Lightning strikes and thunderstorms killed 88 people, injured many and caused widespread damage to property in the last two days in Bihar.
The state disaster management department said 23 districts of the state were gravely affected with Gopalganj accounting for the maximum number of casualties at 13, NDTV reported.
Uttar Pradesh has also been affected by the thunderstorms and has reported casualties.
PM Modi expressed his anguish at the loss of lives and said the state governments are working towards relief and rehabilitation.
"Received tragic news about death of several people in some districts of Uttar Pradesh and Bihar due to heavy rains and lightening. The state governments are engaged in relief work with promptness," he tweeted in Hindi.
Reports from districts said more than 20 people were injured and admitted to hospital. Lightning strikes has also caused widespread damage to houses and properties of the residents.
Bihar Chief Minister Nitish Kumar has announced a compensation of Rs 4 lakh for the families of the victims.
Mr Kumar has also appealed to everyone to remain alert and stay indoors as much as possible during the storms.
Home Minister Amit Shah also expressed grief on the tragedy.
"I am extremely saddened by the death of many people in Uttar Pradesh and Bihar due to heavy rain and lightning. Relief work is going on fast in both the states. I express my condolences to the families of those who have lost their lives in this disaster and wish the injured to be well soon," he tweeted in Hindi.
Bengal Chief Minister Mamata Banerjee also extended her condolences to the victims' families.
Congress leader Rahul Gandhi also tweeted his condolences.
"I am shocked to hear the news of 83 people dying due to lightning in Bihar. May God grant strength to their loved ones to bear this sorrow," he said in a tweet in Hindi.
"I appeal to the Congress workers to extend all possible help to the families of the victims," he added.
The weather office has predicted thunderstorms in all the 38 districts of the state over the next few days along with "heavy to very heavy rainfall" in the northern, flood-prone districts, bordering Nepal.