New Delhi: The India Meteorological Department (IMD) issued advance district-level rainfall forecasts and colour-coded impact-based warnings across Northeast India, including districts of Upper Assam, ahead of the heavy to very heavy rainfall recorded from July 18 to 20, 2026.
The information was provided by Minister of State (Independent Charge) for Earth Sciences Dr. Jitendra Singh in a written reply in the Lok Sabha. The government said the forecasting and early warning system is being strengthened through high-resolution weather models, advanced monitoring systems and improved observational infrastructure.
IMD Issued 7-Day Rainfall Forecasts for Upper Assam
Before and during the heavy rainfall episode, IMD issued district-level forecasts and colour-coded warnings for all districts of Upper Assam, including:
- Jorhat
- Golaghat
- Sivasagar
- Charaideo
The forecasts captured the occurrence of heavy and very heavy rainfall with a lead time of up to seven days. Daily colour-coded warnings were also issued for the following seven days.
The forecasts followed IMD’s standard rainfall classifications, covering different rainfall intensities from light and moderate to heavy and very heavy rainfall, along with spatial categories ranging from scattered to widespread rainfall.
Weather Systems Behind Heavy Rainfall in Assam
The heavy rainfall between July 18 and 20 was associated with multiple synoptic weather systems and atmospheric conditions.
On July 17, the mean sea level monsoon trough extended towards the Northeast Bay of Bengal. At the same time, a low-pressure area was present over Gangetic West Bengal and adjoining areas, while an upper-air cyclonic circulation existed over Northeast Assam and neighbouring areas.
Between July 18 and 20, the upper-air cyclonic circulation shifted towards Central Assam and adjoining Nagaland and persisted during the period.
Another upper-air cyclonic circulation over North Jharkhand and adjoining Bihar, along with an associated trough extending through Gangetic West Bengal, Bangladesh and Tripura, also supported strong monsoonal activity over Upper Assam.
Detailed weather bulletins related to the rainfall episode have been archived by the Regional Meteorological Centre, Guwahati.
IMD Evaluates Forecast Accuracy Across Assam
IMD regularly evaluates its extreme heavy rainfall forecasts across different regions of Assam. The assessment covers West Assam, South Assam, Central Assam and Northeast Assam.
The main performance indicators include:
- Percentage of Correct forecasts (PC)
- Probability of Detection (POD)
- False Alarm Ratio (FAR)
- Critical Success Index (CSI)
The evaluation covers forecast lead times from Day 1 to Day 5.
West Assam
In West Assam, the Percentage of Correct forecasts ranged from 72.14% on Day 1 to 80.94% on Day 5.
The Probability of Detection declined from 0.6345 on Day 1 to 0.1209 on Day 5, while the False Alarm Ratio ranged from 0.738 to 0.9613. The Critical Success Index ranged between 0.1936 on Day 1 and 0.0264 on Day 5.
South Assam
South Assam recorded Percentage of Correct forecasts ranging from 60.48% on Day 1 to 84.68% on Day 5.
The Probability of Detection decreased from 0.525 on Day 1 to 0.0425 on Day 5. The False Alarm Ratio ranged from 0.855 to 0.75, while the Critical Success Index ranged from 0.12 to 0.0425.
Central Assam
In Central Assam, the Percentage of Correct forecasts improved from 66.36% on Day 1 to 80.18% on Day 5.
The Probability of Detection stood at 0.6929 on Day 1 and decreased to 0.0757 on Day 5. The False Alarm Ratio ranged from 0.8114 to 0.8883, while the Critical Success Index ranged between 0.1671 and 0.0429.
Northeast Assam
Northeast Assam recorded a Probability of Detection of 0.7229 on Day 1, declining to 0.1857 on Day 5.
The region recorded a comparatively lower False Alarm Ratio, ranging from 0.3913 on Day 1 to 0.5538 on Day 5. The Critical Success Index ranged from 0.4275 on Day 1 to 0.19 on Day 5.
The Percentage of Correct forecasts in Northeast Assam remained relatively stable, measuring 58.47% on Day 1 and 51.61% on Day 5.
ENSO and IOD Considered in Seasonal Rainfall Forecasts
IMD also considers large-scale climate drivers while preparing seasonal rainfall forecasts for India, including Northeast India.
These include:
- El Niño–Southern Oscillation (ENSO)
- Indian Ocean Dipole (IOD)
- Sea surface temperatures
- Atmospheric circulation
- Other oceanic and atmospheric indicators
IMD uses a combination of dynamical coupled climate models and statistical techniques to generate probabilistic seasonal rainfall forecasts.
El Niño Impact on Northeast India Remains Complex
The government said ENSO and IOD can influence rainfall variability in Northeast India. While El Niño is generally associated with below-normal southwest monsoon rainfall across much of India, Northeast India can experience normal to above-normal rainfall during some El Niño years.
The relationship between El Niño and rainfall in Northeast India is considered relatively weak and complex because rainfall in the region is also strongly influenced by Bay of Bengal moisture transport, monsoon circulation and orographic effects.
Currently, moderate El Niño conditions are prevailing, while the IOD remains neutral, and these factors have been incorporated into IMD’s seasonal rainfall outlooks.
However, seasonal climate conditions alone do not determine flood risk. Short-duration extreme rainfall, river basin conditions and catchment characteristics also play an important role.
CWC Uses IMD Forecasts for Flood Warnings
The government clarified that operational flood forecasting in India is carried out by the Central Water Commission (CWC).
CWC combines IMD’s rainfall forecasts with:
- Hydrological observations
- River gauge data
- Hydrological models
- Observed river conditions
Therefore, while ENSO and IOD help assess seasonal rainfall variability and potential flood conditions, actual flood forecasts depend primarily on observed and forecast rainfall, river conditions and hydrological modelling.
High-Resolution BharatFS Strengthens Forecasting
To improve the spatial resolution and accuracy of forecasts at the sub-district level in Northeast India, an advanced Numerical Weather Prediction system has been operationalised.
The high-resolution New Bharat Forecasting System (BharatFS) operates at a 6-km resolution and is designed to support forecasts at block and panchayat levels.
The system is supported by enhanced computing capabilities through the “Arunika” and “Arka” systems.
Mission Mausam Introduces Advanced Early Warning System
Under Mission Mausam, the government has introduced the advanced Multi-Hazard Early Warning-Decision Support System (MHEW-DSS) for real-time monitoring and forecasting.
The observational network across Northeast India is also being strengthened through the installation of state-of-the-art observing systems.
These measures are aimed at improving the ability to monitor severe weather, provide timely warnings and support decision-making during extreme rainfall and potential flood events.
Read also: Strong El Niño May Intensify Heatwaves and Rainfall Extremes, WMO Alerts
