IMD officially declared the onset of the Southwest Monsoon over Kerala on 4 June 2026 — nine days later than its own April forecast of 26 May, and three days after the climatological normal of 1 June. Widespread heavy rainfall across all 14 Kerala districts on onset day and conditions now favourable for further advancement over the West Coast, parts of Tamil Nadu, Andhra Pradesh coast, and Sub-Himalayan West Bengal / NE States mark a vigorous start to the season. The freshly revised CFSv2 June Initial Conditions forecast, however, brings a sharp downward revision for the full JJAS season — the short-term onset activity masks a deteriorating outlook across most of India except the NW hills.
IMD Press Release — 4 June 2026: "Southwest Monsoon has set in over Kerala today, the 4th June, 2026." Conditions are favourable for further advance of the SW Monsoon into remaining parts of Kerala, coastal Karnataka & Goa, Tamil Nadu, some parts of South Arabian Sea, Andhra Pradesh coast, and Sub-Himalayan West Bengal & NE States during the next 2–3 days.
IMD's April 2026 long-range forecast had targeted 26 May as the Kerala onset date (model uncertainty ±4 days). While cross-equatorial flow strengthened by mid-May through a Bay of Bengal low and the Andaman Sea was activated by 15–17 May, a brief lull in organised convection over the Kerala coast through late May delayed the formal declaration. By 3–4 June all three IMD onset criteria were simultaneously satisfied:
Onset day precipitation — 4 June 2026: Rainfall on the day of declaration was widespread heavy to very heavy across Kerala — all 14/14 districts above 2.5 mm. Thiruvananthapuram recorded 63.4 mm, Alappuzha 58.8 mm, Kollam 53.8 mm, Kottayam 43.7 mm, Thrissur 34.8 mm, Ernakulam 32.5 mm, Pathanamthitta 31.9 mm. Northern districts were light to moderate — Kasaragod 19.3 mm, Kannur 16.4 mm — indicating the onset front was advancing northward. Conditions are clearly favourable for immediate further advance up the West Coast.
Beyond Kerala, the VayuMet-GFS model run initialized at 00 UTC 04 June 2026 provides a 10-day precipitation, wind and temperature outlook through 14 June 2026. Here is the broader regional advance picture:
On onset day (4 Jun), the advance front was already engaging coastal Karnataka: Udupi recorded 22 mm (moderate) and Dakshina Kannada 11.9 mm (light), with the remaining 26 of 31 districts still dry. Conditions are favourable for onset over coastal Karnataka, Goa, Konkan and Maharashtra in the next 2–3 days, consistent with the normal 5–10 June window.
Tamil Nadu was largely dry on 4 Jun except Kanniyakumari which reported moderate rain (25.9 mm) and the Nilgiris / Tenkasi / Tirunelveli belt light rain (3–5 mm). Further advance over the South Peninsula is expected progressively over 5–10 June as the southwest branch deepens northward.
The Bay of Bengal branch was already active over the northeastern flank on onset day — all NE states showed widespread light-to-moderate rain: Assam 34/34 districts, Meghalaya 12/12, Nagaland 12/12, Arunachal Pradesh 25/25, Mizoram 11/11, Tripura 8/8 — all 100% reported above 2.5 mm. Sub-Himalayan West Bengal showed SCT moderate (Kalimpong 23.2 mm, Alipurduar 15.8 mm, Jalpaiguri 12.9 mm). In contrast, Bihar (0/38 districts) and Jharkhand (0/24 districts) were completely dry — the Bay of Bengal trough had not yet extended southwestward into the Gangetic plains.
The CFSv2 (NOAA Climate Forecast System v2) seasonal forecast is generated monthly with fresh Initial Conditions (IC). The correct frame for monsoon assessment is the full JJAS (June–September) season — not June alone. Comparing IC 01 May and IC 01 June across all four JJAS months — organised by IMD standard homogeneous regions — reveals a significantly more alarming picture than the June-only view suggested. The JJAS IC 01 Jun revision is a broad downgrade across almost all of India, with NW India hills the only exception.
Methodological note: The revision between IC 01 May and IC 01 Jun reflects a change in Initial Conditions only — the underlying CFSv2 model (physics, dynamics, parameterizations) is identical in both runs. The shift in the JJAS outlook is driven entirely by the updated atmospheric and oceanic state on 1 June (post-onset SSTs, soil moisture, circulation) versus the state on 1 May. No model upgrade or recalibration has occurred between the two runs.
| State | IC 01 May JJAS (mm) | IC 01 May Anom (mean) | IC 01 Jun JJAS (mm) | IC 01 Jun Anom (mean) | JJAS Revision |
|---|---|---|---|---|---|
| SOUTH PENINSULAR INDIA — Kerala · Karnataka · Tamil Nadu · Andhra Pradesh · Telangana | |||||
| Kerala | 1049 | −26% | 1060 | −45% | ↓ Worsened |
| Karnataka | 851 | +3% | 632 | −32% | ↓ Flipped negative |
| Andhra Pradesh | 917 | +63% | 393 | −25% | ↓ Sharp reversal |
| Telangana | 973 | +26% | 580 | −21% | ↓ Flipped negative |
| Tamil Nadu | 361 | −10% | 230 | −38% | ↓ Worsened |
| CENTRAL INDIA — Gujarat · Madhya Pradesh · Maharashtra · Odisha · Chhattisgarh | |||||
| Gujarat | 113 | −80% | 207 | −55% | ↑ Less deficient |
| Maharashtra | 773 | −26% | 624 | −40% | ↓ Worsened |
| Madhya Pradesh | 225 | −75% | 197 | −72% | → Near unchanged |
| Odisha | 1240 | +4% | 557 | −51% | ↓ Sharp reversal |
| Chhattisgarh | 902 | −24% | 512 | −54% | ↓ Worsened |
| EAST & NE INDIA — West Bengal · Bihar · Jharkhand · Assam & all NE States | |||||
| West Bengal | 1219 | −16% | 720 | −51% | ↓ Sharply worsened |
| Bihar / Jharkhand | 931 / 886 | −14% / −24% | 588 / 542 | −46% / −50% | ↓ Worsened |
| Assam (NE representative) | 1211 | −30% | 831 | −53% | ↓ Worsened |
| Meghalaya / Nagaland | 1202 / 1911 | −53% / +25% | 725 / 1294 | −73% / −20% | ↓ Worsened |
| NW INDIA — J&K · Himachal Pradesh · Punjab · Haryana · Delhi · UP · Rajasthan | |||||
| Himachal Pradesh / J&K | 510 / 449 | −24% / −6% | 597 / 539 | +4% / +32% | ↑ Improved (hills) |
| Punjab / Haryana | 122 / 50 | −74% / −89% | 163 / 75 | −56% / −76% | ↑ Less deficient |
| Delhi / UP / Rajasthan | 18 / 261 / 16 | −96% / −72% / −95% | 37 / 234 / 26 | −89% / −67% / −89% | ↑ Marginally less deficient |
IC = Initial Conditions · Source: CFSv2 IC 01 May 2026 & IC 01 Jun 2026 · JJAS total precip in mm (Jun+Jul+Aug+Sep) · Anomaly = mean of monthly anomaly % across JJAS · Generated 02 May 2026 and 02 Jun 2026. CFSv2 JJAS forecasts carry substantial uncertainty; use as indicative guidance only. NW plains deficiency expected in June — monsoon onset due late June to July.
The IC 01 Jun JJAS update is a near-universal downgrade from the IC 01 May picture. South Peninsular India has seen dramatic negative reversals — Andhra Pradesh collapsed from +63% to −25% and Telangana from +26% to −21% for the full season, suggesting the strong May-IC signal for the east peninsular coast was a model artefact that has now corrected. Central India is broadly below normal with Maharashtra and Odisha both worsening. East & NE India shows the deepest deficits — Meghalaya at −73%, West Bengal at −51%, Bihar at −46% — the weakening Bay of Bengal branch under El Niño conditions is increasingly reflected. The only improvement is NW India hills (HP, J&K) driven by enhanced western disturbance activity. The IMD's 92% LPA below-normal season call is consistent with — and arguably conservative relative to — what the IC 01 Jun JJAS run is now signalling.
Bottom line: Kerala onset declared 4 June — monsoon is active and advancing northward. West Coast and Deccan in good shape for June. East & NE India looks good in the short term over the next few days but faces a below-normal June risk. NW plains remain pre-monsoon through mid-June.