How IMD Declares the Monsoon Onset over Kerala and Monitors Its Advance Across India

Cover image: Monsoon clouds enveloping the Western Ghats near Lavasa, Maharashtra, during the southwest monsoon season. [Photo: Satyaban B Ratna]

Introduction

Every year, the arrival of the southwest monsoon is one of the most eagerly awaited weather events in India. Farmers prepare for sowing, reservoirs await replenishment, and millions of people look forward to relief from the intense summer heat. Consequently, considerable attention is paid to the announcement of the monsoon onset over Kerala, which marks the official beginning of the rainy season over the Indian mainland.

The onset of the southwest monsoon over Kerala (MOK) is considered the official commencement of the monsoon season over the Indian mainland. It serves as an important meteorological milestone and provides the starting point for monitoring the advance of the monsoon across different regions of the country. The normal date of monsoon onset over Kerala is 1 June, although the actual onset date may vary from year to year depending on prevailing atmospheric and oceanic conditions.

Given the widespread interest in the monsoon, many people wonder how the onset over Kerala is determined and how the monsoon subsequently advances across the country. In this blog, I briefly explain the scientific criteria used by the India Meteorological Department (IMD) to declare the monsoon onset over Kerala and describe the normal progression of the monsoon across India.

How Does IMD Declare the Monsoon Onset Over Kerala?

The declaration of monsoon onset over Kerala is not based on a single rainy day. Instead, IMD follows a well-established scientific methodology that combines information from rainfall observations, atmospheric circulation, and satellite-derived cloudiness indicators.  The onset is declared only when all prescribed criteria are satisfied simultaneously after 10 May.

(i) Rainfall Criterion : IMD monitors rainfall at the following 14 designated stations located across Kerala, Lakshadweep, and the adjoining coastal Karnataka region:

Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu, and Mangalore

If at least 60% of these stations (i.e., 9 out of 14 stations) record rainfall of 2.5 mm or more for two consecutive days after 10 May, the rainfall criterion is considered satisfied.

(ii) Wind Field Criterion: The southwest monsoon is characterized by strong moisture-laden westerly winds over the Arabian Sea. For declaring onset, the depth of these westerlies should extend up to 600 hPa over the region bounded by the Equator–10°N and 55°E–80°E. In addition, the zonal wind speed at 925 hPa over 5°–10°N and 70°–80°E should generally be between 15 and 20 knots.

(iii) Outgoing Longwave Radiation (OLR) Criterion: Satellite observations provide valuable information about cloudiness and convection. IMD uses INSAT-derived Outgoing Longwave Radiation (OLR) as an indicator of organized monsoon convection. For the onset declaration, OLR values should be below 200 W m⁻² over the region bounded by 5°–10°N and 70°–75°E. Lower OLR values indicate the presence of deep convective clouds associated with active monsoon conditions.

IMD declares the onset of the southwest monsoon over Kerala only when the rainfall, wind field, and OLR criteria are satisfied simultaneously. The onset date is assigned to the second consecutive day on which the rainfall criterion is met.

The procedure followed by IMD for declaring the onset of the southwest monsoon over Kerala is summarized in Figure 1. The flowchart highlights the sequence of checks involving rainfall, wind field, and OLR conditions that must be satisfied before the onset is officially declared.

Figure 1. Decision flowchart showing the objective criteria used by IMD for declaring the onset of the southwest monsoon over Kerala.

While the declaration of onset over Kerala marks the beginning of the southwest monsoon season over the Indian mainland, the monsoon's story does not end there. Over the following weeks, it advances across the country before eventually covering the entire Indian mainland.

Further advance of the monsoon across India

The declaration of monsoon onset over Kerala marks only the beginning of the southwest monsoon season over India. After reaching Kerala, the monsoon advances gradually across different parts of the country over the next five weeks, normally covering the entire country by around 8 July.

It is important to note that the criteria involving rainfall, wind field, and Outgoing Longwave Radiation (OLR) are used specifically for the declaration of the monsoon onset over Kerala (MOK). These objective criteria help ensure that the onset is associated with the establishment of large-scale monsoon circulation rather than a temporary spell of rainfall.

However, once the onset over Kerala has been declared, the further advance of the monsoon across the country is determined primarily based on rainfall distribution and its spatial continuity, supported by meteorological analysis. This distinction is important because the onset over Kerala marks the establishment of the monsoon system over the Indian mainland, whereas the subsequent advance reflects the gradual spread of monsoon rainfall across different parts of the country.

After crossing the equator and entering the Indian region, the southwest monsoon splits into two major branches owing to the configuration of the Indian peninsula: the Arabian Sea branch and the Bay of Bengal branch (Figure 2).

Figure 2 Schematic illustration of the Arabian Sea and Bay of Bengal branches of the southwest monsoon and their typical pathways across the Indian subcontinent. Together, these branches facilitate the gradual spread of monsoon rainfall across the country.

The Arabian Sea branch advances northward along the west coast of India. It brings widespread rainfall to Kerala, Karnataka, Goa, Maharashtra, Gujarat, and parts of central and northwestern India. The Western Ghats enhance rainfall along the west coast by forcing the moisture-laden winds to rise. 

The Bay of Bengal branch moves towards northeastern India and the eastern Himalayan foothills before advancing westward across the Indo-Gangetic plains. This branch contributes significantly to rainfall over northeastern, eastern, and northern India. 

Together, these two branches are responsible for the gradual spread of monsoon rainfall across the country.

Under normal conditions, the monsoon first reaches the Andaman and Nicobar Islands around 22 May and then arrives over Kerala around 1 June. Thereafter, it progresses northward and westward, covering southern, central, eastern, and northern India in stages before reaching the remaining parts of northwest India by around 8 July.

The normal dates of monsoon onset and advance over different parts of India are shown in Figure 3. It may be noted that these dates represent long-term climatological averages, and the actual advance of the monsoon may be earlier or later in a given year.

Figure 3. Normal dates of onset and advance of the southwest monsoon over India as defined by the India Meteorological Department (IMD). The monsoon normally reaches Kerala on 1 June and covers the entire country by around 8 July. [Source: IMD]

The speed of monsoon advance is not always uniform. In some years, it advances rapidly, while in others it may temporarily stall due to changes in large-scale atmospheric circulation and oceanic conditions. Nevertheless, the normal dates of onset and advance provide a useful climatological reference for monitoring the seasonal march of the monsoon.

The actual onset and advance of the monsoon can differ significantly from the climatological normal. Recent years provide good examples of how the monsoon's progression varies from one year to another. While the onset over Kerala was delayed by about a week in 2023 and occurred earlier than normal in 2025, the subsequent advance across the country evolved differently in response to prevailing atmospheric and oceanic conditions (Figure 4). These examples highlight that the date of onset over Kerala does not necessarily determine the pace of monsoon advance across the rest of the country.

Figure 4. Comparison of the onset and advance of the southwest monsoon over India in 2023 and 2025. While the onset over Kerala was delayed in 2023 and earlier than normal in 2025, the subsequent advance across the country evolved differently in response to prevailing atmospheric and oceanic conditions, illustrating the year-to-year variability of the monsoon system. [Source: IMD]

Every year, meteorologists closely monitor the monsoon's advance because it directly influences agriculture, water resources, hydropower generation, ecosystems, and the livelihoods of millions of people. From its arrival over Kerala to its eventual coverage of the entire country, the monsoon's progression remains one of the most anticipated and closely watched weather events in India.

Why Doesn't the Monsoon Advance at a Constant Speed?

The advance of the monsoon is not like a steadily moving weather front. Some years it progresses rapidly across large parts of the country, while in other years it may pause for several days before resuming its advance.

The speed of advance depends on several atmospheric and oceanic factors, including:

• Strength of moisture-laden cross-equatorial winds over the Arabian Sea.
• Formation and movement of low-pressure systems over the Bay of Bengal.
• Position and strength of the monsoon trough.
• Large-scale atmospheric disturbances such as the Madden–Julian Oscillation (MJO).
• Ocean-atmosphere interactions associated with El Niño and La Niña.
• Regional land-sea temperature contrasts.

Consequently, the monsoon may advance faster than normal in some years and slower than normal in others. These variations are a natural feature of the monsoon system and are closely monitored by meteorologists throughout the season.

Concluding Remarks

The onset of the southwest monsoon over Kerala is one of the most closely watched weather events in India. Behind this announcement lies a carefully designed scientific methodology that combines rainfall observations, wind patterns, and satellite-derived indicators of convection. Once declared, the monsoon embarks on its remarkable journey across the country through its Arabian Sea and Bay of Bengal branches, typically covering the entire Indian mainland by early July.

The southwest monsoon contributes nearly three-fourths of India's annual rainfall and plays a crucial role in agriculture, water resources, hydropower generation, ecosystems, and the national economy. Understanding how the onset is declared and how the monsoon advances across India helps us appreciate one of the most important weather systems influencing the lives and livelihoods of millions of people.

It is also important to remember that an early or delayed onset over Kerala does not necessarily determine the overall performance of the monsoon season. What ultimately matters is the subsequent advance, distribution, and persistence of rainfall during the four-month monsoon season.

Thus, while the onset over Kerala attracts considerable attention every year, it is the evolution of the monsoon across the country that ultimately determines the success of the season and its far-reaching impacts on society, agriculture, water resources, and the economy.