Welcome! This weather analysis provides an insightful look into the shifting weather patterns affecting South India today, September 20, 2025. Please remember that this is a personal weather assessment, and you should adhere to official forecasts and advice from the Indian Meteorological Department and government authorities.
Key Changes in Meteorological Setup
The regional weather setup has seen minor changes since yesterday. The East-West Shear Zone has shifted slightly upwards towards the Andhra regions. Simultaneously, an upper air circulation structure continues to stand over the Myanmar regions.
This upper air circulation is expected to move initially west-northwest and then southwest, eventually settling over the central west Bay of Bengal (specifically, the North Head Bay).
Formation of Low-Pressure System Imminent
The movement of this system will lead to significant development:
1. Strengthening: Over the next 24 to 36 hours, the upper air circulation is expected to strengthen into a low-pressure area, then intensify further into a deep depression, and potentially a severe deep depression.
2. Movement: This intense system is projected to move between Visakhapatnam and Bhubaneswar sometime between September 23rd and 25th.
Monsoon Dynamics: MISO and Convective Rain Decrease
Until this Bay of Bengal system is fully operational (around September 23rd or 24th), heat convection rain (வெப்ப சலன மழை) will persist but is expected to decrease, starting gradually today.
This decrease in convective rain is linked to the strengthening of the system over the central west Bay of Bengal, which consequently increases the moisture flow from the Southwest Monsoon winds. When the intensity of the Southwest Monsoon winds increases, rainfall tends to decrease in rain shadow areas. Conversely, rain is expected to resume in regions like Kerala, Karnataka, and Maharashtra.
The weather picture is further clarified by the Monsoon Intraseasonal Oscillation (MISO). MISO data shows that the period from September 8th to 14th was largely a monsoon break period, which allowed the heat convection rain to gain strength.
However, during the current period (September 15th to 21st), the MISO structure, indicated by a dark blue shade, is positioned between Tamil Nadu and Andhra Pradesh, and along the Western Coastal regions. This movement signifies that the Monsoon is reviving (the convergence zone is moving up from the south).
Looking ahead (September 22nd to 28th), the MISO indicates that the monsoon axis will move further north, entering Central Indian regions, including Gujarat and Maharashtra.
Long-Range Outlook: Break Phase and NEM Dates
Following the northern shift, another break phase in the monsoon is anticipated from September 29th to October 5th. This break is predicted because the deep depression moving through the Maharashtra and Gujarat regions will be pulled away by a western disturbance. During this subsequent break (starting slowly from September 29th), heat convection rain is expected to restart.
The withdrawal of the Southwest Monsoon is set to begin after an upper air circulation moves into the Odisha regions, though this withdrawal may be temporarily hindered before proceeding quickly.
Crucially, the onset of the Northeast Monsoon (வடகிழக்கு பருவமுனை) is forecast to be possible between October 18th and 22nd.
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24-Hour Rainfall Forecast (September 20, 2025)
Kerala
• Moderate to Heavy Rain: Expected in Idukki, Pathanamthitta, Malappuram, Wayanad, Kozhikode, Kannur, and Kasaragod districts.
• Light to Moderate Rain (Isolated): Expected in the eastern parts of Malappuram, Palakkad, Thrissur, Ernakulam, Kottayam, Alappuzha, the eastern and western parts of Pathanamthitta, Kollam, and Thiruvananthapuram.
Karnataka
• Moderate to Heavy Rain: Expected in Bangalore and surrounding areas, Tumkur, Mandya, Mysore, Chamrajnagar, Hassan, Kodagu, Chikmagalur, Chitradurga, Udupi, and Dakshina Kannada districts.
• Moderate to Heavy Rain (Widespread): Expected in Bidar, Kalayana Karnataka regions (Bidar, Gulbarga, Yadgir, Raichur, Bellary), Koppal, Bagalkot, Belgaum, and Bijapur areas.
• Light or Moderate Rain: Expected in other remaining districts.
Andhra Pradesh & Telangana
• Andhra Pradesh (Coastal and Rayalaseema): Widespread moderate to heavy rain is expected in Kurnool, Anantapur, Nandyal, Kadapa, Nellore, Kavali, Tirupati, and Chittoor districts. Moderate to heavy rain is also widely expected in northern coastal districts including Visakhapatnam, Rajamundry, Kakinada, Konaseema, and Godavari districts. Light rain is possible in some other areas.
• Telangana: Moderate to heavy rain is limited to Hyderabad and surrounding areas, Vikarabad, Mahabubnagar, Narayanpet, Vanaparthi, Jagumppa, Kurnool, Nagarkurnool, Nalgonda, Warangal, Kothagudem, Mulugu, and Mahabubabad. Only light rain is possible in other remaining districts.
Tamil Nadu and Puducherry
Due to the northward shift of the East-West shear zone, much of Southern Tamil Nadu is expected to receive light to moderate rain.
• Light Rain (Isolated): Expected in districts including Nagercoil, Tirunelveli, Tenkasi, Thoothukudi, Virudhunagar, Madurai, Theni, Dindigul, Pudukkottai, Sivaganga, Ramanathapuram. This includes western areas like Coimbatore, Valparai, Tiruppur, Karur, Trichy, and Perambalur.
• Moderate to Heavy Rain (Isolated): Rain is expected in Nilgiris, Erode, Krishnagiri, Dharmapuri, Salem, Namakkal, Vellore, Tirupathur, Tiruvannamalai, Kallakurichi, Ranipet, Chennai, Tiruvallur, Kanchipuram, Chengalpattu, Villupuram, Puducherry. Delta districts (Cuddalore, Ariyalur, Thanjavur, Mayiladuthurai, Nagapattinam, Karaikal, Tiruvarur) are also included.
• Chennai Outlook: The expectation is for moderate rain, with isolated heavy rain possible in some areas west of Chennai, but not significantly heavy rainfall overall.
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