Arshad Sharif, 50, had been in hiding since July after he fled Pakistan to avoid arrest. He had been critical of the country's military and the government of Prime Minister Shahbaz Sharif.
Khan was disqualified after an election panel found he unlawfully sold state gifts and concealed assets as premier, officials said. He was ousted in a no-confidence vote in the parliament in April.
Extremely heavy rain fell in the hardest-hit provinces. About 75% more water is falling during the heaviest rainstorms in the region, according to a new scientific analysis.
U.N. Secretary-General Antonio Guterres appealed to the world to help Pakistan after arriving in the country Friday to see climate-induced devastation from months of deadly record floods.