<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/in-mumbai_rain_0.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/in-mumbai_rain_0.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/in-mumbai_rain_0.jpg?v=ap&w=980&h=551 800w" > Rainfall in Navi Mumbai, Sept 2019. (Photo: K.K. Choudhary/BCCL Mumbai)

The southwest monsoon has withdrawn completely, and October heat has set in over western states like Maharashtra and Gujarat. However, even as people in these states bid adieu to the rains and look forward to the relief of cooler months, a new weather system is set to spring a surprise. Model predictions indicate that the low-pressure area over the southeastern Arabian Sea near the Lakshadweep islands is likely to persist till next week, triggering speculation of heavy rainfall over the western states.

As on Friday morning, the low-pressure area prevails over the southeast Arabian Sea, and the related cyclonic circulation is likely to persist through the weekend into next week. The Weather Channel Met team forecasts that the system is likely to bring heavy rain and thunderstorms over southern and southwestern India this weekend.

The Global Forecast System (GFS) model (a popular weather model used by forecasting agencies) indicates that the low-pressure area may intensify into a depression as it moves in a north-to-northeast direction towards the area between Mumbai and Surat. GFS also forecasts moderate to heavy rainfall over many parts of Maharashtra between October 20 and 23.

The India Meteorological Department (IMD) also uses a customised GFS model for medium range (4-10 days) forecasting. As per the GFS model forecast by the IMD, widespread rainfall of 20-70 mm in 12 hours is possible over Mumbai, Pune, Satara, Sangli and Kolhapur on October 21. However, IMD has not issued any official statement about this forecast yet.

While IMD’s model forecasts more rains over Pune division, forecasts from other versions of GFS models show chances of heavier rainfall over isolated regions in Mumbai and Thane on October 23 due to depression. Karnataka and parts of Andhra Pradesh may also receive heavy rains from the prevailing easterly winds carrying moisture from the Bay of Bengal.

“A cyclonic circulation persists over the Arabian Sea with substantial convective activity. This is associated with the Madden–Julian Oscillation (MJO is an eastward moving 'pulse' of cloud and rainfall near the equator that fluctuates on weekly to monthly timescales). Above-normal precipitation is forecast over southern India for the next 2 weeks, and it may push up slightly further north, peaking next week. However, once the MJO moves to the Western Pacific by mid-November, the convective activity will be suppressed, resulting in below-normal rains across India,” says Leon Brown, Head of Global Meteorological Operations at The Weather Company.

Making accurate forecasts about the extent of the expected rainfall is a challenging affair. Climate models have been notoriously inaccurate in their medium and long-term forecasts about the intensification of cyclonic circulations over the Arabian Sea. The oceanic and atmospheric circulations over the next few days would determine the fate of this low pressure. As of now, the conditions seem favourable for the generation of depression over the Arabian Sea.

A look at the history of depressions in the month of October shows that depressions hit the west coast of India very rarely. In the satellite era (which began after 1979)—only one of the seven depressions that formed over the Arabian Sea in the month of October hit the Indian coastline. That was in 1985. So if the conditions remain favourable for this low to intensify further, this could be the first October depression to hit the west coast of India in 34 years.