CHICAGO (CBS) — Hurricane Harvey is the most powerful hurricane to hit the U.S. in a decade, and the first Category 4 storm to make landfill in Texas since 1961.

Here are five things you need to know about Harvey, which was downgraded to a tropical storm Saturday:

1. Texas Gov. Greg Abbott said in a news conference Sunday that he expanded state declaration for 50 counties and federal disaster declaration for 19 counties. With the storm stalling out, millions are now in danger of potential “catastrophic flooding.” More than three feet of rain could fall in some places.

“The federal disaster declaration for those 19 counties has already been granted by the President,” Abbott said. “I do anticipate adding more counties to that list.”

2. President Trump will visit Texas on Tuesday to review the state’s recovery efforts. In a statement from Sarah Sanders, the White House Press Secretary, she said: “We are coordinating logistics with state and local officials, and once details are finalized, we will let you know. We continue to keep all of those affected in our thoughts and prayers.”

3. The standing water caused about 100 delays and 850 cancellations across the country on Saturday alone. William P. Hobby Airport, Houston’s second largest airport, tweeted Sunday that all commercial operations had ceased until further notice.

The ripple effect in air travel even extended into the Chicago region. Just before 3:30 Sunday afternoon, O’Hare International Airport had 57 flight cancellations and Midway Airport had 16.

4. Rescue efforts in Houston are underway, and mounting, as Harvey’s (what could be record-breaking) flood levels continue to rise.

Helicopters, boats and high-water vehicles patrolled Houston neighborhoods, pulling people from their homes and from the water. Officials have made over 500 rescues in Southeast Texas since Harvey hit on Friday.

5. Gov. Abbott said assistance in response to Harvey has been pouring in from across the country. The American Red Cross, for example, has mobilized hundreds of disaster relief workers and truckloads of supplies.

“I want to extend my deep gratitude to our fellow Americans who are pouring in support from other states, and providing resources that are needed to help us deal with these challenges,” Abbott said.