HOUSTON (AP) — Tropical Storm Humberto formed off the Texas coast Wednesday, promising to bring more rain and possibly flooding to a state coming off one of the wettest summers in more than 50 years.

Forecasters warned residents along a 270-mile stretch of coastline extending into southwestern Louisiana to brace for the storm, which was expected to slosh ashore south of Houston overnight Wednesday.

Between 5 to 10 inches of rain was expected, with some spots possibly getting as much as 15 inches. But authorities said evacuations were not necessary.

‘‘This obviously isn’t an evacuation event or hurricane,’’ said John Simsen, emergency coordinator for Galveston County. ‘‘It’s too close to be in a position where it’s going to develop that much.

‘‘However, it looks to be a prolonged rain event and some pretty substantial tropical storm-force winds.’’

Texas has had one of the wettest summers on record, with Houston soaked under the most rain it’s had in a summer since 1942. With the ground already saturated, flooding was likely.

In Austin, Gov. Rick Perry activated 50 military vehicles with 200 soldiers, plus a half-dozen helicopters and two swift-water rescue teams. Other crews from the U.S. Coast Guard were on standby.

‘‘Some areas of our state remain saturated by summer floods, and many communities in this storm’s projected path are at high risk of dangerous flash flooding,’’ Perry said.

Brazoria County Sheriff Charles Wagner said low-lying areas in his coastal county, which includes Freeport, could present problems for residents and ‘‘put water into a lot of houses.’’ No evacuations, however, were recommended.

The storm’s center was predicted to come ashore probably in the Freeport or Galveston areas. Tropical storm warnings were posted from Port O’Connor to Intracoastal City, La.

By late afternoon Wednesday, Humberto was packing nearly 50 mph winds about 50 miles south-southwest of Galveston, moving to the north at 7 mph. It was expected to veer northeast into Louisiana Thursday.

Last month, at least six deaths were blamed on Tropical Storm Erin, which dropped nearly a foot of rain in parts of San Antonio, Houston and the Texas Hill Country.

In 2001, slow-moving Tropical Storm Allison soaked Houston, dumping about 20 inches of rain in eight hours. Some two dozen people died, sections of the city were paralyzed and damage was estimated at some $5 billion.

Humberto’s arrival comes just days after Galveston last Saturday marked the 107th anniversary of the great 1900 storm where more than 6,000 people were killed in what remains the nation’s deadliest natural disaster.

Humberto is the eighth named storm this year and formed from a depression that developed Wednesday morning. A depression becomes a named tropical storm when its sustained winds reach 39 mph and a hurricane when winds reach 74 mph.

Another tropical depression also formed Wednesday far in the open Atlantic, about 1,065 miles east of the Lesser Antilles. It had maximum sustained winds near 35 mph and was moving west-northwest at about 12 mph.

Forecasters said it could become a tropical storm later Wednesday.