This photo made available by NASA shows Hurricane Harvey over Texas on Saturday, as seen from the International Space Station. (Jack Fischer/AP)

Even if Harvey had been a milder storm, spinning lazily across the Gulf of Mexico, there would have been reasons for alarm early last week.

For starters, the jet stream — the air current that meanders across the continent, pushing storms along a familiar path — flowed far north of Texas, and thus when Harvey crashed into the state there was nothing in the atmosphere to shove it somewhere else. Harvey stalled.

After making landfall, the storm took a path that positioned it almost perfectly to drag huge bands of rain out of the Gulf and onto the metropolis of Houston, which is interlaced with rivers and bayous and paved over with impervious urban surfaces. Essentially parked near the coastal town of Victoria, Harvey has dumped trillions of gallons of water across Southeast Texas.

Harvey also proved that the Gulf of Mexico — in late August, in a warming climate — can prove explosive for the development of what is generically called a tropical cyclone. In barely more than a day the storm went from a disorganized tropical depression to a significant hurricane and then all the way up to Category 4 — the second-highest rating on the Saffir-Simpson hurricane intensity scale, which is based on wind speed.

Flooding persists as Harvey downgraded to tropical depression

The result: a record-setting storm that within 24 hours plunged much of the nation’s fourth-largest city and its surroundings under feet of choppy brown water. It threatens to submerge even more of the region during the next few days.

“There are a lot of worst-case-scenario stars that aligned,” said Marshall Shepherd, a professor of atmospheric sciences at the University of Georgia and a past president of the American Meteorological Society. “As bad as it is now, we still have days of this to go.”

The National Weather Service downgraded Harvey to a Category 1 hurricane on Saturday and then to the status of a tropical storm. The rating change might have given people a false sense of security that the worst of the storm was over.

On social media and elsewhere, weather officials and meteorologists emphasized that the danger had not passed and was actually just beginning.

“More people die of inland flooding than any other hurricane threat,” the National Weather Service tweeted multiple times.

“Houston, let me be blunt,” Travis Herzog, a meteorologist for the ABC affiliate in Houston said on Twitter early Saturday. “Prepare for a flood today. Prepare for multiple tornadoes. Do not underestimate #Harvey. Respect the water.”

A stunning amount of rain has fallen over Texas so far, and it is expected to continue for several more days as the storm creeps along, weakening slightly. In general, things will probably get worse before they get better; some areas might see as many as 50 inches of rainfall when all is said and done, wreaking damage that experts predict could lead to years of recovery across the region.

As of Sunday afternoon, the storm had deposited 9 trillion gallons over Southeast Texas, as bands of rain picked up moisture from the Gulf. That’s enough water to fill up the Great Salt Lake twice over, meteorology student Matthew Cappucci wrote for The Washington Post.

“Just how unprecedented is this?” Cappucci wrote. “Well, remember the flooding that New Orleans experienced with Hurricane Katrina? Most places saw about 10 to 20 feet of water thanks to levee failure, inundating about 80 percent of the city. Now, if we took the amount of rainfall that Texas has seen and spread it over the city limits of New Orleans, it would tower to 128 feet in height — roughly reaching as high as a 12-story office building.”

Experts say the lack of “steering currents” to move the storm along is unusual and probably responsible for the scale of the flooding. As of Sunday evening, the center of the storm was virtually parked at a spot about 25 miles northwest of the coastal town of Victoria — crawling southeast at 2 miles per hour toward the Gulf.

With a more common tropical storm, the damage in any one place would be mitigated by the fact that the storms move quickly, spreading the rain over a larger area.

Perhaps making things worse is something called the “brown ocean effect,” which hypothesizes that storms, which typically get their energy over the ocean or another large body of water, can absorb that energy and moisture from rain-soaked land. “The land, in effect, mimics the energy supply of the ocean,” University of Maryland Baltimore County professor Jeff Halverson wrote for the Capital Weather Gang.

Joel Achenbach and Jason Samenow contributed to this report.