A week-long 'triple flood threat' has erupted across several states, bringing potentially life-threatening rainfall to more than 15 million people.

Flood watches have been issued in ten states Tuesday morning as meteorologists warn that the widespread effects of Hurricane Polo are merging with leftover moisture from Hurricane Odalys and a dip in the natural jet stream running across the US to create one massive storm.

This 'triple flood' is expected to bring the heaviest rainfall to Arizona, Colorado, New Mexico, Kansas, Oklahoma and Texas on Tuesday, but the storm threat will continue throughout the week.

The flood watches extend into Utah, Nebraska, Missouri and Iowa, and AccuWeather has predicted that more than two inches of rain could reach Arkansas, Illinois, Indiana, Michigan and Wisconsin by Friday.

'Rainfall rates within the most intense bouts of rainfall [this week] can reach 2-3 inches per hour, which can create a life-threatening situation,' AccuWeather meteorologist Brandon Buckingham said.

AccuWeather's latest forecast has warned that as much as eight inches of rain could flood parts of Colorado, Iowa, Kansas, Missouri, Nebraska, New Mexico, Oklahoma and Texas this week.

However, the weather service's worst-case scenario noted up to 18 inches of rain could fall in isolated areas.

The National Weather Service (NWS) has also issued flash flood warnings in Arizona and New Mexico on Tuesday. They urged anyone in a vehicle to avoid flooded roadways, noting that driving into these conditions is the most common cause of flood deaths. 

NWS added in a statement: 'Flooding of washes and creeks will occur, some dirt roads will become muddy and impassable. Paved roads and underpasses could become flooded as well.'

'Turn around, don't drown when encountering flooded roads. Most flood deaths occur in vehicles.'

AccuWeather Chief Meteorologist Jonathan Porter added: 'In some places, especially in New Mexico, this may be the most dangerous flash flooding risk in the last five years or more, underscoring the importance of watching for and avoiding rapidly rising water.' 

NWS warned that recent storms have already saturated the soil in the Southwest, making it even more difficult for the ground to absorb all the rainfall expected during this week's storm.

As a result, the highest risk of flooding and potential flash floods is projected to stretch across six states on Tuesday, with Arizona, New Mexico, Texas, Oklahoma, Colorado and Kansas all falling into the 'Moderate Risk' zone for excessive rainfall.

Forecasters warn that colder air pushing the jet stream south will end up carrying the moisture from Polo and Odalys east as it swoops into the Midwest and Great Lakes region this week.

The heaviest downpours and thunderstorms are expected to hit central Texas, Oklahoma and Kansas on Wednesday, followed by parts of Arkansas and Missouri on Thursday.

Isolated tornadoes and damaging wind gusts up to 60mph may tear through the South as the remnants of Polo make their way into the US. The major storm, which made landfall in Mexico on Monday, is expected to lose its hurricane status before crossing into the Southwest. 

AccuWeather meteorologist Alex Duffus added: 'This rainfall event will essentially consist of a triple threat combination: moisture leftover from Odalys, moisture transported from Polo and the incoming trough shifting across the West.'

'All pieces will work together to create a life-threatening rainfall event for some locations.'

In mountainous areas, the heavy rains may trigger local mudslides, and AccuWeather has projected that the greatest risk of widespread property damage and injuries will likely be seen across New Mexico and central Texas this week.

The storm has already begun to impact travel throughout the Southwest, with Phoenix Sky Harbor International Airport issuing a ground delay until at least 4pm ET due to thunderstorms.

Flights at the airport have been delayed by an average of 37 minutes as of 11am ET.

Hurricane Polo, a major component of this week's massive storm, struck the California Baja Peninsula in Mexico on Monday, flooding local towns and causing widespread property damage.

NWS has projected that Polo will weaken as it moves over land, but may still be a tropical storm as its eye nears the Texas border Tuesday night.