How could a cluster shower with 100 millimeters of rain completely surprise us? KNMI explains

A day before the mega shower, the KNMI already issued code yellow. Severe thunderstorms are forecast with locally possible strong wind gusts and a lot of precipitation in a short time. The heaviest showers are expected to be 20 millimeters on Friday afternoon.

But when the first rain breaks over land at 5 pm, near Bloemendaal and Velsen, more than twice as much falls there. Shortly afterwards, the KNMI announces code orange for, among other things, North Holland. And then the sky really opens up between Egmond and Zijpe. In a short time, between 80 and 100 millimeters falls there.

A meteorologist from the KNMI explains how we were so surprised by such a huge storm and why it only became so violent when it was above land. “Showers are caused by instability in the air. At the bottom it is warm, and above it cools to such an extent that the air mass becomes unstable and rain falls.”

Clashing winds caused additional problems

And in this case there was something more. Winds collided with each other, pushing the air up even harder and creating showers more quickly. “That this happens is not unique, the amount of precipitation is. It is quite rare that it happens in the summer, but it is possible. The large amounts are often very local. Because the showers did not move on quickly, there could be along one line ( see radar image below) a lot of precipitation is produced,” said the meteorologist.

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The map clearly shows how a part of the province really gets a lot of rain, with 100 millimeters as the peak. While in Amsterdam and the Gooi, for example, hardly any rain fell with 7 millimeters.

Local showers cannot be predicted far in advance

And that, according to the KNMI, is also the reason why such a storm is difficult to predict. “We can expect precipitation. But showers such as on June 18, 2021 have a very local character. So the exact location cannot be predicted far in advance. You can already see that from the large differences on the map.”

Will we see this more often?

Statistically, so much rain in such a short time as here in June 18 occurs once every 500 years. But climate expert Peter Siegmund of the KNMI warns that this does not mean that we now have 500 years of peace. “It can happen again. The chance of it happening locally is of course small, but it is increasing.

He refers to research in which the rainfall between 1990 and 2020 is compared with the period between 1960 and 1990. “There is more rain than in the past. Both during regular and extreme showers. Research shows that it has become much wetter in the Netherlands. Especially in the coastal strip along the North Sea. You can also see that the share of extreme showers in the amount that falls per year is increasing.”

More moisture in the air

Siegmund explains that there is more moisture in the air because the average temperature is getting warmer. “At 1 degree warmer, there is 7 percent more moisture in the air. And more moisture is potentially more likely to reach extremes, that’s just physics.”

But it doesn’t stop there. “It also means that rain falls more quickly. So not only more water falls, but also in a shorter time.”

Coastal area extra sensitive

Research has already shown that due to the warming of the North Sea, the coastal strip is extra sensitive to rain. Not only there is more and more. It has also become wetter to the west of Amsterdam. Exactly what causes it has not yet been investigated, but there are ideas about it. “It may be because it is often warmer there, which leads to rising air movements that cause more rain. Pollution can also promote cloud formation.”