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Forecast for the 2012 Southwest Monsoon Onset over Kerala

May 20, 2012
1. Background

The onset of southwest monsoon over Kerala signals the arrival of monsoon over the Indian subcontinent and represents beginning of rainy season over the region. From 2005 onwards India Meteorological Department (IMD) has been issuing operational forecasts for the monsoon onset over Kerala using an indigenously developed statistical model with a model error of ± 4 days. The operational forecasts issued based on this model during all the last seven years (2005 to 2011) were correct as seen in the table given below.


Actual Onset Date

Forecast Onset Date


7th May

10th June


26th May

30th May


28th May

24th May


31st May

29th May


23rd May

26th May


31st May

30th May


29th May

31sr May

IMD has now prepared the forecast for the 2012 monsoon onset over Kerala.

2. Advance of monsoon over Andaman Sea
The southwest monsoon normally advances over Andaman Sea around 20th May with a standard deviation of about one week. The Inter-tropical convergence zone is showing signs of organization and the cross equatorial monsoon flow is expected to appear over Andaman Sea within next few days resulting increased rainfall activity over the area. The monsoon is likely to advance over Andaman Sea slightly later than its normal date but well within one standard deviation. Past data suggests absence of any one to one association between the date of monsoon advance over Andaman Sea and the date of monsoon onset over Kerala or with the seasonal monsoon rainfall over the country.

3. Forecast for the 2012 Monsoon Onset over Kerala
For predicting the 2012 monsoon onset over Kerala, the model based on Principal Component Regression technique uses the following six predictors:
i) Minimum Temperature over North-west India, ii) Pre-monsoon rainfall peak over south Peninsula, iii) Outgoing Long wave Radiation (OLR) over south China Sea, (iv) Lower tropospheric zonal wind over southeast Indian ocean, (v) upper tropospheric zonal wind over the east equatorial Indian Ocean, and (vi) Outgoing Long wave Radiation (OLR) over south-west Pacific region.
The model suggests that the date of onset of southwest monsoon over Kerala is likely to be on 1st June with a model error of ± 4 days.

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