Storm Poly

By Natasa Strelec Mahovic, Vesa Nietosvaara

On 4 Jul 2023 in the morning hours a process of Rapid Cyclogenesis started over the North Atlantic, southwest of Ireland. The first notion of the onset of cyclonic development could be seen in the WV6.2 micron image in Figure 1, where a dark area is seen over the Atlantic, indicating very dry air protruding from the upper parts of the troposphere, rich on potential vorticity, at the end of a dark (dry) line related to the jet stream. On the other hand, to the right (east) of the dark area there is a bright "leaf" in WV image - a typical appearance in the onset stage of rapid cyclogenesis process. Cyclogenesis occurred within the next few hours. The course of the development and movement of the storm across the Netherlands and Germany can be followed in Figure 2, which shows the Airmass RGB loop. The whole development from a wave formation to rapid cyclogenesis is clearly seen. The zone of maximum winds (Sting Jet) is often located at the very tip of the cloud spiral, which in this case is just arriving at the Netherlands at 05 UTC. High Resolution Visible loop 06-15 UTC (Figure 3) shows the detailed structure of the spiral. The high speed of the clouds around the low pressure centre can clearly be seen.

These types of Atlantic storms, resulting in extreme wind events, are usually forming during winter, most frequently between October and March, so the development of such a powerful wind storm is rather exceptional in July. ECMWF IFS global model maximum wind gust forecast predicted up to 40 m/s gusts at the Dutch coast in the morning hours 06-09 UTC. They coincide well with the frontal spiral seen in the Airmass RGB image at 09 UTC (Figure 4). The forecast of the wind gust speed of the limited area model ICON-D2 (model run from 04 Jul 2023 at 18 UTC)  for 05 Jul at 06 UTC predicted wind gusts of up to 35 m/s over the western coast and even above 40 m/s over the southern part of inland of the Netherlands  (Figure 5 left) At 10 UTC (Figure 5 right) the strongest wind gusts were predicted further to the north of the Netherlands, gusts speed still forecast to reach above 40 m/s.

Most of the severe weather reports related to the storm were indeed severe wind reports, as seen in Figure 6, showing a snapshot from the European Severe Weather Database. Strong winds were also measured over the sea close to the west coast of the Netherlands by ASCAT instrument on Metop-C satellite. Figure 7 shows that sustained NW winds of above 17 m/s were observed over the North Sea towards the Dutch coast.

Rainfall was heaviest in the central and western part of the Netherlands, still not posing a major hazard, with measurements of 20 to 30 mm and locally exceeding 40 mm.

In northern Germany, wind gusts reached 25-30 m/s, causing damages to roofs, uprooting trees and blocking traffic on roads and railways. The strongest wind gusts were observed on the islands in German Bight.

A day after, on 6 July the storm moved further north where it was also viewed by the new geostationary satellite MTG over northern Europe (Figure 8).

Additional content

Two dead as Storm Poly hits Netherlands and 'code red' weather alert urges millions to stay indoors (SKY News)

Rare storm brings death, traffic chaos (Deutsche Welle)

Rare summer storm slamming Western Europe with hurricane-like gusts (Washington Post)