New Orleans’ new storm defence systems will be tested as the USA’s first tropical storm of the year threatens to turn into a hurricane.

A state of emergency has been declared in the state of Louisiana where already high river levels are causing flooding ahead of the storm.

There are fears that colossal rainfall – rather than than wind – could lead to life-threatening conditions, with New Orleans’ $20 billion levees and seawalls predicted to near capacity.

Tropical Storm Barry is currently spinning through the Gulf of Mexico and expected to intensify into a hurricane and make landfall on Saturday local time.

JUST IN: Tropical Storm Barry has formed in the Gulf of Mexico with winds of 40 MPH. Barry is expected to strengthen by later today, potentially becoming a hurricane by late Friday into early Saturday. https://t.co/L9kqPBjBOe pic.twitter.com/it0CCcV0kI

— ABC News (@ABC) July 11, 2019

It is expected to test the limits of New Orleans’ great walls built after Hurricane Katrina killed more than 1500 people in 2005 when the levee system failed and flooded the city.

The new network of levees and walls is billed as one of the largest public works projects in the world.

Authorities have warned that for the first time, the Mississippi River’s levels are already high even before the hurricane makes landfall, which is predicted to occur on Saturday, local time.

Extremely high water levels from record flooding this year could lead to an unprecedented challenge, CNN reports.

“This is the first time we’ve had a tropical system with water levels on the river this high,” said Jeffrey Graschel, a hydrologist with the National Weather Service’s Lower Mississippi River Forecast Center.

Heavy rain has caused flash flooding in parts of New Orleans as a tropical storm intensifies. Photo: AAP

Evacuations are already occurring in parts of the state, while areas of New Orleans are battling high waters and streets are flooded.

Weather forecasters predict as much as 380mm of rain could fall over a 24-hour period.

Tropical Storm Barry has been declared slow moving and low intensity and is not expected to cause widespread damage from winds.

However its crawling pace across the low-lying state – which is also sinking – could dump buckets of rain over vulnerable regions.

The National Guard contingency has been sent to assist with rescue operations.

Parts of New Orleans and the state of Louisiana have been evacuated as a tropical storm bears down in the Gulf of Mexico. Photo: AAP

The river has been sitting at 16 ft (5m) when normally it should be at between 6-8 feet (2.5m).

It could rise to as high as 19 feet (5.8m) but the city is only protected by levees of 20 feet (6m).

A $20 billion system of levees and walls protects New Orleans in the aftermath of Hurricane Katrina. Photo: Getty

Mississippi Governor John Bel Edwards declared a state of emergency and said National Guard troops and high-water vehicles would be positioned all over the state.

“The entire coast of Louisiana is at play in this storm,” Mr Edwards said.