'Thunderstorm is the most challenging segment of weather prediction'

In the month of April and May, north India and northwest India faced strong thunderstorms that killed over 380 people and caused huge financial losses. Thunderstorms in summer season in the region aren’t an unusual weather activity. Then why were these storms so devastating? NIDHI JAMWAL caught up with K J RAMESH, director general of meteorology, India Meteorological Department to understand the nature of thunderstorms, forecasting these severe weather events and the need for faster dissemination of information.

Why is summer season in north India and northwest India associated with thunderstorms and dust storms?

Thunderstorms are expected in pre-monsoon months of March and April that get extended to June in north India. The reason is, though South India starts to see the arrival of monsoon from Kerala in June, it takes 45 days for monsoon to cover north India fully. So, till monsoon arrives, heating continues in north and northwest India. In this hot weather season, when episodes of heat waves (consecutive three to four days of intense heat) are common, thunderstorms or dust storms are a natural heat-abating mechanism. If moisture is available, these storms become thunderstorms; and if there is no moisture, they take the shape of dust storm. This is a usual process during pre-monsoon days.

Why did we have such widespread and severe thunderstorms this summer in north India?

Normally, these thunderstorms and dust storms are highly localised phenomena with a radius of 15 square kilometres only. Most thunderstorms happen after 2.30 pm or early evening in response to three-four days of heating. But, if there is moisture incursion and some other factors, there can be a series of thunderstorms along a particular trajectory. Life cycle of a thunderstorm is maximum four to six hours from genesis to intensification and dissipation.  

Peculiarity of this season’s thunderstorms is the heating in north and northwestern India that has coincided with the movement of Western Disturbances (extratropical storms originating in the Mediterranean region)—which is the main system of bringing rain in non-monsoon period in north India. The western disturbance draws moisture from the Arabian Sea and this moisture incursion leads to intense thunderstorm activity. For instance, we had three such cases one after the other in a sequence. First event was on May 1-2, then May 7-8, followed by May 13-14. These thunderstorms move from west to east and there is more moisture incursion from the Bay of Bengal.

Can the changing pattern of western disturbances be blamed for the severity of recent thunderstorms?

The frequency of western disturbance was less in winter this year as compared to their climatological number. Now they are happening back to back. These western disturbances are global circulation moving from west to east throughout the year and affect the entire globe. 

What parameters are used by the IMD while forecasting thunderstorms? How precise is your thunderstorm warning?

IMD has different forecast assessment systems. Based on a coupled oceanic atmospheric model, which is run once a week for next two weeks, we issue an ensemble outlook that broadly conveys how the weather—temperature, rainfall— will be across the country in the next 15 days. This 15-day outlook is issued once in a week. When the outlook shows some pockets of above normal temperature, it means some heating will take place.

Apart from the 15-day outlook, every day we generate a forecast model for the next one week, which is called the global forecast model. It tells us how the scenario is building up. We get to know where exactly the thunderstorms are developing, how they are moving, and which ones could ultimately create a severe weather event. It also helps us know where there will be rainfall activity in the next 24-48 hours and wind prediction for the next 24 hours.

We follow colour codes to define thunderstorms. Wind speed of less than 35 kmph is light thunderstorm with yellow code; between 35 kmph and 70 kmph is moderate thunderstorm with orange code. More than 70 kmph is severe thunderstorm. Areas where we expect intense precipitation has red code. In all 36 meteorological sub-divisions, we give colour codes for the next four to five days.

But often, IMD’s alerts don’t come true?

Today, we may assess something at yellow code, but tomorrow, with a new forecast and changing conditions, it may get intensified to orange code. Similarly, an orange code can get diluted to yellow.

To give precise weather alert, we have a nowcast (weather in next three hours) desk in our forecasting offices, both in Delhi as well as in state centres. Nowcasting is done simultaneously with Doppler Weather Radars and satellite data. Every 10 minutes, the radar gives a scan of estimated rainfall from that particular cloud passing over a particular location and which way it is expected to move within 250 km radius of radar. If there is no western disturbance, replenishment of thunderstorm will not take place and it would be a localised thunderstorm.

During severe dust storm in Rajasthan on May 2, the Jaipur radar was not functioning. It is a common complaint that radars are down when needed.

Whether radar is there or not, satellite is there. We are doing simultaneous monitoring by both radar and satellite. Radars are anyway not covering the entire country right now. Only 24 Doppler Weather Radars are currently functional in India with three more expected to be installed. Additional 11 radars are in the process of being commissioned across the country by the IAF (Indian Air Force) meteorological directorate. We have already tendered for smaller radars (50-70 km radius) for the hills. Four are expected to come up in Jammu & Kashmir, three in Himachal Pradesh and three in Uttarakhand. We are working to get a scheme of radars in northeast India approved. Slowly, the entire country should get covered by Doppler Weather Radars.

During the recent thunderstorms, people were caught unawares and several of them died. Why do the IMD warnings don’t reach the public and save lives? Can IMD modify its dissemination system? 

Nowcast gives precise information about area and intensity of thunder activity. Based on the possibility of severe weather event in their area, IMD’s state centres send alerts to designated agencies. The colour code warning is issued at least 24 hours in advance. Our centres inform state authorities and emergency response groups through SMS and WhatsApp groups.

Between the thunderstorm of May 2 and May 7, much transformation has taken place in the manner information is disseminated. Through coordinated efforts of National Disaster Management Authority, it was decided that as soon the state centres of IMD issue alert through SMS, the same information would be immediately passed on to media—radio, print and electronic—so that the alert and dos and don’ts can reach wider audience.

How do you intend to change the perception of people towards IMD? It is a common refrain that if IMD says it will rain today, it definitely won’t.

I don’t think that scenario exists now. Most challenging segment of prediction is thunderstorm as it is a very local event. Unlike two-three years back, we now have a combination of fine-scale modelling and round-the-clock monitoring through satellite and radars. Sending SMSes to local authorities and building communication channels with updates associated with the event is a new thing we have built in the last one-and-a half years. Even the concept of colour codes was implemented two years back. Before that, only general warning message used to be issued. Things are improving with increasing computing power.

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