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Informist, Friday, Jun. 6, 2025

LATEST DEVELOPMENTS

The spread and intensity of rainfall over the country declined over the past week as the advancement of southwest monsoon slowed down. The northern limit of the monsoon has been stuck over Mumbai since May 26, while the eastern arm has been stagnant over Sikkim and sub-Himalayan West Bengal since May 29. As a result, central and south India have received below-normal rainfall during the week.

From Sunday to Friday, east and northeast India received 36% above-normal showers, while northwest India received 63% above-average rainfall, data from the India Meteorological Department showed. However, central and southern regions received below-normal rains at 34% and 65%, respectively.

Northeast India continued to receive very heavy to extremely heavy rainfall during the week. This led to several floods, flash floods, landslides, and rockfalls in the area, which resulted in severe damage to infrastructure and loss of lives, several media reports said. The intensity of rainfall over the northeast has been declining since Thursday and is likely to decrease further. Maximum temperatures in northwest India are likely to rise by 3-5 degree Celsius in the next four days, the India Meteorological Department said.

The strong westerly stream, normally seen after the onset of the monsoon, is missing all along the west coast, from Kerala to Konkan, private weather agency Skymet said. The north-westerly winds currently running nearly parallel to the coast do not contribute to any monsoon bursts.

A revival of the southwest monsoon over the country seems likely in the middle of next week, when a monsoon system is expected to form over the Bay of Bengal, and a precursor cyclonic circulation will appear Tuesday, the private weather agency said. While heavy rainfall in northeast India is likely to resume from Jun. 10-15, a revival of the monsoon over most parts of the southern peninsula is seen between Jun. 12 and Jun. 17, it said.

Under the influence of several weather systems, a wet spell is likely over most parts of the country in the next seven days, the weather department said. Maximum temperatures in central and east India are expected to rise in the next five days.

The following table gives sub-division-wise rainfall during Jun. 1-6 in the country (in mm): 

MONSOON PROGRESS

This year, the southwest monsoon hit the Kerala coast on May 24, eight days ahead of its normal date of Jun. 1, and by May 29 the monsoon current covered southern and northeast India entirely, and parts of east and central India. Normally, beginning Jun. 1, the southwest monsoon advances northwards and covers the entire country around Jul. 8. The withdrawal of the monsoon starts around Sept. 17, and it withdraws from the entire country around Oct. 15.

ONSET CRITERIA

At present, the department uses criteria that was adopted in 2016 for declaring the onset of the monsoon over Kerala, which is based on the daily rainfall at 14 stations in Kerala and the neighbouring area, along with wind-field and outgoing long-wave radiation over the southeast Arabian Sea.

If, after May 10, 60% of the 14 designated stations--Minicoy and Amini in Lakshadweep, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, and Kannur in Kerala, and Kudulu and Mangalore in Karnataka--report rainfall of 2.5 millimetres or more for two consecutive days, the onset over Kerala will be declared on the second day, provided the wind-field and outgoing long-wave radiation conditions are met.

The outgoing long-wave radiation measures the amount of terrestrial radiation released into space and, by extension, the amount of cloud cover and water vapour that intercepts that radiation in the atmosphere.

The criteria emphasise the sharp increase in rainfall over Kerala along with the setting up of large-scale monsoon flow and extension of westerlies up to 600 hectopascal. 

However, the department declares the monsoon onset or progress dates for other regions operationally in a subjective manner, considering the sharp increase in rainfall and its characteristic sustenance for a few days and associated changes in the atmospheric circulation features.

LONG-RANGE FORECAST

The India Meteorological Department has projected the southwest monsoon to be above normal at over 106% of the long-period average of 870 millimetres, which is based on historical data from 1971 to 2020. The forecast assumes a modelling error of plus or minus 4%, according to the second-stage long-range forecast for the Jun-Sept southwest monsoon season. The department had earlier predicted rainfall for Jun-Sept at 105% of the long-period average with a model error of plus or minus 5%.

The agency said that on average, it expects above-normal rainfall over most parts of the country this year except northeast India, where rainfall is expected to be lower than normal. Central and south peninsular India are expected to receive above-normal rainfall at 106% of the long-period average, while rainfall over the northwestern parts of the country is likely to be normal at 92-108% of the long-period average.

According to the bureau's forecasts for the season, there is 59% chance of above-normal monsoon rains and 31% chance of normal showers. The chances of below-normal precipitation stand at 8% and those of deficient showers are just 2%. 

The weather bureau predicts rainfall in June to be above normal at 108% of the long-period average. Normal to above-normal rainfall is very likely over most parts of the country in June. However, some southern parts of peninsular India and parts of northwest and northeast India are expected to record below-normal rainfall.

The latest climate models forecast that weak negative Indian Ocean Dipole conditions are likely to persist during the southwest monsoon period. The Indian Ocean Dipole is the difference in sea surface temperatures between the western and eastern tropical Indian Ocean. Usually, a negative Indian Ocean Dipole leads to less rainfall in India, while a positive dipole results in better rainfall.

The weak negative conditions will not affect the country's southwest monsoon as much as the El Nino-Southern Oscillation, India Meteorological Department Director General Mrutyunjay Mohapatra said. The weather department sees neutral El Nino-Southern Oscillation conditions prevailing over the equatorial Pacific region during Jun-Sept. 

The El Nino-Southern Oscillation refers to the fluctuation between El Nino and La Nina conditions in the Pacific Ocean. El Nino, which means Little Boy in Spanish, is a climate pattern marked by an unusual warming of the surface waters in the Pacific Ocean leading to drier, hotter weather in India. La Nina, which means Little Girl in Spanish, is the opposite of El Nino. It is an oceanic and atmospheric phenomenon that is associated with the colder sea surface temperatures. La Nina is usually associated with good rains in India. 

The department's long-range forecasts are as follows:

Private forecaster Skymet has predicted a 'normal' southwest monsoon at 103% of the long-period average with an error margin of plus or minus 5%. The agency had predicted a weak start to the monsoon this year but expects the rainfall to increase from July. It does not see any chance of the occurrence of El Nino. 

PRE-MONSOON RAINFALL

While the monsoon arrived on May 24, the weather department said rainfall during May would not be factored into the southwest monsoon. The country received above-normal rainfall during the pre-monsoon period in Mar-May. India received 185.8 millimetres of rainfall during Mar-May, 42% above the normal for the period. Last year, it received 108.7 millimetres rainfall during Mar-May, 8.1% below the normal for the period.

RECAP 2024
The country received 934.8 millimetres of rainfall in Jun-Sept last year, 8% above the normal weighted average of 868.6 millimetres. The monsoon had hit the Kerala coast on May 30, two days ahead of the scheduled date, and covered the entire country by Jul. 2, six days ahead of the normal date of Jul. 8. The weather bureau's forecast for the monsoon onset over Kerala last year was May 31.     

Overall, monsoon rainfall was above normal for the season and the spatial distribution was uniform. The rainfall in June was below-normal at 89% of the long-period average. However, the country received above-normal showers during the rest of the southwest monsoon period. Rainfall was 109% of the long-period average in July, 115% in August, and 112% in September.

Rainfall during the season was 107% of the long-period average in northwest India, 119% in central India, 86% in east and northeast India, and 114% in the southern peninsula. Of the total 36 meteorological subdivisions, two subdivisions received large excess, 10 received excess, and 21 recorded normal rainfall. Three subdivisions received deficient rainfall during Jun-Sept last year – Arunachal Pradesh, Punjab, Jammu & Kashmir, and Ladakh.

RESERVOIR LEVELS

The water level in 161 key reservoirs was 55.334 billion cubic metres as of May 29, which is higher than 53.578 billion cubic metres as of May 22. The water level in these reservoirs is 30.33% of the total live storage capacity, up from 29.37%. The current level in these reservoirs is 28.68% higher than a year ago and 24.73% above the average for the last 10 years.

Before the southwest monsoon advanced into northeast India, during Mar. 1 and May 24, the region received 13% below normal showers at 280.0 millimetres. However, in a week's time, rainfall in the region rose to 378.3 millimetres, which is 1% above the normal of 373.1 millimetres. Of the 27 reservoirs in the eastern region, one reservoir was completely full, and six reservoirs had a capacity of 81%-70%. 

During the corresponding period last year, the situation was completely different as a few reservoirs in southern India were completely dry. At one point in time, total water level in reservoirs across the country fell to a five-year low. This was because of an erratic monsoon during Jun-Sept 2023. However, the situation improved as the southwest monsoon started early in 2024, and staggered rainfall replenished water levels in these reservoirs. 

The current live storage capacity of these 161 reservoirs is 182.444 billion cubic metres. Water in these reservoirs, which is used for irrigation and generating electricity, is replenished during the monsoon season.

SOWING

Sowing of kharif crops, which depends on rainfall during the Jun-Sept period, is yet to begin in most parts of the country. A few states, such as Gujarat and Karnataka, have reported the start of kharif sowing.

TERMS

LARGE EXCESS: Percentage departure of actual rainfall from normal rainfall is 60% or more 

EXCESS: Percentage departure of actual rainfall from normal rainfall is 20-59% 

NORMAL: Percentage departure of actual rainfall from normal rainfall is -19% to +19%

DEFICIENT: Percentage departure of actual rainfall from normal rainfall is (-)59-20%

LARGE DEFICIENT: Percentage departure of actual rainfall from normal rainfall is (-)99-60%

NORMAL: Percentage departure of actual rainfall is within +/-10% of the long period average

BELOW NORMAL: Percentage departure of actual rainfall is <10% of the long period average

ABOVE NORMAL: Percentage departure of actual rainfall is within >10% of the long period average

EXCEPTIONALLY HEAVY RAINFALL: Rainfall realised in a day is a value near about the highest recorded rainfall at or near the station for the month or season. However, this term will be used only when the actual rainfall amount exceeds 120 mm. 

EXTREMELY HEAVY RAIN: Amount of rainfall realised in a day is >/= 244.5 mm

VERY HEAVY RAIN: Amount of rainfall realised in a day is 124.5-244.4 mm

HEAVY RAIN: Amount of rainfall realised in a day is 64.5-124.4 mm

MODERATE RAIN: Amount of rainfall realised in a day is 7.6-35.5 mm 

RED ALERT: Very heavy rainfall of over 204.4 mm in 24 hours

ORANGE ALERT: Heavy rainfall of 115.6-204.4 mm in 24 hours 

YELLOW ALERT: Warning to be aware of worsening weather conditions that could disrupt daily life

End

Internet links:
India Meteorological Department - www.imd.gov.in
National Centre for Medium Range Weather Forecasting - www.ncmrwf.gov.in
Indian Institute of Tropical Meteorology - www.tropmet.res.in.

Reported by Shreya Shetty

Edited by Ashish Shirke

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