Places in Central Tamil Nadu, particularly to the East of Palghat Gap, do not see much rains during Southwest Monsoon. Strong westerly winds create a funnel effect at Palghat gap reducing convection over places further east. Similarly during Northeast Monsoon period they depend on favourable transit of disturbances for sustained rains. About 125 kms separate Trichy with Nagapattinam but Trichy gets nearly 40% lesser rains than Nagapattinam. Go further west to Karur Paramathi the annual rainfall drops to about 60 cms. This indicates how Central Tamil Nadu along with adjoining areas of West TN is much different to rest of the state. It is interesting to note IMD observatories at Trichy & Karur Paramathi have recorded 30% and 60% excess rains this year.
Occasionally favourable wind pattern may bring thunderstorm possibility to parts of Central Tamil Nadu which brings rains. Last night we saw parts of Trichy, Perambalur, Ariyalur and Thanjavur district see rains. These storms moved from West to East indicative of the influence of Westerly Trough over Peninsular India. Talking about Westerly Trough the satellite image over the past 3 days bring a fascinating view of this interaction. Satellite images of the past 48 hours show how the convection has shifted East gradually. The depression currently about 375 kms to the ENE of Chennai has bulk of its convection to its East as the trough worked its way through.
This has also brought about a reduction in rains over coastal areas of Tamil Nadu. The last batch of rains happened yesterday morning over Chennai and suburbs. Over the next couple of days the depression will continue to remain around Central Bay area. Gradually as the westerly trough influence weakens Easterlies will start to nudge it back towards Peninsular India. It remains to be seen if this SW movement brings about any rainfall event for coastal areas of TN.
But parts of Central Tamil Nadu, adjoining interior delta and South TN may see thunderstorms today. Wind convergence over these areas along with Westerlies may bring moderate rains in a few places for the next day or two. North TN is expcted to be dry due to winds from the North created by the circulation. The land winds may keep temperatures lower during the nights with slightly warmer afternoons. There are indications peak NEM season is over for North TN. But it may be prudent to wait and see for a couple of days before calling off NEM for North TN.