As we get closer to the middle of October “When is Northeast Monsoon Onset” is the question that lingers win everyone’s mind. Just like Southwest Monsoon Onset IMD has a set of conditions for Northeast Monsoon onset too. The criteria listed by IMD for declaring onset is as below.
- Withdrawal of Southwest monsoon up to Latitude 150N
- Onset of persistent surface easterlies over Tamil Nadu Coast
- Depth of easterlies up to 850 hPa over Tamil Nadu Coast
- Fairly widespread rainfall over coastal Tamil Nadu, South Coastal Andhra Pradesh and adjoining areas
Currently Southwest Monsoon continues to persist over large parts of the country. It is likely to withdraw over the next couple of days over most of Central India. Peninsular India is already seeing a fair bit of Easterlies. Chennai and suburbs received rains from the sea yesterday morning in a classic NEM type thunderstorm activity. The developing Low Pressure Area over SE Arabian sea is likely to strengthen Easterlies further. This LPA is the first of the disturbances that is likely to influence Peninsular India over the next two weeks.
A weak cyclonic circulation over South Bay has a 50:50 chance of becoming a Low Pressure Area towards the end of the week. This potentially could also trigger Northeast Monsoon onset in 3 / 4 days time from now. Weather models indicate there is a slight chance this could be an LPA off North TN / South AP coast which could bring about a strong onset. Meanwhile the 3rd circulation could drift into Andaman sea area from South China Sea in a couple of days time.
This circulation depending on its location of arrival is also likely to move in a NW direction towards Peninsular Coast. A more detailed outlook on this circulation will be done in due course once it moves into Bay. Nevertheless the three circulations are likely to play a crucial role in enhancing Easterlies. With enhanced Easterlies there could be more moisture pushed into Peninsular India as well. Sub seasonal weather models indicate above average rainfall anomaly for Peninsular East coast during the upcoming week.
Starting today we can gradually see an increase in rains along the coast of Tamil Nadu. Interior areas will continue to see thunderstorms driven by Easterlies for the next couple of days. As the circulations strengthen and start influencing weather the countdown for Northeast Monsoon Onset can begin. Chennai and suburbs will see on and off rains during the morning hours over the next couple of days. Towards end of the week rains may be more widespread with moderate to heavier spells seen as well.