Northeast Monsoon Rainfall outlook through December

The Northeast monsoon was finally declared to have started across southern India back on 28 October and overall rainfall has been below normal in many parts of southern India through early November.

The slow start to the Northeast monsoon was expected due to development of a La Nina in the equatorial Pacific and the corresponding strong positive Southern Oscillation Index or SOI in August and September. A strongly +SOI prior to the onset of the Northeast monsoon, especially in September, usually means a late start to the Northeast monsoon season and tends to favor less rain early in the season.

The SOI teleconnection tends to flip late in the season where a +SOI in December tends to favor more rain and a slower withdrawal. The SOI has weakened some in October and early November, but remains positive. The weakening trend will have to be monitored, but weak to moderate La Nina conditions should favor the SOI staying positive through the end of 2020 and into early 2021.

In the nearer term, this week will start on a dry note across southern India with rainfall expected to return to Andhra Pradesh, Tamil Nadu, Karnataka and Kerala late this week. Meanwhile, an MJO pulse is expected to enter the Indian Ocean next week, which should improve opportunities for rainfall later in November.

The 200 mb velocity potential shown above also favors the idea of improved rainfall across southern India and Sri Lanka during the second half of November. The latest Extended Range model guidance from the IMD shown below seems to agree with the velocity potential and MJO teleconnection.

For the month of November much of southern India and Sri Lanka is expected to average near normal for rainfall with active and break periods over the next two weeks or so. More consistent rainfall may arrive toward the end of November.

There are no signs of tropical development in the northern Indian Ocean in the near term, but the chance for low development in the Bay of Bengal or Arabian Sea should improve late in the month as an MJO pulse pushes into the Indian Ocean.

Looking ahead to December, the chance of the SOI remaining positive into December and perhaps the MJO remaining over the Indian Ocean in early December should favor good rainfall at least the first half of the month.

Several multi-model ensemble climate models shown above all tend to favor the idea of good rainfall in southern India and Sri Lanka to end 2020. Teleconnections tend to agree with the climate models, so we are forecasting a wetter-than-normal December across much of Andhra Pradesh, Tamil Nadu, Kerala and southern Karnataka into Sri Lanka.

There is a chance for tropical lows in the southern Bay of Bengal which can enhance rainfall in these areas. The risk of a cyclone forming over the Bay of Bengal appears to be low at this point, but there is a chance for a well-marked low or depression.

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