The advisory was issued by the director health services KMC.
According to him, the temperature will likely soar to 40 C for three days April 22-24.
He also ordered setting up heatstroke wards at all government-run hospitals to cope with emergency cases during heatstroke.
Read more: Heatwave death toll in Karachi reaches 1260
Heat wave had killed more than 1100 people in Karachi IN June 2015.
The heat wave coincides with major power cuts, leading to harsh criticism of the provincial government and K-electric, the company that supplies electricity to Karachi.
Related read: Ways to protect yourself from a heatstroke
Temperatures had touched 44 degrees Celsius (111 degrees Fahrenheit) in the steamy karachi during those days, up from a normal summer temperature of 37 C (99 F).
Rangers had set up medical camps at several points in the city where they are providing water and anti-dehydration salts.
The provincial government was largely criticized by opposition parties for not managing the crisis, which had been exacerbated by severe power cuts.