Mr. Edwards activated 3,000 members of the National Guard, 1,100 of whom were deployed to New Orleans, where there was a storm surge warning. Flood warnings were in effect for Lafayette and Baton Rouge.

Rain, said city and state officials, will be the biggest threat from Barry. Up to 20 inches could fall in some places. In New Orleans, the aging pumps have proved vulnerable to break downs and power failures in recent years.

Mayor LaToya Cantrell of New Orleans urged people to shelter in their homes beginning on Friday night. She said the city was breaking from its traditional practice of handing out sandbags during severe weather so that sand would not clog storm drains.

The intense rains that are expected this weekend, and the deluges causing the Midwestern floods that have engorged the Mississippi, are consistent with the effects of climate change, in which a warmer atmosphere holds more moisture and can release it in thrashing downpours.

Ghassan Korban, the executive director of New Orleans’s Sewerage and Water Board, which runs the city’s water pumps, cautioned on Friday that the city “could have a repeat” of the widespread flooding seen earlier in the week, when a strong storm on Wednesday dumped up to nine inches of rain in some neighborhoods, temporarily turning streets all over town into shallow rivers.

Beyond the rain, officials with the Army Corps of Engineers will be closely watching how high the storm surge pushes up the Mississippi River. As of Friday morning, the river level stood at just above 16 feet, close to the low point of 20 feet for some stretches of the levees. Forecasters projected late Friday that the river would crest at about 17 feet, lower than the 19 feet projected earlier.