Out With the Monsoon, In With a Storm — A Bay of Bengal Low Brews Near 20 September

Two stories are unfolding at once in this run, and they're connected. Over the northwest, pressure is climbing and the monsoon trough is losing its grip — the model's clearest hint yet that the Southwest Monsoon is beginning its long goodbye. Over the Bay of Bengal, the opposite is happening: a low-level circulation is organising near the Andaman Islands, with gusts brushing Gale Force and a genuine closed low developing on the mean sea level pressure field by the back half of this outlook — squarely inside the 18–22 September window that puts 20 September right in the middle of it. Mainland India, for once, is quiet almost everywhere else — which only makes both signals easier to see.

The Quietest Mainland in Weeks — Except for One Place

Cumulated Rainfall (mm) and anomaly vs 1991–2020 LTM · GFS IC 13 Sep 2026 00 UTC · Left: 13–17 Sep · Right: 18–22 Sep

Central and Northern India Go Quiet — the Real Rain Is Offshore

Day 1–5 shows a largely flat rainfall anomaly across central and northern India — no organised belt of Heavy Rain anywhere on the mainland. By Day 6–10, a genuinely intense rainfall bullseye (300+ mm) develops over the Andaman Sea and the islands themselves, with the anomaly there among the strongest signals in the whole 10-day run. Coastal Andhra Pradesh, Odisha, and Tamil Nadu pick up a modest positive anomaly late in the period as the system's outer influence edges toward the mainland, but this remains, first and foremost, an offshore event.

The Monsoon Begins Its Long Goodbye

Mean Sea Level Pressure and anomaly vs 1981–2010 NCEP R2 LTM · GFS IC 13 Sep 2026 00 UTC · Left: 13–17 Sep · Right: 18–22 Sep

Pressure Rises Over the Northwest as the Monsoon Low Fades

The Day 1–5 mean field still shows the familiar monsoon low (1002–1004 hPa) sitting over Rajasthan — but the anomaly panel tells the real story: a positive MSLP anomaly (+2 to +7 hPa) is already building across Pakistan, Rajasthan, and northwest India. By Day 6–10, that low visibly weakens and shrinks on the mean map while the positive anomaly strengthens further — pressure rising exactly where the monsoon trough should be holding firm, a textbook precursor to withdrawal. IMD's own withdrawal criteria (five consecutive dry days, an anticyclone establishing at 850 hPa, and moisture depletion in satellite imagery) require live observations to confirm, but this model signature is consistent with those conditions starting to assemble over the northwest, right around the region's usual mid-to-late September withdrawal window.

Mean 2 m Temperature (°C) and anomaly vs 1991–2020 LTM · GFS IC 13 Sep 2026 00 UTC · Left: 13–17 Sep · Right: 18–22 Sep

Clearing Skies, Rising Temperatures Over the Northwest

A warm anomaly (+3 to +5°C) sits over Rajasthan, Punjab, and Haryana in both periods, alongside the much larger seasonal warming already underway over the Himalayan foothills as summer cloud cover clears. It isn't heatwave-intensity warming, but the direction matters more than the magnitude here: skies clearing and temperatures nudging above normal over the same northwest India footprint where the pressure is rising is exactly the combination you'd expect as the monsoon's cloud and rain shield lifts away.

Mean 200 hPa Wind (m/s) and speed anomaly vs 1991–2020 LTM · GFS IC 13 Sep 2026 00 UTC · Left: 13–17 Sep · Right: 18–22 Sep

The Tropical Easterly Jet Fades, the Westerlies Edge Back

The Tropical Easterly Jet (TEJ) — the upper-level easterly flow that powers an active monsoon — looks visibly weaker over peninsular India in both periods compared to peak-season runs, while the subtropical westerly jet continues to sit close to its autumn position over the Himalaya. That's the seasonal upper-air transition working as expected. The far more striking feature on this chart, though, is a very large positive wind-speed anomaly (+10 to +14 m/s) building over the Bay of Bengal and Andaman Sea — upper-level divergence strong enough to support real convective development below it.

The Bay Wakes Up — a Low Taking Shape Near Andaman

Mean 850 hPa Wind (m/s) and speed anomaly vs 1991–2020 LTM · GFS IC 13 Sep 2026 00 UTC · Left: 13–17 Sep · Right: 18–22 Sep

A Closed Cyclonic Circulation Wraps Up Over the Bay by Day 6–10

Day 1–5 shows an unremarkable low-level wind field. By Day 6–10, a distinct cyclonic circulation is visible in the wind vectors over the Bay of Bengal near 12–15°N, with wind speeds reaching 15–17 m/s wrapping around it — paired with a strong positive speed anomaly (+8 to +10 m/s) that lines up almost exactly with the closed low on the MSLP chart above. Three independent layers — MSLP, 850 hPa flow, and 200 hPa divergence — are all pointing at the same location during the same five days. That kind of vertical agreement is not something this model shows lightly.

The district data over Andaman & Nicobar backs this up directly. Very Heavy Rain (Orange) shows up at Nicobars on 17 September, North & Middle Andaman on 18 September, and South Andaman on 20 September — the alert effectively walking across the island chain as the circulation organises and shifts. Forecast gusts reach 79 km/h at South Andaman on 17 September and 75 km/h at North & Middle Andaman on 20 September — using the standard gust-to-sustained-wind ratio (gust ≈ 1.3–1.5× sustained), that implies sustained winds in the 50–61 km/h range, consistent with Depression-strength organisation. IMD has not declared any classification for this system as of this run, and none is implied here — what the model shows is a well-organising low-level circulation, not yet a named or classified system.

What This Means for the Northeast Monsoon

None of this happens in isolation. The Northeast Monsoon — the northeasterly low-level flow that waters Tamil Nadu, southern Andhra Pradesh, and Sri Lanka from around October — needs the Southwest Monsoon to withdraw and a continental high to build over northern India before it can properly establish. The pieces on this run's charts are consistent with that sequence starting to move: pressure rising over the northwest, the TEJ fading, and — perhaps most tellingly — a Bay of Bengal system already showing signs of organising this early. Bay cyclogenesis in the September–October transition is one of the more reliable early markers that the low-level pattern is shifting toward its post-monsoon configuration. It is too early to call any of this a confirmed Northeast Monsoon onset signal, and IMD's normal onset over Tamil Nadu isn't due until mid-October, but the ingredients on this run point in a favourable direction rather than against it.

Heavy Rain Alert Districts — 13–21 September 2026 (Model Guidance)

Date	State	Key Districts	Alert Level
17 Sep	Andaman & Nicobar	Nicobars	Very Heavy
18 Sep	Andaman & Nicobar	North & Middle Andaman	Very Heavy
20 Sep	Andaman & Nicobar	South Andaman	Very Heavy
20 Sep	Andaman & Nicobar	North & Middle Andaman	Heavy
14 Sep	Gujarat	Ahmadabad	Heavy
21 Sep	Karnataka	Dharwad, Gadag, Koppal	Heavy
16 Sep	Meghalaya	East Garo Hills, North Garo Hills, South Garo Hills	Heavy
14 Sep	Tamil Nadu	The Nilgiris	Heavy

Model guidance only — verify against latest IMD district bulletins. Districts flagged at the model's lower "watch" tier along the Tamil Nadu, Andhra Pradesh, and Odisha coast from 19 Sep onward are not shown here but are discussed above.

VayuMet Outlook — 13–22 September 2026

13–17 September — Quiet Mainland, Building Bay

• Mainland India: Isolated Heavy Rain only — Ahmadabad (Gujarat) and The Nilgiris (Tamil Nadu) on 14 Sep, the Garo Hills belt of Meghalaya on 16 Sep. No Orange or Red anywhere on the mainland.
• Northwest India: The monsoon low over Rajasthan weakens through the period as the positive MSLP anomaly builds — the model's clearest withdrawal precursor signal so far this season.
• Andaman & Nicobar: Nicobars crosses into Very Heavy Rain (Orange) on 17 Sep with gusts near 44 km/h, as the low-level circulation begins organising.

18–22 September — the Bay System Peaks, Withdrawal Signals Firm Up

• Andaman & Nicobar: North & Middle Andaman Very Heavy Rain on 18 Sep (180 mm), South Andaman Very Heavy Rain on 20 Sep with gusts to 79 km/h — the peak of this run's most significant weather.
• East coast mainland: Watch-tier signals reach Tamil Nadu's northern coast (Chengalpattu, Viluppuram), south Odisha (Malkangiri, Koraput), and north Andhra Pradesh (Alluri Sitharama Raju) from 19 Sep — the outer influence of the Bay system, not yet a confirmed landfall track.
• Northwest India: The MSLP anomaly and warm temperature anomaly both strengthen further, and the Tropical Easterly Jet continues to fade — the withdrawal picture becomes more coherent, not less, through the back half of this run.
• North interior Karnataka: Dharwad, Gadag, and Koppal pick up Heavy Rain by 21 Sep, a separate, more routine signal unrelated to the Bay system.

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