The southwest monsoon has made an early arrival over Kerala and North-eastern states on Thursday with heavy rains over the region. This marks the beginning of the four-month monsoon season during which India receives nearly 70% of the annual rains.
Normally, the monsoon sets in over Kerala on June 1 and advances over most part of northeast India by June 5. However, this time, the seasonal rains have begun over South India as well as the north-eastern states around the same time. While it arrived over Kerala two days before the normal date, it was six days early over the northeast. The onset is also in sync with the IMD’s forecast of May 31 (+/-4 days).
It has also advanced into most parts of Lakshadweep area, most part of south Arabian sea and some parts of central Arabian sea and some parts of south Tamil Nadu. The Northern Limit of Monsoon on Thursday passes through Kannur (Kerala), Coimbatore & Kanyakumari (Tamil Nadu) in the south and Agartala (Tripura), Dhubri (Assam) in the east. The Northern Limit of Monsoon on Thursday passes through Kannur (Kerala), Coimbatore & Kanyakumari (Tamil Nadu) in the south and Agartala (Tripura), Dhubri (Assam) in the east.
How IMD Predicts Monsoon
The monsoon normally arrives over Kerala around June 1, and covers India by July 8, as per the long period records maintained by the IMD from 1970 to 2019. However, this date varies each year due to the year-to-year variations in the meteorological conditions.
So, the weather department has set some predefined conditions of cloud cover, wind pressure, moisture content, which it monitors before declaring the onset date. This criterion was revised in 2016, and is now based on the daily rainfall of 14 stations over Kerala and neighbouring area along with wind field and Outgoing Longwave Radiation (OLR) over southeast Arabian Sea, which basically helps to determine the cloud cover/moisture.
According to it, there should be a sharp increase in rainfall over Kerala in at least 60% of the available 14 stations enlisted – Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu and Mangalore. If the rainfall of 2.5 mm or more is recorded for two consecutive days, then the onset over Kerala can be declared on the second day.
But this should also be accompanied with the setting up of large-scale monsoon flow and extension of westerlies (winds) up to 600 hPa. The cloud zone along the coast is also measured with satellite images, as well as the water vapour imageries, which are also studied to assess the extent of moisture incursion.
“During past two days, cloud cover has increased over Southeast Arabian Sea, the depth of westerlies over Southeast Arabian Sea also extends upto 4.5 km above mean sea level. There has been widespread rainfall over Kerala during past two days. Considering all the above satisfied conditions, Southwest Monsoon has set in over Kerala on the May 30,” announced the weather department on Thursday.
The monsoon onset/progress dates for other regions and states is declared by IMD operationally in a subjective manner considering the sharp increase in rainfall and if it sustains for at least a few days and associated changes in the atmospheric circulation features.
Monsoon Normally Covers India by July 8
According to the long-period records of 1970-2019, the monsoon normally covers the rest of the country by July 8, however, it is subject to changing weather conditions. It normally reaches Delhi around June 27. This time, its progress after June 15 and arrival over Northwest India could be somewhat sluggish.
“We are expecting somewhat less rainfall over some parts of Northwest India in June – Jammu and Kashmir, Himachal Pradesh, parts of Punjab, Haryana and Uttar Pradesh. So, the temperatures will remain high during day and night with more heatwaves. Due to the onset of monsoon, the humidity levels will also increase,” said the IMD Chief M Mohapatra. However, the IMD may update this forecast over the next two weeks.
Meanwhile, most parts of South India and adjoining parts of Central India are expected to witness above-normal rains in June, indicating its normal progress at least during the first half. Monsoon normally reaches Mumbai and Kolkata around June 11, and advances over Udupi Karnataka by June 4.
This time, IMD is expecting normal to above-normal rains over most parts of the country except for some north-eastern states as well as northern parts of Jammu and Kashmir, Himachal Pradesh and Uttarakhand, as well as many parts of Odisha, southern Gangetic West Bengal and southern parts of Jharkhand and Chhattisgarh.
