Houston, Sep 14 (IANS): Tropical Storm Nicholas, with gusty winds and storm surge, is posing a major rainfall flood threat to parts of southern US states Texas and Louisiana, meteorologists have forecasted.

The storm could become a strong tropical storm or Category 1 hurricane prior to making landfall along the middle Texas coast as soon as Monday night, according to the US National Hurricane Center (NHC), Xinhua news agency reported.

"The storm has been very changeable, even this morning," Dan Reilly with the National Weather Service said on YouTube.

"The forecast continues to be tweaked."

NHC satellite imagery showed the storm rotating just off the Gulf Coast earlier on Monday ahead of landfall, according to a Houston Chronicle report. The storm stretches along the entire Texas coast and beyond, encompassing more than half of the Gulf of Mexico.

A hurricane watch has been in effect for parts of Texas since the weekends.

Heavy rain and flash flooding are a major concern in parts of Texas and Louisiana, said a report from U.S. weather channel, noting flooding has already closed some roads along the middle Texas coast.

Stronger wind gusts might cause some tree damage and scattered power outages in spots. The winds could also down some trees, especially where soil becomes saturated from Nicholas' heavy rain, the meteorologists warned.