5 reasons to closely watch Hurricane Harvey

It started raining Thursday in some parts of Texas. It might not stop until next week, thanks to lumbering Hurricane Harvey.

Harvey is an unusual tropical cyclone, one that will bring different concerns to different areas vulnerable to such a huge rainmaker with incredibly powerful winds.

Track the storm

The damaging effects of Harvey, expected to be a Category 3 hurricane by the time it makes landfall, will depend on where a person lives, hurricane specialist John Cangialosi of the National Hurricane Center said.

"We want people to remember tropical cyclones come with a whole package of potential problems," he said Thursday from Miami. Coastal residents may be more concerned about storm surge, whereas people who live inland will look skyward for days. People in some areas near the eye will have to deal with "devastating, if not catastrophic winds," he said.

Here are some of the factors that make Harvey a storm to be concerned about.

Speed

"Slow motion is almost always a bad thing," Cangialosi said. Harvey is not moving fast and once it gets to Texas it will spend several days going hardly anywhere. Cangialosi said cyclones are often pushed away by low-pressure systems but there is a high-pressure ridge that will block Harvey from going far.

That area of Texas is also very flat and often swampy, so Harvey will weaken slowly. The center is predicted to stay just north of Corpus Christi for four days.

The currents that are steering the storm are very weak, Cangialosi added.

A storm stalling is pretty unusual, he said, but not "completely rare."

Rainfall

The storm will continue to pull moisture from the Gulf of Mexico and could pour 35 inches of rain on some parts of Texas.

"Some of the heaviest rains will fall near coastlines, but the threat of inland flooding is significant," Cangialosi said.

The hurricane center said some inland areas like the Texas Hill Country could see 5 to 12 inches, while much of the coast will see 15 to 25 inches of rain.

Cangialosi said the focus is on Texas but rain could spread north and east into Louisiana.

Storm surge

If Harvey's eye hits a high tide, things could be very bad for people along the coast. The hurricane center said the water level could be as much as 12 feet above ground. It will bring with it destructive waves.

There are storm surge warnings for 300 miles of coastline.

High tide Saturday morning is at 4:08, according to the National Weather Service.

Flooding

The National Weather Service office in Houston is warning that Harvey could bring 2 to 4 inches of rainfall an hour during parts of the storm. Houston is the fourth most populous city in the United States and as the region grew, prairie areas that took large amounts of runoff disappeared, a Rice professor told CNN affiliate KPRC last year.

"When we put concrete down, when we build drainage channels, we concentrate that water and dump it off the land where it used to be stored, we push it downstream on ourselves. That's been the story of Houston development," said civil engineering professor Jim Blackburn.

Rainfall will be a concern through much of south Texas.

When CNN spoke to Cangialosi, he warned as much as 30 inches of rain could fall. A few hours later, an updated storm forecast raised the potential total to 35 inches.

Winds

The fastest sustained winds will be 111 mph or more at landfall, but gusts will blast the coast even harder.

The National Hurricane Center said, "Devastating damage will occur" during a Category 3 hurricane. It warns that homeowners could see major damage and roof decking and gables could fly off. Many trees will come down.

Hurricane-force winds on Thursday extended up to 25 miles from the center of the storm. As the storm gains energy, that distance will increase.