🌦️ 1. OBSERVED WEATHER & SYNOPSIS (12–18 SEP)

The past week delivered high-impact extremes, highlighted by the active Bay-origin low tracking inland and unleashing extremely heavy rainfall and flood risks over Gujarat and central India. Meanwhile, the monsoon withdrawal window officially commenced around 19th September 2026 from extreme west Rajasthan, marking the traditional end-of-season retreat.

Across the south peninsula, a prominent north-south trough associated with shear zone as I had already predicted last week developed with and active convergence zones driving localized thundershowers over interior Karnataka, Tamil Nadu, and eastern pockets of Kerala.

SW monsoon performance as on date shows around 52% of the country's climatical subdivisions receiving normal to excess rainfall while the rest remaining in deficit. The biggest deficit figures in Meghalaya, Arunachal Pradesh, Andhra Pradesh,& Bihar where the deficit ranges from.35% to 60%, while Telengana,Punjab remains deeply stressed with deficit falling in the range 30-35%.

The background ElNino state seems to be showing its true colours with the monsoon already on the retreat much earlier than it should be as I had already doubted in my last week projection with rainfall dropping appreciably in the southern peninsular India.

This is a wind transition period and it is also the time when the lower level westerlies of SWM go weak. Remember it is a strong ELNino background state which could mean an earlier exit of the monsoon. This would make for easterly anomalies and shear zones to gain momentum over the southern peninsula as is now happening.

The easterly anomalies are always present but hiding due to the strong westerlies. Once the westerlies weaken or when the monsoon starts retreating, means when the monsoon trough or the thermal equator which had been residing over north India starts shifting southwards in sync with apparent solar transition to autumn equinox around September 22 nd, the first signs of wind reversal starts to appear and gradually gain momentum. The north- east monsoon would move centre stage roughly a month after the autumn equinox.

🌀 2. THE EVOLVING SYNOPTIC SETUP (19–25 SEP)

Andaman Low: A fresh cyclonic circulation emerging over the north Andaman Sea is organizing into a new low-pressure area around 19th- 20 th September, tracking west-northwestwards toward the south Odisha-north Andhra Pradesh coasts.This low should not be strictly construed as a monsoon depression as the basic criteria for a.monsoon depression will not be met .

Monsoon Withdrawal Watch

Climatological retreat is underway from extreme west Rajasthan and adjoining northwest plains, clashing dynamically with fresh moisture pulses pushing up from the east coast.

Shear & Convergence Zones: Active north-south wind convergence extending from Telangana and interior Karnataka down to south interior Tamil Nadu continues to generate robust peninsular thundershowers.

🔮 3. FORECAST PHASE BREAKDOWN
Phase 1 (19–21 September)

Active thundershower spells across Telangana, Coastal Andhra Pradesh, Karnataka, interior Tamil Nadu, and Kerala. Fairly widespread rainfall builds across Gangetic West Bengal, Odisha, and Northeast India as the new system develops.

Phase 2 (22–25 September): The newly formed east-coast low tracks inland, setting up a fresh late-season moisture surge across central and eastern India, while western and northwestern grids stay predominantly dry under the expanding withdrawal line.

🌴 4. SOUTH PENINSULA & KERALA OUTLOOK

Kerala: Humid and muggy conditions give way to afternoon/evening thundershowers driven by local convergence. Fairly widespread rainfall activity remains likely over central and high-range belts (Wayanad, Idukki, Pathanamthitta).

Tamil Nadu & Karnataka: Active north-south convergence keeps interior districts (Salem, Madurai, Coimbatore, and south interior Karnataka) primed for frequent, organized evening thundershowers.

🎯 5. REGIONAL SUMMARY TABL

East & Northeast India (Odisha, Gangetic West Bengal, Sikkim, Assam, Meghalaya)
Fresh low of Andaman-origin system tracking west-northwest toward Odisha/AP coasts.

🔴 Fairly Widespread Heavy rains in all these regions from 22 nd to 25 th September.

South Peninsula (Telangana, Coastal AP, interior Karnataka, Interior TN)
Active north-south shear zones and wind convergence zone triggering widespread thundershowers.

🟠 Widespread Thundershowers & Pockets of heavy rainfall : Vizag, Kurnool, Warangal , Vijayawada, Ramagundam, Hyderabad, Bengluru , Madhurai, ( Sept 21-23)

Kerala & Mahe (High Ranges & central Kerala)

Localized sea-breeze convergence and regional moisture pull.
🟡 Intermittent Afternoon Spells / Lightning Risk in eastern high ranges and central Kerala.

Northwest & Central India (West Rajasthan, Punjab, Haryana, Delhi)

Monsoon withdrawal phase in full swing; dry-air dominance coupled with boundary layer dynamics caused by a weak western disturbance causing some rainfall over NCR Delhi, west Uttarpradesh and west Madhya Pradesh between 24-26 th September.

🟡 Subdued / Progressive Dry-Out

Bihar, Jharkhand Chattisgarh
The fresh low over central Bay of Bengal could trigger heavy thundershowers over these regions.
Backloading the final lap of rains in these regions before monsoon retreat begins.

🌊 3. ENSO, IOD & MJO CLIMATE UPDATE

El Niño: The central-eastern equatorial Pacific continues to warm rapidly, with Niño-3.4 weekly readings exceeding +2.0°C. NOAA’s Climate Prediction Center highlights a greater than 90% chance of a very strong event (potentially peaking as a "Super El Niño" by late autumn/winter).

Indian Ocean Dipole (IOD): A positive IOD phase is actively strengthening into the September–November season, with recent weekly index values pushing near +0.65°C. This positive dipole provides crucial seasonal cushioning and moisture support, partially offsetting El Niño's drying influence.( See Pic-1 & Pic1-a)

Madden-Julian Oscillation (MJO): The MJO wave which had been indiscernible and weak and residing over western hemisphere is likely to quickly traverse to its Indian Ocean phase by the end of September 2-3 rd week which could whip up cyclonic activity in the Bay of Bengal. The MJO remains in a phase that favors convective pulsing across the eastern Indian Ocean and Maritime Continent, supporting cyclogenesis over the Andaman Sea and head Bay of Bengal while modulating regional vertical velocity fields.