Meteorologists have warned of potentially heavy rainfall across Nepal from Thursday and urged people to remain alert as a weather system develops over the Bay of Bengal.

According to the Meteorological Forecasting Division of the Department of Hydrology and Meteorology, Nepal is currently under the partial influence of both the monsoon and westerly winds. The monsoon trough remains south of its normal position. A low-pressure area has formed over the northern Andaman Sea and adjoining areas and is expected to strengthen and move west-northwest, the division said.

Binod Pokharel, an associate professor at the Central Department of Hydrology and Meteorology at Tribhuvan University, said several forecast models point to the possibility of exceptionally heavy rain.

“After looking at several forecast models, the probability of heavy rain appears high. Some models show more than 400 millimetres of rain, with one showing as much as 741 millimetres,” Pokharel said in a social media post. “That is an enormous amount of rain. It could fall across many parts of Nepal or in just a few places, but either way, the consequences could be serious.”

The influence of a system developing over the Bay of Bengal is expected to increase from September 24 to 28, potentially bringing substantial rainfall to Nepal. Meteorologists expect the system to overlap with a westerly weather system. If the two systems reinforce each other, rainfall could become more intense as they draw moisture towards Nepal.

The India Meteorological Department says a low-pressure system is developing around the Andaman Sea. 

Its influence could increase weather activity over the Bay of Bengal and send more monsoon moisture towards Nepal, meteorologists said.

Sanjib Adhikari, a meteorologist at the Meteorological Forecasting Division, said the eastern system could interact with the weather system to the west. “Based on the initial track, there is a possibility of very heavy rainfall in Sudurpaschim and central Nepal,” Adhikari said. He urged people to monitor weather information and take precautions as the situation develops.

Whether a cyclone forms over the Andaman Sea or not, the system could still affect Nepal, Adhikari said.

Pokharel likewise said it remained uncertain whether the system would develop into a cyclone, but the threat of heavy rain in Nepal remained. He said many areas could receive repeated spells of heavy rainfall from east to west.

If the weather system moves slowly, the rain could continue for an extended period, he said.

The Department of Hydrology and Meteorology classifies rainfall of less than 10 mm in 24 hours as light, 10 mm to less than 50 mm as moderate, 50 mm to less than 100 mm as heavy, 100 mm to less than 200 mm as very heavy, and more than 200 mm as extremely heavy.

Nepal’s monsoon normally withdraws around October 2. Adhikari said the current weather pattern could keep the monsoon active until around that date.

Last year, the monsoon withdrew from Nepal much later. The Department of Hydrology and Meteorology says it fully withdrew on October 10, 2025, eight days later than the normal withdrawal date.

This year, the monsoon entered Nepal on June 19, six days later than the normal onset date of June 13.

Nepal has also experienced several episodes of heavy October rainfall in recent years. Pokharel said heavy rain has become a recurring feature of October over the past five years. Heavy rainfall in October 2022, 2024 and 2025 caused significant loss of life and property.

Meteorologists also expect El Niño to alter rainfall patterns this year. The World Meteorological Organisation says El Niño is now firmly established and is expected to strengthen into a very strong event. Its latest assessment gives a near-100 percent probability that the phenomenon will persist through February 2027. 

For September-November, the WMO expects El Niño to drive significant shifts in rainfall and temperature patterns worldwide. It has also warned of increased risks of extreme weather, including floods, drought and extreme heat.