Tamil Nadu is unique — it depends on the Northeast monsoon. Unlike the rest of India, Tamil Nadu receives more rainfall from the Northeast monsoon (Oct–Dec) than the Southwest monsoon (Jun–Sep). The Western Ghats block SW monsoon moisture from reaching the eastern plains. The 2026 SW monsoon outlook is below-normal nationally, but the NE monsoon 2026 forecast will be updated closer to October.
| District / City | Day 1–2 (mm) | Day 3–4 (mm) | Day 5–7 (mm) | Confidence |
|---|---|---|---|---|
| Chennai | 0–5 | 5–20 | 10–35 | Moderate |
| Coimbatore | 2–8 | 5–20 | 10–30 | Moderate |
| Madurai | 1–5 | 3–15 | 8–25 | Moderate |
| Tiruchirappalli | 1–5 | 3–12 | 8–22 | Moderate |
| Salem | 2–8 | 5–18 | 10–28 | Moderate |
| Tirunelveli | 2–8 | 5–18 | 12–35 | Moderate |
| Vellore | 0–4 | 3–12 | 8–22 | Moderate |
| Nilgiris (Ooty) | 5–18 | 12–35 | 18–50 | Low–Moderate |
Note: GFS underestimates orographic rainfall over the Western Ghats (Nilgiris, Palani Hills). Table reflects SW monsoon period climatology — during the NE monsoon (Oct–Dec) coastal Tamil Nadu sees significantly higher totals.
For Tamil Nadu, the SW monsoon is a secondary rainy season. The Western Ghats block the Arabian Sea moisture, placing most of central and northern Tamil Nadu in a rain shadow. Only the Nilgiris and Palani Hills receive heavy SW monsoon rainfall through orographic enhancement. IMD's 2026 national forecast of 92% of LPA with El Niño suppression means even the SW contribution to Tamil Nadu will be below-normal in 2026.
The NE monsoon is Tamil Nadu's primary rainy season — onset is normally around 20–25 October. Chennai and coastal districts receive 60–65% of their annual rainfall during this period. Bay of Bengal low-pressure systems and depressions forming in October–November are the primary rain-bearing systems. El Niño years historically have a mixed influence on the NE monsoon — some El Niño years have brought above-normal NE monsoon rainfall to Tamil Nadu. The 2026 NE monsoon outlook will be issued by IMD in September 2026.