Southwest Monsoon: IMD predicts rainfall in Kerala, Lakshadweep; thundersquall in Karnataka

The India Meteorological Department (IMD) issued a forecast predicting rainfall over Kerala and Lakshadweep, signalling the imminent arrival of the Southwest...

What Happened

The India Meteorological Department (IMD) issued a forecast predicting rainfall over Kerala and Lakshadweep, signalling the imminent arrival of the Southwest Monsoon over the Indian subcontinent.
Thundersquall activity was also predicted for Karnataka, with isolated heavy rainfall warnings issued for coastal and interior districts.
IMD had initially forecast monsoon onset over Kerala around May 26, 2026 (±4 days); however, the monsoon failed to materialise on schedule and the revised window shifted to June 2–4.
For the 2026 season overall, IMD has projected total monsoon rainfall at 92% of the Long Period Average (LPA) — the first below-normal forecast since 2023.

Static Topic Bridges

The Southwest Monsoon: Mechanism and Significance

The Southwest Monsoon is a seasonal large-scale wind system that brings the majority of India's annual rainfall from June to September. It originates from the southwest over the Indian Ocean and the Arabian Sea, driven by the differential heating of land and sea during summer. India receives approximately 70–80% of its annual rainfall during this four-month window, making the monsoon the single most critical climatic event for agriculture, water resources, and the broader economy.

The monsoon has two branches: the Arabian Sea branch (which hits the Western Ghats first) and the Bay of Bengal branch (which advances along northeastern India and up the Gangetic Plain).
The normal onset date over Kerala is June 1, with a standard deviation of approximately 7 days — meaning onset between May 25 and June 8 is within the range of natural variability.
IMD's operational forecast for onset date has been issued annually since 2005 and has shown high accuracy over more than two decades.
India's agricultural calendar — sowing schedules for kharif crops such as paddy, maize, cotton, and pulses — is timed around monsoon onset.

Connection to this news: The forecast for rainfall over Kerala and Lakshadweep is the pre-onset signal; the actual declaration of onset requires multiple meteorological criteria to be satisfied simultaneously.

IMD's Monsoon Onset Declaration Criteria

IMD does not declare monsoon onset on the basis of rainfall alone. It uses a multi-parameter framework established to ensure the declaration reflects a genuine shift in the atmospheric circulation, not a transient weather event.

Rainfall threshold: At least 60% of 14 designated weather stations across Kerala (including Thiruvananthapuram, Kochi, Kozhikode, Mangalore, and Minicoy) must record 2.5 mm or more of rainfall on two consecutive days after May 10.
Wind criterion: Sustained westerly winds at 925 hPa (lower troposphere) should reach 15–20 knots over the area bounded by 5°–10°N latitude and 70°–80°E longitude.
Depth of westerlies: The westerly flow must extend up to the 600 hPa level (mid-troposphere) in the equatorial Indian Ocean box (0°–10°N, 55°–80°E).
Cloud cover: INSAT-derived Outgoing Longwave Radiation (OLR) values must fall below 200 W/m² in the box 5°–10°N, 70°–75°E, indicating deep convective cloud cover.

Connection to this news: The forecast of rainfall in Kerala and Lakshadweep indicates that some criteria may be approaching threshold levels, but the delayed onset suggests that wind and cloud cover conditions had not yet been fully met.

Long Period Average (LPA) and Below-Normal Monsoon Classification

IMD measures annual monsoon performance against a Long Period Average — a statistical baseline calculated over 50-year periods. The current LPA for June–September all-India rainfall is 87 cm (updated periodically). IMD classifies seasonal rainfall in five bands relative to the LPA.

Normal: 96–104% of LPA
Above Normal: 104–110% of LPA
Excess: above 110% of LPA
Below Normal: 90–96% of LPA
Deficient: below 90% of LPA
At 92% of LPA, the 2026 forecast falls in the "Below Normal" category.
A below-normal monsoon increases risks of kharif crop shortfall, reservoir depletion, and rural distress — with implications for food inflation and rural demand.

Connection to this news: While the immediate news is about onset timing, the 92% LPA forecast is the more consequential figure for policymakers, given its downstream effects on agriculture, power (hydro generation), and water security.

El Niño and Indian Ocean Dipole (IOD) — Monsoon Modifiers

Two major oceanic-atmospheric systems modulate the strength of India's monsoon: El Niño–Southern Oscillation (ENSO) in the Pacific, and the Indian Ocean Dipole (IOD) in the Indian Ocean.

El Niño (warming of the central-eastern Pacific) is historically associated with suppressed monsoon rainfall over India; La Niña tends to enhance it.
A positive IOD (warmer western Indian Ocean relative to the eastern part) generally compensates for El Niño's adverse effects on Indian rainfall.
IMD issues its first long-range forecast in April and a second, updated forecast in June, incorporating the latest ENSO and IOD readings.
The 2026 below-normal forecast is consistent with neutral-to-weak El Niño conditions in the Pacific alongside a weakly negative IOD.

Connection to this news: Delayed onset and a below-normal season forecast are early signals for district administrations and state governments to activate contingency crop plans and monitor reservoir levels closely.

Key Facts & Data

Normal monsoon onset over Kerala: June 1 (±7 days)
IMD's initial 2026 forecast: onset around May 26 (±4 days); revised to June 2–4
2026 seasonal forecast: 92% of LPA — classified as Below Normal
LPA for June–September all-India rainfall: 87 cm
Onset declaration requires: 60% of 14 stations record ≥2.5 mm for 2 consecutive days + wind and OLR criteria
Designated onset stations include: Thiruvananthapuram, Kochi, Kozhikode, Mangalore, Minicoy, among others
India's share of annual rainfall from SW Monsoon: ~70–80%
IMD operational onset forecast track record: issued annually since 2005