A couple of weeks back just before Northeast Monsoon picked up over Delta coast we had made a post talking about how November end is an ideal period for Tamil Nadu. Over the years Bay of Bengal has churned out many cyclones during the last week of November / early December. This year Cyclonic Storm Fengal has got added to the list after making landfall near Pondicherry on 30th November. What made Cyclonic Storm Fengal unique was the constant twists and turns in path.
Weather models struggled under complex steering dynamics which in real time were even worse than model estimates. Not only did models struggle with the steering dynamics but it also struggled with rainfall estimates too. Interior areas like Tiruvannamalai, Krishnagiri got dumped with rains that are very rarely seen. Coastal districts like Villupuram, Cuddalore and Pondicherry got battered by high intensity rains while it stayed off the coast. In a way the enhanced rainfall was also a reflection of the growing trend due to a warm North Indian Ocean.
Both Arabian Sea and Bay of Bengal have been seeing abnormally warm waters for the past few years. This is reflecting in enhanced rainfall episodes under right conditions. With poor steering a stationary Fengal was able to draw on the moisture from the seas to dump a lot of rains in its path. As we get closer to mid December giving medium term weather inferences is fraught with risk. The risk of being caught out by sudden change in monsoon dynamics. The switch is extremely dramatic and quick during El Nino years while La Nina years tend to gradually taper off.
Cyclonic Storm Fengal was a forecaster’s nightmare right through its life span. A similar scenario may once again unfold as an area of disturbance persists over South Andaman Sea. Just like Fengal models are putting out divergent forecasts on its intensity and path. The divergent forecasts is also reflecting in the IMD’s extended Cyclogenesis outlook. While it infers no probability of cyclogenesis based on overall environmental conditions and model guidance it also mentions a LPA to form on 7th December over South Bay of Bengal.
Just as the case with Fengal models are in tight agreement about the circulation reaching East of Sri Lanka over the next 48 to 72 hours. Subsequently there is a divergence based on intensity estimates. One set of models taking it westward over southern parts of Sri Lanka as a weak disturbance. Another set of models intensify it into a depression and bring it towards Coastal TN. The rainfall outlook also changes depending on how the movement is. In the first scenario Delta to South TN may see more rains while in the second scenario Delta to North TN may see more rains.
While we will keep a watch on the developing circulation rainfall probability and potential impact zone may have to wait for another day or two. Meanwhile on the immediate weather front mostly dry conditions to prevail across Tamil Nadu. Cool winter like conditions to persist during the morning hours under the influence of land winds over places like Chennai.