Monsoon Onset in Kerala

May 31, 2011; 12:28 PM ET

IMD OBJECTIVE GUIDELINES SHOW KERALA ONSET MAY 29, 2011

The India Meteorological Department (IMD) have now said that the SW Monsoon of 2011 began in the country on May 29, based upon their objective criteria, in the southwestern state of Kerala.

Southern Kerala, along with neighboring Tamil Nadu, is normally the first part of India to feel the breath of the rain-laden South West Monsoon.

The determining criteria are three. These base Monsoon onset upon rainfall, wind direction and outgoing radiative flux.

First, the IMD say that it must rain significantly (2.5 mm or more) for two consecutive days at 60 percent of the available 14 specifically named weather observation sites in Kerala.

Next, winds must be westerly in a layer reaching upward to 600 mb on two consecutive days within a designated swatch (mostly over water) between the 10th parallel and the Equator. And winds should be “of the order of” 15-20 knots at 925 mb,

As for radiative flux, INSAT Outgoing Longwave Radiation should lower below 200 watts/meter for two straight days.

All three of these criteria have apparently been met as of May 29, 2011, according to IMD.

Incidentally, there have been some heavy falls of rain in Kerala. At Kochi, rainfall since May 28 has been 310 mm, or 12.2 inches, if I have done the math right.