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Advancing monsoon clouds and showers in Aralvaimozhy, near Nagercoil, India. Photo by w:user:PlaneMad

Monsoon Forecast: Onset of Southwest Monsoon Around May 30 Likely

Every year, 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 and as per the practice, ESSO-India Meteorological Department (IMD) has been issuing forecasts using an indigenously developed statistical model, using six predictors.

The predictors to forecast wouthwest monsoon include:

i) monsoonMinimum 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 operational forecasts for the event issued during all the last nine years (2005 to 2014) were correct as seen in the table given below.

Year Actual Onset Date Forecast Onset Date
2005 7th June 10th June
2006 26th May 30th May
2007 28th May 24th May
2008 31st May 29th May
2009 23rd May 26th May
2010 31st May 30th May
2011 29th May 31st May
2012 5th June 1st June
2013 1st June 3rd June
2014 6th June 5th June

The southwest monsoon over the Andaman Sea normally takes place around 20th May with a standard deviation of about one week.  But in the last few days, enhanced convection and increased rainfall activity has been observed over the Bay of Bengal and the Andaman Sea.

The cross equatorial flow is likely to strengthen and deepen over the area indicating favourable for advance of the the monsoon over the Andaman Sea in the next 3-4 days. Absence of any one to one association of the date of monsoon advance over Andaman Sea with the date of monsoon onset over Kerala indicates link with the seasonal monsoon rainfall over the country.

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