Tropical Storm Cristobal could soon renew its strength by uniting with another storm system coming from the west to form one giant cyclone, forecasters warned, as missing boaters were found safe and sound.

In Louisiana, two boaters were found Monday afternoon in good condition after their boat sank in a deepwater straight near Slidell on Sunday, officials said.

The two told deputies they survived in the water by clinging to one life jacket Sunday afternoon and Sunday night before they were able to reach the shore Monday morning, St. Tammany Parish sheriff’s Sgt. Suzanne Carboni said in a statement.

The pair then swam across a marshy area and were found sitting on the porch of a camp, Carboni said.

“We were largely spared,” Louisiana Gov. John Bel Edwards said.

Cristobal provided the state with “a good test” of overall hurricane response and preparedness, particularly combined with ongoing coronavirus response efforts, the governor said.

President Trump agreed to issue an emergency declaration for Louisiana, officials said.

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After drenching much of the South, forecasters said they expected the remnants of Cristobal to unleash fierce winds, heavy rain and thunderstorms on much of the Midwest by Tuesday.

A very strong storm system sweeping out from the Rocky Mountains is expected to meld with Cristobal in the next couple of days, said Greg Carbin, who has overseen forecasts at the Weather Prediction Center in College Park, Maryland.

“The two will eventually merge into a large cyclone,” Carbin said. “It’s a pretty fascinating interaction we’ll see over the next couple of days.”

Isabelle Schneidau, left, gesturing to the camera as she walked in a rising storm surge with Mont Echols, center, and L.G. Sullivan, right, after checking on their boats in the West End section of New Orleans on Sunday.

Wind gusts of up to 45 mph are expected in Chicago by Tuesday night, the National Weather Service added. Boaters were being warned of gale-force winds on nearby Lake Michigan on Tuesday and Wednesday.

High winds could be felt from Nebraska to Wisconsin, forecasters said. In parts of Iowa, Minnesota and South Dakota, the gusty winds and low humidity will bring the threat of wildfires in areas with dry grass, National Weather Service forecasters warned. Any blazes that start will spread rapidly, they said.

Cristobal weakened into a depression early Monday after inundating coastal Louisiana and ginning up dangerous weather along most of the U.S. Gulf Coast, sending waves crashing over Mississippi beaches, swamping parts of an Alabama island town and spawning a tornado in Florida.

Heavy rainfall and a storm surge continued posing a threat across a wide area of the Gulf coast after Cristobal made landfall Sunday afternoon packing 50-mph winds between the mouth of the Mississippi River and the since-evacuated barrier island resort community of Grand Isle.

Rudy Horvath handing a piece of wood up to his wife, Dawn Horvath, as their home, a boathouse in the West End section of New Orleans, took on water from storm surge in Lake Pontchartrain on Sunday.

At 5 p.m. ET Monday, the storm was centered about 110 miles north of Monroe, Louisiana, with top winds of 35 mph. It was moving north at 18 mph.

Cristobal’s remnants could be a rainmaker for days. Its forecast path takes it into Arkansas and Missouri by Tuesday, then through Illinois and Wisconsin to the Great Lakes.

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“It’s very efficient, very tropical rainfall,” National Hurricane Center Director Ken Graham said in a Facebook video. “It rains a whole bunch real quick.”

In their last update on Cristobal from the hurricane center, forecasters said up to 15 inches of rain could fall in some areas, and could cause significant river flooding across the mid and upper Mississippi Valley.

The Associated Press contributed to this report.