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Failure by numerical models in prediction of monsoon advance over Delhi rare: IMD

Facing criticism over consecutive errors in forecasting the advancement of the Southwest Monsoon over Delhi, the India Meteorological Department (IMD) on Monday said such type of failure by numerical models in the prediction of monsoon over the capital is “rare and uncommon”.

The weather department said its latest model analysis had indicated that the moist easterly winds in the lower level from the Bay of Bengal would spread to northwest India, covering Punjab and Haryana by July 10, leading to the advancement of monsoon and an increase in rainfall activity over the region, including Delhi, from July 10 onwards.

Accordingly, the moist easterly winds have spread to northwest India, the IMD said in a statement.

These moisture-laden winds have led to an increase in cloudiness and relative humidity.

The winds have also led to a revival of monsoon over the region and the occurrence of fairly widespread or widespread rainfall activity over east Rajasthan, Himachal Pradesh, Uttarakhand, Jammu and Kashmir and scattered rainfall over Punjab and west Rajasthan, the Met office said.

However, it did not cause significant rainfall activity over Delhi, even though there was rainfall activity over places around the national capital.

It rained even in the desert district of Jaisalmer and Ganganagar — the two last outposts for monsoon to cover — and in the peripheral areas of Delhi, but not in the capital.

“Such type of failure by numerical models in the prediction of monsoon advance over Delhi is rare and uncommon,” the IMD said.

“It is needless to mention that IMD has predicted well with high accuracy about the advance of monsoon over Delhi in the recent past years and also the advance of monsoon over different parts of the country during monsoon 2021, about four to five days ahead,” it added.

On June 13, the IMD had said the Southwest Monsoon would reach the national capital by June 15.

The wind system continued to advance over the country till June 13, in association with favourable atmospheric circulation and a low-pressure system over the Bay of Bengal after the onset of monsoon over Kerala on June 3, it pointed out.

By June 13, it had covered most parts of the country except northwest India.

On June 13, numerical weather prediction (NWP) models suggested favourable conditions with moist lower-level easterly winds reaching northwest India, which would have helped monsoon advance over most parts of Madhya Pradesh, the remaining parts of Uttar Pradesh, Delhi, Haryana and Punjab in the subsequent 48 hours.

“Accordingly, a press release was issued on June 13 indicating the likely advance of monsoon into Delhi by June 15,” the IMD said.

On June 14, however, the weather analysis based on satellite and NWP model consensus indicated the approach of a trough in the mid-latitude westerly winds, leading to the weakening of the easterly winds over northwest India.

Due to an adverse influence of this mid-latitude westerly winds, the further advancement of monsoon into the remaining parts of northwest India, including Delhi, was not expected, the IMD said.

Accordingly, it issued an updated press release on June 14 indicating that the further advancement of the Southwest Monsoon into the remaining parts of northwest India, including Delhi, would be slow and delayed.

“However, this development of interaction with westerlies could not be anticipated by the weather prediction models,” the IMD explained.

On June 16, another press release was issued indicating a delay in the advancement of monsoon into Delhi and a slow progress into some parts of northwest India.

Accordingly, monsoon advanced into some more parts of northwest India by June 19. It did not progress further due to weak or break conditions from June 20.

Press releases were issued on June 22, 24, 26 and 30 and July 1 indicating such delays in the advancement of monsoon into the remaining parts of northwest India, including Delhi, and weak monsoon conditions over the country. 

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