NEW DELHI — A punishing pre-monsoon heatwave pushed temperatures past 47 degrees Celsius across a 2,000-kilometer arc of South Asia on Tuesday, shattering April records in more than two dozen cities and overwhelming hospitals, power grids and municipal water systems from Sindh through the Indo-Gangetic Plain into West Bengal.

The India Meteorological Department said the central Indian city of Nagpur reached 47.4 degrees Celsius shortly after 3 p.m. local time, the highest April reading in its 132-year observational record. Jacobabad, in Pakistan’s Sindh province, recorded 48.2 degrees Celsius, less than half a degree shy of its all-time April high. New Delhi’s Safdarjung station, the capital’s official benchmark, hit 45.6 degrees, the city’s hottest April day since 1941.

“What we are seeing today is not the upper edge of a normal April, it is something well beyond that envelope,” said Dr. Mrutyunjay Mohapatra, director-general of the India Meteorological Department, in a televised briefing from Mazagon Dock in New Delhi. “We are issuing red-category warnings across nine states, and we are advising the public to treat the next 72 hours as a public-health emergency.”

Indian authorities reported at least 73 suspected heat-related deaths since Friday, the highest cumulative toll for an April heatwave on record, with the largest clusters in Bihar, Uttar Pradesh and Madhya Pradesh. In Pakistan, the National Disaster Management Authority confirmed 41 deaths across Sindh and southern Punjab and said field hospitals had been set up in Sukkur, Larkana and the outskirts of Karachi to relieve pressure on overstretched emergency rooms. Bangladesh’s directorate of health services reported 14 fatalities, mostly day laborers and rickshaw pullers, across Dhaka, Chittagong and Khulna.

The heat dome, which forecasters at the European Centre for Medium-Range Weather Forecasts described as one of the most intense pre-monsoon patterns observed in the region this century, is being driven by a stubborn ridge of high pressure parked over central India. Subsiding air beneath the ridge has compressed and dried the lower atmosphere, suppressing convection and allowing surface temperatures to climb day after day. Forecasters said the pattern was likely to hold through at least Friday before a weak western disturbance arrived to break it.

“You have a dome that simply will not move, sitting on top of land that is bone-dry after a weak post-monsoon,” said Dr. Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology in Pune. “There is no soil moisture left to evaporate. Every joule of energy goes straight into heating the air. This is the textbook recipe for what we used to call once-in-a-decade heat, and we are now seeing it almost every year.”

The strain on power grids has been severe. India’s Central Electricity Authority said peak demand on Tuesday afternoon climbed to 251 gigawatts, breaking the previous all-time national record set last May. Rolling blackouts of two to six hours were reported across Uttar Pradesh, Bihar, Jharkhand and parts of Maharashtra, and the city of Lucknow imposed an emergency demand-response order on industrial consumers. In Pakistan, K-Electric, which serves Karachi, announced four-hour load-shedding rotations after a generating unit at the Bin Qasim complex tripped offline in the late afternoon. Sindh provincial authorities ordered all government offices and most private workplaces in Karachi closed Wednesday and Thursday.

Water systems are buckling alongside the grid. In New Delhi, the Delhi Jal Board said reservoir levels at Wazirabad had fallen so low that the city’s water-treatment capacity would be cut by roughly 18 percent through Friday, forcing tanker deliveries into hundreds of unauthorized colonies. The Bihar State Disaster Management Authority said more than 4,200 villages were reporting acute drinking-water shortages, and a senior official, speaking on condition of anonymity because he was not authorized to discuss internal planning, said the state was considering invoking emergency provisions to commandeer private tube wells.

“My wife and I take turns standing at the tanker queue from four in the morning,” said Ramesh Yadav, a 52-year-old auto-rickshaw driver in the Seemapuri neighborhood of east Delhi, who said his family had been receiving fewer than 40 liters of water a day for the past week. “If we miss a turn, we have nothing. And if we stand in the sun too long, we collapse.”

The Indian Council of Medical Research issued an advisory directing all district hospitals across the affected states to expand triage capacity for heatstroke and severe dehydration. Field doctors in Patna, Varanasi and Jaipur told reporters that emergency-room volumes had doubled or tripled since the weekend and that wait times for non-critical cases had stretched past six hours.

Agriculture officials warned of significant losses to standing wheat and mustard crops in the late-harvest pockets of Punjab and Haryana, and to mango orchards across Uttar Pradesh and southern Pakistan. Wholesale onion and tomato prices in major mandis spiked sharply on Tuesday.

“The numbers we are being given are almost certainly an undercount,” said Dr. Dileep Mavalankar, director of the Indian Institute of Public Health in Gandhinagar, who pioneered some of South Asia’s earliest urban heat-action plans. “Many of the dead are coded as cardiac, renal or respiratory failure. The heat is the trigger, but the death certificate hides it.”

Forecasters said the western disturbance expected to arrive Friday night would bring scattered thunderstorms across the northwestern plains over the weekend, with temperatures likely to fall 6 to 9 degrees in the worst-affected districts. Even so, climate scientists cautioned that relief would be partial and temporary, with the monsoon onset over Kerala still more than a month away. Authorities in India and Pakistan said expanded emergency measures, including possible additional workplace closures, would be announced city by city as the heat dome’s track became clearer.