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Severe weather at the weekend: "We only know which locations will be affected 30 to 90 minutes in advance."

2021-07-22T18:03:30.627Z

In Germany, thunderstorms and heavy rain have been announced again. The situation differs significantly from the situation before the most recent disaster, says a DWD expert. What to watch out for now.



Enlarge image

Storm in Essen at the beginning of July 2021: "I don't want to trivialize anything with a view to the places affected."

Photo: Gottfried Czepluch / imago images

SPIEGEL:

Mr. Friedrich, are we facing the next disaster this weekend?

Andreas Friedrich:

There are new storms, yes.

Overall, however, we have a different general weather situation than during the floods caused by low »Bernd«.

Several days of heavy rain and especially continuous rain came together with huge amounts of precipitation over a larger area, which then led to the crossing of rivers and streams and the violent floods.

Now at the weekend we are expecting warm, humid air again and a low, but unlike "Bernd" that will not settle over days, but will be driven by stronger air currents.

SPIEGEL:

What does that mean exactly?

Friedrich:

We get typical summer thunderstorms.

There will be short-term and locally limited storms on Saturday in the southwest and on the edge of the Alps, which will spread northeast in the evening and on Sunday night, i.e. also in the direction of Rhineland-Palatinate and North Rhine-Westphalia.

It can also lead to hail, squalls and heavy rain with more than 40 liters of rain per square meter in one or two hours.

The thunderstorms occur so locally that it will pour and thunderstorm in one place and the sun may already be shining again in the next.

SPIEGEL:

According to recent experiences, alarm bells are ringing for many people when it comes to heavy rain.

Right?

Friedrich:

There is no need to panic, but storms should not be played down either.

In Germany, warm and humid air can be expected well into the coming week and, accordingly, showers and thunderstorms again and again.

We will continue to monitor the situation closely in the coming days and issue a short-term warning if necessary.

SPIEGEL:

How does the average consumer know whether a severe weather warning threatens flooding with serious consequences?

Friedrich:

That is difficult.

Ultimately, the question is how the expected precipitation will affect the water levels of rivers and streams.

That goes beyond the responsibility of us meteorologists.

We can only tell how much rain is coming from the sky.

At least that provides a rough indication of the overall situation.

SPIEGEL:

Which one?

Friedrich:

If you look at the weather maps on the website of the German Weather Service (DWD) or in the warning app before a storm, you can see how the purple-colored areas are distributed.

These are the regions with the highest warning level, where thunderstorms and a lot of precipitation can be expected in a short time.

The overview shows, for example, when thunderstorms only occur locally, i.e. maybe in two or three communities.

Then no major flood disaster caused by tidal waves in rivers is to be expected.

SPIEGEL:

Nevertheless, a lot of water falls in the affected areas in a short time.

What does a severe weather warning mean for municipalities?

Friedrich:

Locally, heavy rain can cause landslides, smaller streams can overflow their banks, and basements can be flooded.

What exactly happens, however, depends on the specific conditions on site.

SPIEGEL:

Do you have an example?

Friedrich:

It makes a difference whether someone lives on a hill without a watercourse or in a valley near a small stream, in which there can be a flash flood or, in extreme cases, a landslide.

Whatever applies: basements should be avoided in such extreme weather situations.

An overloaded sewerage system can fill them up even if there is no river or stream nearby.

For all further measures you should pay attention to the local information provided by the emergency response teams.

They'll probably take a closer look now.

SPIEGEL:

What role does it play in the current risk assessment that it has recently rained a lot in North Rhine-Westphalia?

Friedrich:

Again heavy rain would have negative effects more quickly because the soil is already very saturated with water.

New precipitation then seeps away poorly and runs above ground into streams or rivers.

This is now less explosive than with the low »Bernd« because, as I said, it only happens very locally.

But with a view to the affected places, I don't want to play down anything.

There can be landslides, a small stream can overflow its banks and a mudslide can go through the village.

We have had such pictures over and over again in the past few weeks.

However, we do not yet know whether there will be any more storms at all in the last affected regions at the weekend.

If we're lucky, the thunderstorms just pass.

SPIEGEL:

When will it be clear where the thunderstorms are discharging?

Friedrich:

It won't be much more specific until Saturday morning.

Then we can narrow down the regions affected by the storm and the duration of the events more precisely and then also issue advance information.

We know which exact locations will be affected and to what extent, unlike in the case of the low »Bernd«, but only 30 to 90 minutes beforehand.

To do this, we have to briefly monitor storm cells on the radar.

SPIEGEL:

In connection with the most recent flood disaster, there is currently a lot of discussion about failure of civil protection.

Should the German Weather Service have warned more clearly?

Friedrich:

As a federal authority, we naturally discussed the topic. There will be numerous meetings in the coming weeks, also with the flood control centers of the federal states. They analyze on site, based on our precipitation forecasts, how the water levels of rivers will change. In contrast to the European flood warning system, which we read a lot about at the moment, they also determine the values ​​for smaller rivers such as the Aar.

However, I cannot say what the problem is in the current case, because I do not know exactly how information was provided in the affected locations. But you have to realize that the flood was an event that is to be expected in the region much less frequently than every hundred years. Our climatologists have just analyzed that. This certainly contributed to the fact that the regions were not prepared for a flood of such magnitude.

Source: spiegel

All tech articles on 2021-07-22

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