Despite widespread rainfall across Kerala and a yellow alert issued in all 14 districts for May 23, the India Meteorological Department (IMD) has not yet declared the official onset of the southwest monsoon. This delay is due to the IMD’s strict scientific criteria that must be met before making such an announcement.
IMD’s Criteria for Declaring Monsoon Onset:
Rainfall: After May 10, at least 60% of 14 specific weather stations (including Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudlu, and Mangalore) must record a minimum of 2.5 mm of rain or more for two consecutive days. The declaration can be made on the second day if the other conditions are also satisfied.
Wind Patterns: There should be a consistent presence of westerly winds up to a height of 600 hPa (approximately 4.2 km) across the equatorial region between 0°–10°N latitude and 55°–80°E longitude. Additionally, wind speeds between 15 to 20 knots (around 28 to 37 km/h) should be observed up to 925 hPa (about 750 m height) in the region between 5°–10°N latitude and 70°–80°E longitude, as confirmed through satellite data.
Outgoing Longwave Radiation (OLR): The OLR values, which are monitored via INSAT satellites, should be below 200 W/m² in the area between 5°–10°N latitude and 70°–75°E longitude.
All three of these parameters—rainfall, wind, and OLR—must align for the IMD to declare the official onset of the monsoon in Kerala. Until then, even though heavy rains are occurring, the public is advised to remain alert and rely only on verified updates to avoid misinformation regarding the monsoon’s arrival.
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