A rainfall update for South TN just as Summer gets ready to show up its face over Peninsular India may surprise a few. But for long term weather observers it may not be a surprise as such considering the prevailing La Nina Conditions. A quick look at depressions during March over the past 75 years will show La Nina signature ver well. 3 out of the 4 years when we have seen a depression during March has seen La Nina conditions. The fourth one was a neutral year which eventually translated into a weak El Nino. As recent as 2022 we saw two depressions during the month of March in Bay of Bengal.
The movement of the circulations also give an idea on why rainfall may be a hit or miss. Unlike NEM season when WNW / NW movement is seen during Marcha more South to North movement is seen. This would mean the genesis of the circulation holds the key to rainfall prospects. In 2018 the depression found a sweet spot over Comorin Sea bringing widespread rains over the state. Even interior places in Vellore / Tiruvannamalai districts of North TN saw 5 to 7 cms rains. Thoothukudi in South TN though recorded 20 cms due to the depression in Comorin Sea.
Sub seasonal weather forecasts were consistently indicating this rainfall episode around Feb. end / March beginning for the past few weeks. As mentioned earlier when La Nina conditions prevail it is not unusual for Cyclonic circulations to form over Equatorial waters of Bay. The East to West moving ER wave makes it conducive for the circulation to strengthen over South Bay. Models are expecting the circulation to move along the equatorial waters south of Sri Lanka. It is essential to point out it may be prudent to keep a watch on possible South to North movement when it reaches Comorin Sea area. Models though are not expecting this scenario as of now.
As things stand based on short term model outputs parts of Delta can see widespread moderate rains. South TN may see widespread rains with few places seeing heavy rains around 28th February / 1st March. The presence of a Anti Cyclonic Circulation over Central India may restrict the rainfall impact for places in North TN. This also explains why models do not expect a South to North movement for this cyclonic circulation. Currently places like Chennai can expect cloudy skies with light rains likely around Friday and Saturday. Depending on the movement and strengthening of the circulation West interior areas can see light to moderate rains.