Know the guidelines followed to declare southwest monsoon onset

On Friday, the India Meteorological Department (IMD) announced the onset on southwest monsoon over Kerela. The monsoon onset over Kerela is said to be eight days before the normal date of June 1.

As per IMD, this was the earliest date of monsoon onset over Kerala after the year 2009 when it was set in over Kerala on 23rd May 2009.

Normally, the southwest monsoon makes its onset over Kerala by June 1 and covers the entire country by July 8. The monsoon starts retreating from northwest India around September 17 and withdraws completely by October 15.

Also Read: Southwest Monsoon sets in over Kerala 8 days before normal date of June 1: IMD 

But do you know that there are certain guidelines which are followed to declare onset of southwest monsoon over Kerela and its further advancement over the country?

Southwest Monsoon Declaration: The Guidelines

Rainfall

If after 10th May, 60% of the available 14 stations enlisted, viz. Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu and Mangalore report rainfall of 2.5 mm or more for two consecutive days, the onset over Kerala be declared on the second day, provided the following criteria are also in concurrence.

Wind field

Depth of westerlies should be maintained upto 600 hPa, in the box equator to Lat. 10°N and Long. 55°E to 80°E. The zonal wind speed over the area bounded by Lat. 5-10°N, Long. 70-80°E should be of the order of 15 - 20 Kts. at 925 hPa. The source of data can be RSMC wind analysis/satellite derived winds.

Outgoing Longwave Radiation (OLR)

INSAT derived OLR value should be below 200 wm-2 in the box confined by Lat. 5-10°N and Long. 70-75°E, tweeted the India Meteorological Department.