Swollen by unseasonal Midwest rainfall during the past month, the Mississippi River is now expected to crest at 15.5 feet on July 20 and 21 at the Carrollton Gage in New Orleans, according to a 28-day forecast issued Wednesday (July 8) by National Weather Service's Lower Mississippi River Forecast Center.

The river will reach 15 feet at the Carrollton Gage on Tuesday, which will trigger Army Corps of Engineers "phase II flood fight" rules that will increase restrictions on construction near the river levees. The corps and local levee districts also will increase their inspections of levees to at least once a day.

The corps instituted its phase I flood fight rules on March 17, which allowed construction on or within 1,500 feet of levees only with a waiver, and required inspections of levees three times a week. The new restrictions will halt all work within 1,500 feet of the levee, including excavations and pile-driving, said corps spokesman Ricky Boyett.

Boyett said the corps has been averaging a phase II flood fight once every three years.

"However, it is not common for us to enter a phase II this late in the season," he said. "Additionally, as you know, this forecast cannot project (additional) rainfall occurring over the 28 days, so this information could continue to change if we see more rainfall in the valley."

As of 6 p.m. Tuesday, the corps had identified 91 active inspection sites along the Mississippi and Atchafalaya rivers in Louisiana, including five that it lists as medium priorities.

These include 14 sand boils, with 11 at Point Pleasant, on the east bank of the Mississippi just above Empire; and 20 seepage areas, including two in the Lake Borgne sector, which includes the Mississippi's east bank between Chalmette and Pointe a la Hache; one on the western levee along the Atchafalaya River; 10 in the Upper Pontchartrain sector on the east bank of the Mississippi in St. James, St. John and St. Charles parishes; and two in the Upper West Mississippi sector, which includes the west bank levee between the Morganza Spillway and the Port Allen lock in Pointe Coupee and West Baton Rouge parishes.

Both the east bank and West Bank regional levee authorities also have stepped up their inspections.

Surveillance patrols will be conducted daily along levees controlled by the Southeast Louisiana Flood Protection Authority-East, said authority executive director Robert Turner. He said floodgates that would be affected by the rising water also will be closed.

"We haven't found any problems with our (Mississippi River) levees," Turner said. "Typically, with the river going over 15 feet, we would expect to find a few localized areas of seepage. These areas usually occur in the same places with every high river. When they occur, we monitor them to make sure they are not having a negative impact on levee stability."

On the West Bank, the staff of the Southeast Louisiana Flood Protection Authority-West will increase its inspections to twice daily, seven days per week, said Susan Maclay, president of the authority.

"Inspections will be performed by two inspectors in separate vehicles with one inspector beginning at the St. Charles Parish line and the other at the Plaquemines Parish line," she said.

"The inspectors will alternate beginning points for each inspection. All floodgates will be inspected and made ready as necessary. The condition of all levee crowns will be reviewed to determine if aggregate surfacing repair is required," Maclay said. "In this instance, the river is projected to fall rapidly (4.5 feet in 14 days) so SLFPA-W will maintain vigilance throughout this period. We are also notifying all permit holders that all subsurface work (including pile driving) must stop for the duration of the high water event."

The Governor's Office of Homeland Security and Emergency Preparedness is monitoring the river and keeping in touch with both corps and local levee officials, a spokesman said Wednesday.

The rising river also is likely to trigger additional restrictions on river traffic by the U.S. Coast Guard. When the river is at 15 feet and rising, vessels entering South Pass from the Gulf of Mexico are required to keep at a minimum speed of 10 mph. The Coast Guard may also consider implementing other traffic rules after conferring with its waterway partners.

Vessels have been restricted to one way traffic around Algiers Point since the river rose above 8 feet at the Carrollton Gage, officials said.

According to river forecasters, the Mississippi will still be at 10.8 feet at the Carrollton Gage on Aug. 4, the last day of the 28-day forecast, which also is well into the most active period of the the Atlantic hurricane season.

Fortunately, this year's hurricane season has been forecast to be below average in activity, in part because of the same weather anomaly that seems to be triggering the above-average rainfall: El Nino.

That's a term used by climatologists to describe warmer than normal surface water conditions in the east and central Pacific Ocean. Those water conditions are linked to brisker upper level winds in the Atlantic that shear off the tops of clouds that could be forming into tropical depressions or storms.

This year, there also has been an active Bermuda High system over the Atlantic that has extended into the Gulf of Mexico, again limiting the development of tropical waves into depressions or tropical storms.

The present high pressure conditions are expected to last at least another six days, according to several forecast models used by hurricane experts.

That's good news, since at 15.5 feet,  even a relatively weak hurricane, like the Category 1 Hurricane Isaac that hit New Orleans in 2012, could add enough storm surge to the Mississippi to cause it to overflow along the "sliver by the river" in Uptown.

The river rose from 3.79 feet above sea level on the morning of Aug. 28, 2012, before Isaac made landfall, to a peak of 9.51 feet at 8 a.m. on Aug. 29, or just under 6 feet.

While the official flood stage at the Carrollton Gage is 17 feet, floodwalls protect the city of New Orleans to water levels of 20 feet, according to the National Weather Service.

If a similar rise were to occur when the river crests on July 14, it would result in a water level of a foot or more higher than that protection level.

If a storm were to form in the Gulf, corps officials might consider opening either the Bonnet Carre Spillway or the Morganza Spillway, or both, to lower the river level. Opening either of the spillways could take several days, however, and the process has never been done to protect from surge.