Tamil Nadu rain: IMD heavy-rain warning runs to 10 November, next one is for 15 November

The report's summary says rain occurred at many places over Tamil Nadu, Puducherry and Karaikal. Maximum temperatures fell by 2 to 3 degrees Celsius at a few places and were below normal by the same margin at isolated places. Madurai Airport recorded the state's highest maximum, 34.0 degrees, and Karur Paramathi the lowest minimum on the plains, 22.0 degrees. Uthagamandalam recorded 11.2 degrees in the hills.

The report also gives the temperature changes. Maximum temperatures fell appreciably, by 2.1 to 4.0 degrees, over the Thanjavur, Chennai, Kanyakumari, Nagapattinam, Ramanathapuram and Tiruvallur districts and Karaikal, and markedly, by more than 4 degrees, over Cuddalore district. Maximums were below normal by 1.6 to 3.0 degrees over Chennai, Kanyakumari, Nagapattinam, Ramanathapuram and Thoothukudi districts and Karaikal, and appreciably below normal over Tirunelveli district. A cooler day is what heavy cloud and rain would be expected to bring.

The cause is an upper-air cyclonic circulation over the Comorin area and its neighbourhood, at 0.9 km above mean sea level, which persists from the day before. A trough that ran from it to the east-central Arabian Sea has become less marked.

Thunderstorms with lightning and gusty winds of 30 to 40 kmph are likely at one or two places over Tamil Nadu, Puducherry and Karaikal on both days. For Chennai and its neighbourhood the local forecast is partly cloudy skies with light rain in some areas on 9 and 10 November, a maximum of 32 to 33 degrees and a minimum of 24 to 25 degrees.

The week ahead: no heavy-rain warning until 15 November

The heavy-rainfall warning section of the report names only three dates: 9, 10 and 15 November. For 11 to 14 November the report forecasts light to moderate rain at isolated places, mostly in the delta and southern districts, and mainly dry weather over several districts, but no heavy-rain warning.

On 15 November the report has heavy rain at isolated places over Thanjavur, Thiruvarur, Mayiladuthurai, Nagapattinam and Karaikal, and moderate rain at a few places over a longer list that includes Chennai, Tiruvallur, Chengalpattu, Kancheepuram, Ranipet, Villupuram, Cuddalore, Tiruchirappalli and Puducherry. A forecast made six days ahead can change, and the department updates its report every day.

How to read the warnings

The words that describe how widely rain will fall are also defined. In the department's usage, rain at isolated places covers up to a quarter of the stations in an area, at a few places covers 26 to 50 per cent, at many places 51 to 75 per cent, and at most places 76 to 100 per cent. So a warning of heavy rain at isolated places over eight districts does not mean all eight districts will be soaked, only that one or a few spots in them will cross the heavy-rain threshold.

The department grades rain by the amount that falls in 24 hours. Light rain is 2.5 to 15.5 mm, moderate rain is 15.6 to 64.4 mm, heavy rain is 64.5 to 115.5 mm, very heavy rain is 115.6 to 204.4 mm and extremely heavy rain is more than 204.4 mm. The 107.4 mm at Kayalpattinam was therefore in the heavy band, near the top of it.

A forecast of heavy rain at isolated places means that a few spots will get it and most will not. The warning describes a risk for a district, not a certainty for every locality in it. That is why the same report can give Chennai only light rain while a district to the south has a single day above 100 mm.

For anyone travelling or planning work in the delta districts or the far south, the practical steps are to check the district colour on the department's map, to watch for notices from the district authorities, and to allow extra time on 10 and 15 November. This article does not include school closures or other local decisions, since the report does not mention them.