On the back of the failure of Northeast Monsoon 2018 and the ground water resources depleting over most parts of Chennai most citizens of the city are looking to the skies for some spell of rains to mitigate the upcoming summer.
Regular weather observers know the period between January to May is typically the off season for Chennai though interior areas of Tamil Nadu start receiving rains from late February / Early March on account of Veppa Salanam or changing winds as precursor to Summer Monsoon. So how off is the off season for Chennai? A COMK Study based on daily rainfall recordings from 1901 to 2018 at the Airport Observatory in Meenambakkam.
While monthly average rainfalls are one way of looking at off season rainfall pattern we thought it could be an interesting exercise to understand instances of rainfall events over a day across the months of January to May to infer what to expect. The table pretty much confirms what to expect during this period. For the 5 months between 1st January to 31st May between 1901 to 2018 only 101 days Chennai AP has recorded more than 10 mm during a 24 hour period. If we were to look at as a statistical probability its possibly little less than 3 days in a year or less than 2% probability for any given day to record more than 10 mm during the off season period. 1909 and 1995 were possibly the better years in terms of off season rains with both recording 10 days each when at least 10 mm was recorded in Meenambakkam observatory.
Looking at the significant rainfall events during the off season period one can clearly see a pattern, most extreme rainfall events have been associated with synoptic conditions like Depression / Cyclones in Bay of Bengal etc. 15 times Chennai AP has recorded more than 10 cms rains during the off season period with most connected to disturbance in Bay.
It is interesting to note May 1943 saw Pondy record nearly its Northeast Monsoon average in 4 days during the month of May due to a slow moving cyclone. Similarly 1952 May saw Ponneri record 33 cms in one day which even during Northeast Monsoon days does not happen often. 2016 saw Cyclone Roanu brush the Chennai coast as a Deep Depression giving very good rains around the TN Assembly Election time.
We need a lot of luck for a good spell of Off Season Rains, Lets hope Nature provides an answer in the form of a Miracle to tide over the drinking water crisis