Temperatures have risen since Monday night and the cooling winds from the Arabian Sea have stopped, metrological official Shahid Abbas told dpa.
The mercury was set to hit 45 degrees on Wednesday, which would make it the hottest day this summer for the coastal city of more than 20 million people, Abbas said.
More than 100 people died due to heatstroke last week, said Anwar Kazmi, a spokesman for the Edhi Foundation, which runs a network of ambulances and several morgues.
Commuters cool off from the heatwave as they are showered by a water jet on the roadside in Karachi.
No deaths have been reported so far this week, Kazmi said, but he added that the foundation's 34 rescue centres, ambulances and morgues have been put on high alert.
The health department has advised people to stay indoors and avoid exposure to the sun, local official Nasir Durrani said.
There have been high temperatures elsewhere in the country, with 10 cities registering 48 degrees on Monday. The current wave is likely to subside from Thursday night, Abbas said.
In 2015 more than 2,000 people died in Karachi when a heatwave struck in the holy month of Ramadan.
