'Mumbai to get heavy showers on Aug 5'

S R Kalsi, additional director general of meteorology, has warned that there would be "very heavy rainfall" in or around Mumbai "any time on or after August 5."

In an exclusive interview to rediff.com correspondent Ehtasham Khan, Kalsi talks about the devastating rainfall that hit Mumbai on July 26. He calls it "a cloudburst," a phenomenon in which there is very heavy rainfall, though for a brief perioud. Expressing surprise over the phenomenon, he says it was "an unusual cloudburst lasting close to 24 hours" which has bewildered weathermen.

What happened on July 26 in Mumbai, when the city received a record 94 cm of rain?

Well, June and July are the months of peak rainfall activity. Especially in July, we have very heavy rainfall in Mumbai and parts of Konkan [in Maharashtra]. This is normal. But the rainstorm which was witnessed in Mumbai on July 26 was highly devastating. It was about 94-95 cm of rainfall at Mumbai airport. This exceptionally heavy rainfall was confined to about 20-25 km of radius. Our observatory located at Colaba in south Mumbai recorded about just 7 cm of rain. Many areas along the Western Ghats recorded 25-30 cm of rainfall near the sea.

Terrific Tuesday: Mumbai copes with a calamity

This was mainly because of the rainstorm and sustained Westerly progress which produce some kind of convergence at the post and that seems to have supported and sustained the strength and direction [of the cloud] which delivered this intense showers.

All the showers that were received that day were accompanied by thunderstorm.

But that kind of intense shower lasted for a day or so and then there was some respite for nearly three days. And in association with another low pressure area that had evolved into a deep depression, increased monsoon activity in Maharashtra by July 30. And again on July 31, there was a substantial increase [in the rainfall] but certainly not of that order. There was about 30 cm of rainfall in Mumbai and other parts of Maharashtra.

But why was the heavy rainfall mostly confined to Mumbai and not all along the Western coast?

Generally they [heavy rains] are also controlled by what is called the cyclonic sheer. The monsoon sheer zone was centered somewhere 21-22 degrees north. In the area where there is lot of cyclonic sheer, such kind of strong vertical velocity could be generated by the convergence in the friction body there. Then only such heavy rainfall occurs. This area was Mumbai. There are other causes also like strong monsoon flow. All these things aided in the very heavy rainfall that we witnessed in Mumbai on July 26 and later.

In the media, the heavy rainfall is being termed as torrential rain. What exactly is it? Was the rainfall on July 26 a cloudburst?

Torrential rainfall in the layman's language is a very heavy rainfall.

So was that a cloudburst?

The type of storm we saw on July 26 had very small dimension of 25-30 km. And that could be termed under the category of what we call cloudburst. The first one [July 26] could be termed as cloudburst because particularly after 14:30 IST, we received very intense showers accompanied with thunderstorm. This clearly shows this was some kind of very violent. It was seen organized on a much smaller scale and therefore in all likelihood it appears it was a cloudburst.

But it was a very unusual kind of cloudburst. Cloudbursts generally do not have that prolonged longevity. They are very short-lived. Whereas in this case, the phenomenon was noticed for 12-18 hours… the intense rainfall. Therefore it was a unique type of cloudburst.

All the ingredients that we see in a cloudburst were available there on that day. It was very highly localized. Compared to this, if you see the rainfall in the last 24 hours [since July 31], there is marked difference. Now it is what we call the large-scale rainfall. It is not accompanied with that kind of convection that we noticed on July 26.

So were the rainfall on July 31 and later an usual phenomenon?

It is very much usual for the monsoon.

What is the forecast now? How will be the weather in the coming few days?

The rainfall which is currently being observed in parts of Konkan is expected to continue. It has already extended into Gujarat and parts of south Rajasthan. It is also moving towards Madhya Pradesh. The heavy rainfall we are having now will continue for another two days. Then there will be some respite. This respite will be just for a day or two. And again there will be very heavy rainfall.

What are states you think will be affected?

All of central India, Maharashtra and Gujarat.

Looking at the unusual event of July 26, do you think there is some change in the weather trend?

No. I don't think so because it was a highly localized system and very sluggish and slow. It cannot be linked to any climate change.

Why was Indian Meteorological Department not able to forecast this devastation?

We did forecast that there will be a large-scale rainfall on July 26. Even in advanced countries, they do what is called time-now-casting. It means telling people what is happening now. There is no forecasting. At the most, it can be detected about an hour before. That much we also did. We had informed on July 26 that something of this sort is happening. Even for Sunday and Monday, we had predicted that there is going to be heavy to very heavy rainfall.

Do you think the present system of weather forecasting and monitoring is equipped enough to tackle such situations?

We are in the process of revamping, up-gradation and modernization. It is under different stages of development.

When are we going to have a very heavy or somewhat devastating rainfall again?

Anytime after August 5.