Thaw may bring about flooding

Ice clogs the Delaware at Trenton. The river crested there at 20.76 feet, then receded to 19 feet. As ice shifts, it can pile up near river bends and bridges. When the jams break, water gushes downstream. The National Weather Service warned people to keep clear of the river: "The ice jam is an extremely unstable situation, as the ice could break without a moment's notice."
Ice clogs the Delaware at Trenton. The river crested there at 20.76 feet, then receded to 19 feet. As ice shifts, it can pile up near river bends and bridges. When the jams break, water gushes downstream. The National Weather Service warned people to keep clear of the river: "The ice jam is an extremely unstable situation, as the ice could break without a moment's notice." (APRIL SAUL / Staff Photographer)
Posted: January 11, 2014

The region is about to experience at least a mini-January thaw, but it might extract a price.

Along ice-clogged portions of the Delaware River, flood warnings and watches were posted for Friday morning, and the National Weather Service said temperatures rocketing to 60 on Saturday might conspire with heavy rains to set off more-widespread flooding.

On Thursday, the Coast Guard dispatched an ice-breaking harbor tug to the Delaware near Trenton to break up ice locked in place by the region's coldest outbreak in 20 years.

It was uncertain what effect the icebreaker was having, but the river level at Trenton had fallen to 19 feet late Thursday, down from 20.76 feet Wednesday evening, said Sarah Johnson, a meteorologist at the National Weather Service Office in Mount Holly.

Flood level there is 20 feet.

Whether that was a trend was unclear. "It's fluctuating," said Johnson.

Water behind the jam, about a mile south of the Route 1 bridge near Trenton, has been flowing over the riverbanks, flooding roadways on both sides as well as the Capitol garage in Trenton.

The weather service said river-and-stream ice was common throughout the region. As ice shifts, it tends to pile up near bends in streams and at bridge abutments.

When the jams break precipitously, water gushes downstream.

Johnson said that even though temperatures were expected to rise above freezing Friday morning after some wintry precipitation, the ice jams might survive into Saturday, when the air is to warm dramatically and heavy rains fall.

"That's our concern," she said.

The weather service warned people to keep clear of the river. "The ice jam is an extremely unstable situation," it said, "as the ice could break without a moment's notice."

In the short term, a flood watch was in effect until 8 a.m. Friday along the Delaware for Philadelphia, Bucks County, and portions of Burlington County.

In addition, a freezing-rain advisory was posted until 9 a.m. Friday, but temperatures were due to rise after midmorning.

Once the ice is gone, it shouldn't be back for a while, with no below-freezing readings due until Wednesday morning.


twood@phillynews.com

610-313-8210

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