IMD uses four colours for its district warnings: green means no warning, yellow means be updated, orange means be prepared and red means take action. For Chennai, an orange alert is the level at which most households and offices should start changing plans, as Saturday’s deluge in the north of the city showed.
On 17 and 18 October, IMD placed Chennai, Tiruvallur, Chengalpattu, Kancheepuram, Tiruvannamalai and Villupuram under orange alert, ETV Bharat reported. Parrys and Kathivakkam each recorded more than 30 cm overnight. Here is how to read the warnings properly and where to find them.
The four colours and what they ask of you
| Colour | IMD label | What to do in Chennai |
|---|---|---|
| Green | No warning | Normal routine. |
| Yellow | Be updated (Watch) | Heavy rain possible at isolated places. Check forecasts before travel. |
| Orange | Be prepared (Alert) | Move vehicles from low spots, charge phones, stock essentials, avoid subways at night. |
| Red | Take action (Warning) | Stay indoors unless told to move. Follow evacuation orders. |
The labels come from IMD’s own warning legend. In everyday reporting, the colours are often paraphrased as be aware, be prepared and take action. A Deccan Herald explainer notes that red is reserved for weather certain to disrupt travel and power and pose a significant risk to life.
The colours are based on expected impact, not a fixed rainfall number. That is why an orange alert for Chennai in a week when lakes are full means more than the same colour after a dry spell.
Rainfall words decoded
IMD bulletins use standard terms. Its seven-day forecast for the subdivision defines them:
- Heavy: 64.5 to 115.5 mm in 24 hours (roughly 7 to 11 cm).
- Very heavy: 115.6 to 204.4 mm (12 to 20 cm).
- Extremely heavy: more than 204.4 mm (21 cm or more).
The spread of rain is also coded. Isolated means 1% to 25% of stations, scattered 26% to 50%, fairly widespread 51% to 75% and widespread 76% to 100%. So “heavy rain at isolated places” means a few spots, not the whole city. Thunderstorm codes such as TSL (thunderstorm and lightning) and TSLG (with gusty winds) also appear in the tables.
The rain day runs from 8.30 am to 8.30 am. A report saying Chennai got 16 mm on average does not cancel out 300 mm in one neighbourhood.
District warnings and nowcasts
There are three products worth bookmarking:
- District-wise warnings: IMD’s district map shows colours for five days, from today to four days ahead. Click Chennai rather than relying on a state summary.
- Nowcasts: short-range warnings for the next few hours, district by district and station by station. They are issued when thunderstorms build up and are the best guide for the evening commute.
- Regional Meteorological Centre Chennai: the RMC Chennai page carries the city forecast, a seven-day forecast, an impact-based forecast, radar images and 24-hour rainfall data.
For alerts on your phone, the state’s TN-Alert app sends location-based warnings, with distinct sounds for different alert levels.
Cyclone warnings use a different ladder
When a cyclone approaches, IMD issues a four-stage warning: a pre-cyclone watch about 72 hours ahead, a cyclone alert (yellow) at least 48 hours before bad weather reaches the coast, a cyclone warning (orange) at least 24 hours ahead and updated every three hours, and a post-landfall outlook (red) at least 12 hours before landfall. These sit alongside the district rain colours.
A real example from this week
The forecast IMD Chennai issued at 1 pm on 17 October flagged heavy to very heavy rain at isolated places over Chengalpattu district for the first day and widespread rain across the subdivision. By that evening, north Chennai had extremely heavy rain in pockets: Parrys and Kathivakkam crossed 300 mm, while the city average for the 24 hours was far lower. The next morning, IMD placed Chennai and five neighbouring districts under orange alert and eight more districts, including Ranipet, Vellore and Cuddalore, under yellow, ETV Bharat reported.
The takeaway is that “isolated” can mean your neighbourhood. When an orange alert covers Chennai, plan as though the heaviest rain could fall on your street.
Common mistakes
- Treating a yellow alert as no alert. Isolated heavy rain can still flood your street.
- Reading forwarded images without dates. Check the issue time on IMD pages.
- Assuming school closures follow automatically. District collectors decide those separately.
- Ignoring night forecasts. Monsoon showers here often peak in the evening and around midnight.
Quick answers
What does an orange alert mean for Chennai?
It means be prepared: heavy to very heavy rain is likely and can disrupt travel and services.
How much rain is “very heavy”?
IMD defines it as 115.6 to 204.4 mm in 24 hours.
Where can I see Chennai’s district warning?
On IMD’s district-wise warnings map and on the RMC Chennai website.
How long is a nowcast valid?
Nowcasts cover the next few hours only. Check the issue time printed on each one.
Information checked against official sources on 18 October 2026. Fees, dates and rules can change; confirm on the linked official pages before acting.
Sources
- Warning colour legend (India Meteorological Department)
- District-wise warnings (India Meteorological Department)
- Regional Meteorological Centre Chennai (IMD)
- 7-day subdivision forecast issued 17 Oct 2026 (IMD Chennai)
- Four stage cyclone warning (RSMC New Delhi, IMD)
- What do IMD’s colour-coded weather warnings mean (Deccan Herald)
- Several areas inundated, orange alert for six districts (ETV Bharat)
- TN-Alert app listing (TNSMART / RIMES)
