TropPath10pmOct4

A tropical depression in the Caribbean was expected to strengthen to Hurricane Nate by the time it enters the Gulf of Mexico on Saturday with landfall occurring somewhere between southeast Louisiana and Florida on Sunday, the National Weather Service said.

The storm shifted slightly westward by about 30 miles during the day Wednesday and is forecast to make landfall Sunday near Destin, Florida, said National Weather Service forecaster Phil Grigsby.

"But there's still a big spread and a lot of uncertainty," Grigsby said. 

Grigsby said that uncertainty means the storm could make landfall anywhere between southeast Louisiana in the Plaquemines Parish area and eastward along the Gulf Coast to the Tallahassee area of Florida. 

He cautioned that residents from Florida to Louisiana should "keep an eye on it until we know better where it's going."

At 10 p.m. Wednesday, the system was moving northwest near 6 mph, a motion expected to continue through early Thursday. It's expected to shift more northerly and gain speed on Thursday and Friday. Maximum sustained winds were near 35 mph.

The storm was forecast to enter the Gulf on Saturday approximately 1 p.m., with 80 mph winds, said Grigsby, who works in the Weather Service's New Orleans/Baton Rouge regional office.

Grigsby said the storm was still a tropical depression about 4 p.m. Wednesday.

Alek Krautmann, another National Weather Service meteorologist working in the regional office, said that Louisiana residents likely will begin seeing the effects of the storm this weekend as chances for rain increase, along with strong easterly winds that will bring higher tides along the coast.

"From Louisiana to Florida, we need to closely monitor this every day for the next several days to see where it will go," Krautmann said of the path the storm will take.

While it's the 16th tropical depression this year, it will be the 14th named storm, a category that includes hurricanes and tropical storms, when it becomes Tropical Storm Nate, said Barry Keim, Louisiana's state climatologist. The average, he said, is 12 named storms, he said.

"This certainly ranks among the busier seasons," Keim said.

The year 2005, the year of hurricanes Katrina and Rita, when "sea surface temperatures were running very high," holds the historic record of having the most named storms, with 18 at this point in October that year, he said.

Among this year's named storms were the five major hurricanes Irma, Harvey, Maria, Lee and Jose, each bringing major destruction and loss of life.

Very warm sea surface temperatures in the tropical Atlantic and relatively low wind shear — winds blowing at very high altitudes — are two factors figuring into the active 2017 hurricane season, Krautmann said.

"An increase in wind shear can blow apart storms," Krautmann said.

And, he said, 2017 is not an El Nino year, referring to the periodic warming of water in the Pacific that impacts weather patterns globally.

One effect of El Nino is an increase in "winds at upper levels that can suppress hurricanes," Krautmann said.

"Is global warming making it worse? There are lots of opinions on that," Keim said. 

There's one model of continuous warming of the planet that shows "there's likely to be the same or fewer number of storms, but the ones we get would be stronger," he said.

Also to be considered, he said, is a long-recognized weather pattern called the Atlantic Multi-Decadal Oscillation, characterized by periods of low hurricane activity for 30 to 40 years, followed by periods of great activity, with warmer sea temperatures, for another 30 to 40 years.

The 1920s to the 1960s was a period of high hurricane activity while the 1960s to the mid-1990s was not, Keim said.

"We're in this active cycle that started in 1995," he said.

While the hurricane season officially begins every year on June 1 and ends Nov. 30, it typically peaks on Sept. 10, Keim said. The weeks before that point, from mid-August to the first week in October, are considered the "prime time for named storms. After that, the chances diminish."

"But they happen," he said.   

As far back as the mid-1800s, Louisiana has been impacted by five hurricanes that came in October, including the deadly Cheniere Caminada hurricane on Oct. 2, 1893, that devastated the island by that name and killed 2,000 people.    

The most recent October hurricane was Hurricane Lili that made landfall in Louisiana on Oct. 3, 2002. 

Hurricane Juan in 1985 lingered in Louisiana over three days at the end of October, holding the latest landfall record in Louisiana since record-keeping began in the state in 1851. 

"We were still dealing with it on Halloween night," Keim said.

As the seasons began to change with the approach of fall, so do the origins of hurricanes, Krautmann said.

"Earlier in the (hurricane) season, they start as tropical waves off the African coast," he said.

As fall approaches, trade winds in the Atlantic begin to weaken, minimizing those conditions, he said.

"Later in the season, hurricanes tend to start in the western Caribbean, where the water is still very warm," he said. 

Follow Ellyn Couvillion on Twitter, @EllynCouvillion.