Originally Published: 13 SEP 21 03:51 ET

Updated: 13 SEP 21 12:43 ET

By Madeline Holcombe and Jackson Dill, CNN

(CNN) -- Tropical Storm Nicholas is making its way toward Texas and Louisiana, threatening to drench the region for much of the week and flood some communities.

Having slowed to near a standstill in the Gulf of Mexico, the storm could strengthen significantly. A hurricane watch is in effect from Port Aransas to San Luis Pass, Texas, the National Hurricane Center said, meaning hurricane conditions -- with sustained winds of at least 74 mph -- are possible within 48 hours.

Storm surge will also be a risk, with 2 to 5 feet forecast through parts of Texas and Louisiana. A storm surge warning is in effect from Port Aransas to Sabine Pass, Texas, and for Galveston Bay, Aransas Bay, San Antonio Bay and Matagorda Bay; it indicates a danger within 36 hours of life-threatening inundation from rising water moving inland.

Nicholas had sustained winds of 60 mph storm and was 45 miles northeast of the mouth of the Rio Grande, the hurricane center said in an 11 a.m. ET update. The tropical storm was still trying to develop as it moved slowly northward toward the Texas coastline.

The center of Nicholas will pass Monday morning near the coasts of northeastern Mexico and south Texas and move onshore late Monday afternoon or evening along the coast of south or central Texas, the hurricane center said.

Almost 10 million under flash flood watch

The risk for flash flooding from Nicholas is increasing, with a level 4 of 4 risk for excessive rainfall along the coast today between Galveston Bay and Matagorda Bay.

There is "an extremely moist tropical airmass is in place" across these regions, resulting in predictions of high rainfall totals, the Weather Prediction Center said. Nearly 10 million people are under flash flood watches.

Some places will get much more rain than others.

"Rainfall amounts in excess of 10-15 inches with isolated higher totals are expected through the end of the day on Tuesday in some locations," the National Weather Service office in Houston said.

"Rather than fall in an even distribution, heavy rainfall will put down significant totals in relatively short periods, which enhances the threat of flash flooding."

Some spots could get up to 20 inches, CNN meteorologist Dave Hennen said.

Heavy rain will be the main threat, with a widespread 5 to 10 inches expected for southwest Louisiana. Across middle and upper coastal Texas, the storm is expected to produce 8 to 16 inches, the hurricane center said.

"This storm has the potential for widespread flash flooding. Houston can easily have problems with 4 to 5 inches of rain," said CNN Meteorologist Tom Sater. "More than that will create bigger problems."

There is also considerable flood threat through Wednesday in areas from Corpus Christi, Texas, through the Houston metro area and portions of western Louisiana, including Lake Charles.

Louisiana governor declares state of emergency

Louisiana Gov. John Bel Edwards declared a state of emergency Sunday ahead of Nicholas, he said in a news release.

"The most severe threat to Louisiana is in the Southwest portion of the state, where recovery from Hurricane Laura and the May flooding is ongoing. In this area heavy rain and flash flooding are possible. However, it is also likely that all of South Louisiana will see heavy rain this week, including areas recently affected by Hurricane Ida," Edwards said.

Nicholas may impact efforts to restore power after Ida, Edwards said. More than 117,000 customers statewide still had no power Monday morning, according to PowerOutage.us.

Houston prepares by lowering Lake Houston

Houston is preparing for moderate to heavy rain overnight from Nicholas, Mayor Sylvester Turner said Monday morning.

"This is a storm still with some unpredictability, but we know this is going to be primarily a rain event," he said.

Everyone should wrap up what they're doing by sundown so they can be home safe when the storm comes through, he said.

The city is preparing for the rain by lowering Lake Houston by 1 foot and deploying high-water equipment throughout the city because officials cannot pinpoint which locations will get more rain, the mayor said.

Houston Independent School District schools will be canceled Tuesday due to inclement weather, according to the school district's website. All after-school activities Monday are canceled as well. The district is the largest district in Texas, with over 275 schools.

Nicholas, the Atlantic hurricane season's 14th named storm, formed in the Gulf of Mexico Sunday. Typically, the 14th named storm doesn't form until November 18, Brian McNoldy, senior research associate at University of Miami's Rosenstiel School, said in a tweet.