The weatherman has warned of heavy to very heavy rainfall at isolated places in Bihar over the next couple of days.

The warning comes after Patna witnessed the season's heaviest rainfall of 107.6mm in 24 hours.

The rain started around 11.30pm on Thursday night and continued till Friday morning. Intermittent rain kept lashing the city throughout the day on Friday. "The axis of monsoon trough has now shifted to the Himalaya foothills," Patna Met director Sumendu Sengupta said, giving reasons for the sudden downpour. "That explains the light to moderate rains, coupled with heavy spells in the entire state. Besides, formation of an upper air cyclonic circulation over northern parts of Uttar Pradesh and adjoining Bihar are also aiding effective rainfall activity in the state."

"Southwest monsoon is active over Jammu and Kashmir, Punjab, Bihar and sub-Himalayan Bengal & Sikkim," the India Meteorological Department (IMD) said in its weather outlook on Friday. "Heavy to very heavy rain occurred at isolated places over Bihar, Himachal Pradesh, Uttarakhand, sub-Himalayan Bengal & Sikkim." The IMD attributed the rain to the axis of monsoon trough at mean sea level, which continues to run close to the foothills of the Himalayas along with an upper air cyclonic circulation that lies over sub-Himalayan Bengal and the neighbourhood and extends up to 1.5km above mean sea level. It warned of heavy to very heavy rain in the next few days.

Patna Met department scientist Anand Shankar confirmed the same. "There would be widespread rain across Bihar in the next five days. The intensity of rain will be higher tomorrow and day after."

Since 8.30am on Thursday, Patna had recorded 107.6mm of rain. The figure for Darbhanga was 46mm, Buxar 70mm, Bhagalpur 18mm, Gaya 15.4mm, Chhapra 10.6mm, Muzaffarpur 9.8mm, Gorakhpur 15.9mm and Bahraich 3.2mm. According to Skymet Weather intensity of rain will slow down next week. Sengupta confirmed the same, attributing it to the trough line shifting to southern states.

Foothills of both Uttar Pradesh and Bihar have recorded some good spells of rain in the last 24 hours, the intensity being greater over Bihar in contrast to Uttar Pradesh.

Heavy rains coupled with rains lashing neighbouring Nepal have, meanwhile, triggered fears of flash floods in the state.

According to information received from the Central Water Commission, water level of the river Bagmati is expected to rise by 142 cm while and that of Kosi by 36cm. Mahananda, too, showed a rising trend with its water level expected to rise by 32cm by Saturday. Rains are expected in the catchment areas of all rivers till Saturday morning.

The Central Flood Control Room in Patna claimed all embankments in the state were safe.