As Philadelphia and its suburban counties prepared for flooding and high winds from Hurricane Irene this weekend, Mayor Nutter warned Thursday that the storm could be the worst to hit the area in a half-century.

Rainfall could exceed seven inches, "which would send all streams, creeks, and the Schuylkill into flood stage," Nutter said. "Flash flooding in the street can also be expected. Tidal flooding along the Delaware River also is possible."

At 6 p.m. Thursday, Delaware Gov. Jack A. Markell declared a state of emergency and ordered nonresidents to evacuate coastal areas. He urged residents in beach communities to evacuate as soon as possible.

Philadelphia might experience sustained winds between 40 and 50 m.p.h. and gusts up to 60 m.p.h., which "could cause significant damage or downage to trees, power outages, and other infrastructure disruptions," Nutter said.

"This storm is coming," he said. "The only question is its severity. If you live in an area that is prone to flooding, prepare to evacuate as needed."

Peco Energy has placed extra personnel and crew on standby and was working to arrange for support from Commonwealth Edison in Chicago.

The city will provide another update Friday and possibly declare a severe weather emergency, depending on the storm track. Already, Philadelphia's rainfall this month has totaled 13.11 inches, surpassing the previous record for any month - 13.07 inches, set in September 2009.

The National Association of Insurance Commissioners, citing the hurricane, canceled its planned summer meeting of 1,500 members next week in Philadelphia.

The same caution and preparation were reported in the four suburban counties in Pennsylvania.

Delaware County's Emergency Operations Center will open late Saturday, and there will be additional staffing for the 911 center, said Chad Brooks, chief of operations for the Department of Emergency Services.

"The best thing is prevention," he said. He recalled the flash flooding that killed four people in Pittsburgh last week, and cautioned that people should not drive into low-lying or flooded areas.

In Bucks County, Neshaminy Creek is almost always the first to flood, spilling over in Northampton, Middletown, Bristol, and Bensalem Townships and several small boroughs along the way.

That has been of much less concern in recent years, however, because remediation programs have removed or elevated most of the threatened houses in flood-prone areas. Until the flood stage along the Neshaminy exceeds 14 feet, there is usually minimal damage, said John Dougherty, the county's emergency services director.

If the Delaware River overflows, however, a far wider array of communities is in harm's way. The riverside towns of Yardley and Morrisville, along with low stretches of Lower Makefield and Bensalem, are among the first to take on water, followed closely by New Hope, Riegelsville, and other communities along River Road from Bristol northward.

Especially damage-prone from even moderate flooding are recently repaired sections of the historic Delaware Canal, which was ravaged by three successive floods in the last decade only to suffer further harm in moderate flooding last spring.

In Chester County, "we are telling people what everyone is saying: Be prepared - the storm is going to hit. Watch out for flooding. If you come upon a flooded road, turn around," said Patty Mains, spokeswoman for the county's Department of Emergency Services. "Don't drown, turn around."