The monsoon is likely to arrive on the Kerala coast on May 31, the India Meteorological Department (IMD) announced on Friday. The normal monsoon onset date over Kerala is June 1 with an error window of four days. The monsoon arrival comes as an emerging cyclone brews over the Arabian Sea. Forecasters say the cyclone is currently a ‘depression’ or a pre-cyclone located about 30 km off Lakshadweep. Current weather models say the depression will become a cyclonic storm by Saturday evening and Tauktae — as the storm will then be formally called — will move along the western coast and reach the Gujarat coast by May 18.
In the interim, it could intensify into a ‘Very Severe Cyclonic Storm’ that is characterised by wind speed of over 120 kmph. This would then make it a grade 3 cyclonic storm and two steps short of what is called a ‘Super Cyclonic storm’ with wind speed of over 200 kmph.
It is common for storms to brew in the Bay of Bengal and the Arabian Sea ahead in May or in the month ahead of the monsoon onset.
Heavy rains
Already heavy rains have set in over Lakshadweep, Kerala Tamil Nadu , Karnataka and Goa and this is expected to continue for the next few days.
The IMD has forecast a normal monsoon during June-September, largely premised on relatively cool temperatures in the Central Pacific and ruling out the possibility of an El Nino. “Normal” rainfall refers to a range: 96%-104% of the Long Period Average (LPA) of 88 cm.
Last year, the IMD announced a monsoon onset date over Kerala of June 1 whereas the monsoon actually arrived on June 5. There is no correlation between the date of onset of the monsoon and the actual quantum of rain that is received during these months.
To herald the monsoon onset, initial monsoon rains first occur over south Andaman Sea and the monsoon winds then advance across the Bay of Bengal. The emerging storm, according to the IMD’s press note on Friday, has strengthened the monsoon winds.
“The cross equatorial south westerlies have temporarily strengthened over the Arabian Sea and is expected to deepen over the Bay of Bengal from May 20 and a sustained rainfall activity is likely over the south Bay of Bengal and Andaman and Nicobar Islands from May 21. Hence the monsoon advance over Andaman and Nicobar Islands is very likely around May 21 2021,” it says.
While various meteorological factors — including a minimum amount of rain over Kerala and certain wind flow speeds — determine the IMD’s decision to officially declare the onset of the monsoon, a rule of thumb says that it takes about 10 days from the monsoon’s arrival over the Andaman and Nicobar Islands for it to reach Kerala.
Since 2005, the monsoon has arrived within the error margin of the IMD’s weather models except for 2015.
