NEW ORLEANS -- A cold, grey day greeted revelers gathering Tuesday along parade routes as New Orleans' famed Carnival season headed to a crest with the unabashed celebration of Mardi Gras.

The first street marching groups -- including clarinetist Pete Fountain's Half-Fast Walking Club -- were to begin their marches along oak-lined St. Charles Avenue and into the business district. Later, the floats of the Zulu and Rex parades and hundreds of truck trailers decorated by family and social groups would wind down St. Charles Avenue.

Rain was forecast to begin in the late morning, but revelers were still expected to gather by the tens of thousands in the French Quarter, where the bawdy side of Mardi Gras was expected to be on full display.

Mark Nelson of St. Louis said he would be in the mix even in a downpour. It's his first Mardi Gras.

"That's why God made washing machines," said Nelson, who was sipping on a daiquiri as he enjoyed the sounds of trumpeter Kermit Ruffins and the Barbecue Swingers, who performed at the Lundi Gras festivities Monday along the Mississippi River.

Parades were scheduled to roll throughout south Louisiana and in coastal Mississippi and Alabama, sharing the traditions brought by French colonists in the 18th century.

In Louisiana's bayou parishes, riders on horseback would go from town to town, making merry in what is called the Courir du Mardi Gras.

The merriment must come to a halt at midnight, when the solemn season of Lent begins. New Orleans police were expected to sweep down Bourbon Street at midnight in the annual ritual of letting revelers know the party is over for another year.

The Zulu krewe's 2014 Witch Doctor, Derek Rabb, said he was charged with praying for the krewe's good health and good weather on Mardi Gras. "By God's grace, there will be sun," he said.

When out of costume, Rabb works at New Orleans-based Entergy Corp. A member of the organization for the past eight years, he said being in such a high-profile position has been an experience he won't soon forget.

"It's been a whole lot of fun," he said. "It's allowed me to meet some really interesting people."