A girl walks through waves as Hurricane Harvey approaches the Texas coast Friday. Harvey grew rapidly, accelerating from a Category 1 hurricane early in the morning to a Category 4 by evening.

CORPUS CHRISTI, Texas — With time running out, tens of thousands of people fled Friday from the path of an increasingly menacing-looking Hurricane Harvey as it took aim at a wide swath of the Texas Gulf Coast that includes oil refineries, chemical plants and dangerously flood-prone Houston, the nation’s fourth-largest city.

Texas Gov. Greg Abbott warned that the monster system would be “a very major disaster,” and the forecasts drew fearful comparisons to Hurricane Katrina, one of the deadliest ever to strike the U.S.

“We know that we’ve got millions of people who are going to feel the impact of this storm,” said Dennis Feltgen, a spokesman and meteorologist for the National Hurricane Center. “We really pray that people are listening to their emergency managers and get out of harm’s way.”

Fueled by warm Gulf of Mexico waters, Harvey grew rapidly, accelerating from a Category 1 early in the morning to a Category 4 by evening. Its transformation from an unnamed storm to a life-threatening behemoth took only 56 hours, an incredibly fast intensification.

Landfall was predicted for late Friday or early Saturday near Rockport, a fishing-and-tourist town about 30 miles northeast of Corpus Christi.

If it does not lose significant strength, the system will come ashore as the fiercest hurricane to hit the U.S. in 13 years and the strongest to strike Texas since 1961’s Hurricane Carla, the most powerful Texas hurricane on record.

Aside from the winds of 130 mph and storm surges up to 12 feet, Harvey was expected to drop prodigious amounts of rain — up to 3 feet. The resulting flooding, one expert said, could be “the depths of which we’ve never seen.”

Galveston-based storm surge expert Hal Needham said forecasts indicated that it was “becoming more and more likely that something really bad is going to happen.”

At least one researcher predicted heavy damage that would linger for months or longer.

“In terms of economic impact, Harvey will probably be on par with Hurricane Katrina,” said University of Miami senior hurricane researcher Brian McNoldy.

“The Houston area and Corpus Christi are going to be a mess for a long time.”

Federal health officials called in more than 400 doctors, nurses and other medical professionals from around the nation and planned to move two 250-bed medical units to Baton Rouge, La.

Other federal medical units are available in Dallas.