May weather will not likely hit early monsoon onset: Experts

One important factor for a normal monsoon over the country is formation of a so-called “heat low” over Northwest India

May has seen unusual weather in India, marked by frequent thunder- and dust-storms and below normal maximum temperatures over most parts of the country, but experts are convinced that this anomaly will not affect the onset of the Southwest Monsoon, which is expected to arrive early, in the last week of May.

 Monsoon rainfall during June to September, over the country is likely to be “above normal” at 105% of LPA with a model error of ± 5%, IMD has said in its long range forecast. (Reuters)

One important factor for a normal monsoon over the country is formation of a so-called “heat low” over Northwest India. This creates a trough of low pressure that sucks moist air from the monsoon trough . In the absence of this, the monsoon may be deficient.

Right now, there is no heat-low.

“Due to frequent western disturbances and moisture incursion, we are seeing intermittent thunderstorm activity and above normal rainfall. During May, temperatures have largely been below normal or normal over most parts of the country except Rajasthan where a heat wave is being recorded in some areas,” said M Mohapatra, director general IMD.

But that could change.

“During the last week of May we are expecting heat wave area to expand covering Haryana and parts of Uttar Pradesh,” Mohapatra added.

To be sure, the absence of a heat low, or a weak one, could still mean a poor monsoon.

“As forecast by us we are expecting an early onset of monsoon over Bay of Bengal and over Kerala around May 27 but onset cannot be linked to quantum of rain. After onset, how much rainfall will be received needs to be seen and monitored,” Mohapatra explained.

 Monsoon rainfall during June to September, over the country is likely to be “above normal” at 105% of LPA with a model error of ± 5%, IMD has said in its long range forecast.

“It appears that as forecast by IMD, the onset of monsoon will happen much earlier than normal. I do not see the temperatures in May impacting the onset. We cannot say anything about the quantum of rain right away,”added M Rajeevan, former secretary, ministry of earth sciences.

The Southwest Monsoon further advanced over some more parts of south Arabian Sea, Maldives and Comorin area; South Bay of Bengal; some more parts of central Bay of Bengal and some parts of Northeast Bay of Bengal on Monday.

“Conditions are favourable for further advance of Southwest Monsoon over some more parts of south Arabian Sea, remaining parts of Maldives and Comorin area; some more parts of South Bay of Bengal; some more parts of central Bay of Bengal, and some more parts of northeast Bay of Bengal during next 2-3 days,” IMD forecast.

HT reported on May 9 that frequent, slow-moving western disturbances -- essentially, cyclones originating in the Mediterranean Sea and moving east -- in the first week of May have resulted in an unusually cool, rainy start to what is usually the peak summer month for northwest India. Western disturbances normally do not influence the Indian region in summer, they are mainly associated with winter rain.

IMD’s extended range forecast shows widespread rainfall over Kerala and peninsular region between May 22 and 29.

On April 30, IMD forecast a higher number of heat wave days and above normal temperatures over most parts of the country in May.

“Temperatures are high above 46 degree C over some stations, but heat low has not formed yet. Temperatures may drop by 2 to 3 degrees C in next 3 to 4 days. As there are chances of isolated thunderstorm activities over Northwest India,” said Mahesh Palawat, vice president, climate and meteorology at Skymet Weather.

Palawat added that while a heat low is an important component of a good monsoon, so far models do not indicate the monsoon will be impacted. “It is needed for the initial push, however, other prevailing factors, such as active weather systems at the time can compensate for that.”

Data also reflects the unusual May being recorded in northwest India. For instance, Delhi is yet to record a single heatwave day this month. It saw four such days in April. It has also recorded 92.7mm in monthly rainfall — over three times the normal long-period average of 30.7mm. “While April was fairly dry, May has been the opposite with short but intense spells a common occurrence, primarily due to western disturbances impacting one after the other in the form of a cyclonic circulation,” Palawat added, stating such activity is expected to continue this week in the region too.