Bay of Bengal Depression Explained as Odisha Braces for Rain

IMD expects the well-marked low-pressure area over the Bay of Bengal to become a depression within about 24 hours and a deep depression after that, bringing it to the north Andhra Pradesh and south Odisha coasts around Wednesday morning, September 23. The heaviest rain is forecast for Wednesday, when four districts of south Odisha, Kandhamal, Gajapati, Ganjam and Puri, are expected to see extremely heavy rainfall of more than 20 cm. Fishermen have been told to return to the coast. None of this makes it a cyclone, and the labels IMD uses are precise enough that they are worth reading closely.

What IMD Has Said So Far

As of Monday, September 21, the system is described as a well-marked low-pressure area over the east-central and west-central Bay of Bengal, a stage below a depression. IMD’s outlook is that it becomes a depression over the west-central Bay within 24 hours and a deep depression over the west-central and adjoining northwest Bay in the 24 hours after that. Reports put it near the north Andhra Pradesh and south Odisha coasts around the morning of Wednesday.

North coastal Andhra Pradesh is forecast to get heavy to very heavy rain on Tuesday and Wednesday. South Odisha, Chhattisgarh and Telangana are forecast to see heavy showers over three days, with the extremely heavy spell reserved for south Odisha on Wednesday. Winds of 55 to 65 km/h are forecast along the south Odisha and north Andhra coasts around landfall, and up to 75 km/h over the west-central and northwest Bay. Further south, the Regional Meteorological Centre in Chennai has forecast heavy rain at isolated places in 21 Tamil Nadu districts on Monday, with light to moderate rain and thunderstorms likely in Chennai in the evening or night.

Depression, Deep Depression, Cyclone: What the Labels Mean

IMD classifies a system in the Bay of Bengal by its maximum sustained wind speed, measured in knots. The scale is what separates a rainy low-pressure system from a named cyclone.

IMD category	Sustained wind (knots)	Roughly in km/h
Depression	17 to 27	31 to 50
Deep depression	28 to 33	50 to 61
Cyclonic storm (gets a name)	34 to 47	62 to 88
Severe cyclonic storm	48 to 63	89 to 117
Very severe cyclonic storm	64 to 89	118 to 165
Extremely severe cyclonic storm	90 to 119	166 to 220
Super cyclonic storm	120 and above	221 and above

Sources round the knot-to-km/h conversion slightly differently, so the km/h column can differ by a point or two between publications. What matters here is where the forecast stops. IMD’s outlook ends at deep depression, one stage below a cyclonic storm, and the coastal wind forecast of 55 to 65 km/h sits inside the deep depression band. A system is named only once it reaches cyclonic storm strength, so unless it strengthens beyond the current forecast, this will be tracked by its category and not by a name.

How Much Rain Is “Extremely Heavy”

IMD’s rainfall words are also defined by numbers, measured over 24 hours.

IMD term	Rain in 24 hours
Heavy	64.5 to 115.5 mm
Very heavy	115.6 to 204.4 mm
Extremely heavy	204.5 mm and above

That is why the Odisha forecast reads “more than 20 cm”: 204.5 mm is the threshold for the top category, so Kandhamal, Gajapati, Ganjam and Puri are being warned of the highest rainfall band IMD uses, not simply a lot of rain. Rain that heavy in a day is what turns rivers and low-lying streets into problems, which is why the warning is about the district and the day, not the system’s label.

What Is Not Settled Yet

The exact landfall point and hour, and whether the system reaches deep depression strength before the coast, will only be known as the depression forms. The reports checked also do not say which colour alerts (yellow, orange or red) are in force for each district, so anyone in the affected districts should check IMD’s district-wise warnings and their state disaster authority directly. This article will be updated as IMD issues its next bulletins.

IndiaRealTime’s live pages carry conditions for the districts in the forecast: Puri and Ganjam in Odisha, Visakhapatnam on the Andhra coast, and Chennai, plus the state pages for Odisha and Andhra Pradesh. For how IMD builds and words these forecasts, see IndiaRealTime’s guide to how IMD weather forecasts work, and for the season this system sits in, why the monsoon withdrew two days late in a season short on rain.

FAQs

Is this a cyclone?
No. IMD’s forecast is for a depression and then a deep depression. A cyclonic storm needs sustained winds of at least 34 knots.

Where and when is landfall expected?
Near the north Andhra Pradesh and south Odisha coasts around the morning of Wednesday, September 23, according to reports of IMD’s outlook.

Which Odisha districts face the heaviest rain?
Kandhamal, Gajapati, Ganjam and Puri, where extremely heavy rainfall of more than 20 cm is forecast for Wednesday.

What does “extremely heavy rain” mean?
IMD uses it for 204.5 mm or more of rain in 24 hours, its highest rainfall category.

What is the difference between a depression and a deep depression?
Both are below cyclone strength. A depression has sustained winds of 17 to 27 knots, a deep depression 28 to 33 knots.

Are fishermen affected?
Yes. Reports say fishermen have been advised to return to the coast because of rough seas.

Sources

• The Week: IMD alert, deep depression in Bay of Bengal to intensify, heavy rain in four states
• LatestLY: Tamil Nadu rain alert, heavy rain forecast in 21 districts
• International News and Views: Weather today, September 21, 2026, IMD rain alert
• Wikipedia: Tropical cyclone intensity scales, IMD classification
• India Meteorological Department: district-wise warnings

Updated September 21, 2026.