:15 UTC).

The animated gifs in Figure 4 and 5 illustrate the difference between present capabilities of current Meteosat Second Generation visible channels (nominally at 3km resolution, 15-minute time step) and the future FCI visible channels (nominally at 1km resolution, 10-minute time step), both in a full-disc scanning mode. It is obvious that most of the cloud-top dynamics are lost with reduced spatial resolution, although the difference in the time step is not crucial. In this case the two scenarios are simulated using MSG HRV channel data in rapid-scanning mode (nominally 1km resolution, 5-minute time step).

 Simulated scenario showing 1 km, 10 min time step
Figure 4: Simulated scenario showing 1km, 10-minute time step
 Simulated scenario showing 3 km, 15 min time step
Figure 5: Simulated scenario showing 3km, 15-minute time step

Addtional content

Flash floods and hail hit Thessaloniki trapping tourists in White Tower (Greek City Times)
Flash floods in Thessaloniki, Greece (World Weather/YouTube)
Significant flash flooding in Thessaloniki Greece this afternoon May 10 (Cameleon/YouTube)