Heavy rain in Kerala, but monsoon onset not yet declared; here's why

The official declaration of the monsoon's arrival in Kerala is still pending due to specific criteria.

Thiruvananthapuram: While signs of the monsoon are active in the state, with a yellow alert issued for fourteen districts in Kerala on May 23, the India Meteorological Department (IMD) is yet to officially declare the monsoon's onset. This is due to specific criteria that the IMD follows. Let's delve into these criteria in detail.

IMD Criteria for Declaring Monsoon Onset

1. Rainfall Amount

After May 10, if 60% of the following 14 stations (Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudlu, Mangalore) record 2.5 mm of rainfall or more for two consecutive days, the monsoon onset over Kerala is declared on the second day, considering the following additional criteria.

2. Wind

Westerly winds should be present up to 600 hPa (4.2 km) altitude in the region between the equator and 10°N latitude and 55°-80°E longitude. Wind speeds should be between 15 and 20 kts (27.8-37 km/hr) up to 925 hPa (750 m) altitude in the region between 5°-10°N latitude and 70°-80°E longitude. This is assessed using satellite data.

3. Outgoing Longwave Radiation (OLR)

OLR values recorded by INSAT satellites should be below 200 wm-2 in the region between 5°-10°N latitude and 70°-75°E longitude.

The IMD officially declares the monsoon onset over Kerala only when all these indicators align. However, with rainfall already active in many parts of the state, public vigilance is crucial. In the absence of an official declaration, it's important to avoid misinformation.

Information courtesy: Rajeevan Erikkulam (Meteorologist - Kerala State Disaster Management Authority)