"We're forecasting widespread rains in the country, especially in flood-affected areas," Qamar-uz-Zaman Chaudhry, director general of Pakistan's meteorological department, said.
More than 1,600 people been killed by the floods, which started last week when torrential monsoon downpours hit the north-west of the country.
Swollen rivers are carrying a huge volume of water south, raising fears that further destruction lies ahead.
In Sindh province, half a million people have been evacuated in anticipation of the arrival of the torrent of floodwater.
"We are seeing a number of preparations being made across Sindh province," Al Jazeera's Imran Khan reported from the southern coastal city Karachi.
"So far 500,000 people have been evacuated. Nearly 250,000 homes have been destroyed across the provinces bordering Sindh.
"The floods are coming further south. Nobody knows whether the floodwaters will reach Karachi, but severe flood warnings have been issued."
The poor weather forecasts have grounded helicopters flying rescue missions to affected areas.
US military personnel were forced to abandon flights to stranded communities in the upper reaches of the hard-hit Swat Valley, as storm began to dump more rain on the stricken region, where many thousands are living in tents or crammed into public buildings.
About 85 US military personnel are taking part in the relief activities, along with six helicopters that have been flown over from Afghanistan, where some 100,000 American troops are based battling the Taliban.
Meanwhile 30,000 Pakistani soldiers are rebuilding bridges, delivering food and setting up relief camps in the northwest, which is the main battleground in the fight against al-Qaeda and the Taliban.
Foreign countries and the United Nations have pledged millions of dollars to help reconstruction efforts.
Yusuf Raza Gilani, Pakistan's prime minister, appealed for international aid during a televised address to the nation after visiting flooded areas.
"Pakistan has been hit by worst floods of its history," he said. "At this time of crisis I would like to appeal to the international community to support Pakistan to help alleviate the suffering of the flood affected people."
Criticism of Pakistan's government has intensified in recent days, with particular ire directed at Asif Ali Zardari, the president, who went ahead with state visits to Europe at the height of the disaster.
|The worst affected areas are in red; moderately affected areas are in yellow. Evacuations are under way in the striped area of Southern Sindh province
Zardari's absence has provoked angry reaction from many of those affected, and the row has intensified debate over his leadership of the country, where the security situation has been deteriorating and corruption is rampant.
""Our president prefers to go abroad rather than supervising the whole relief operation in such a crisis," Ghulam Rasool, a resident of the town of Sukkur, said.
"They don't care about us. They have their own agendas and interests."
Aid organisations are struggling to cope with scale of the disaster.
Food supplies are running low in some areas and health experts warn that the conditions are ripe for the spread of disease.
Many ordinary Pakistanis have simply been left to fend for themselves in the aftermath of the flooding.
"We don't have anything, no one has given us even a single penny," Khair Mohammad, a cattle-breeder who has been displaced by the disaster, said.