Officials in California were racing against the weather Tuesday, struggling to shore up the Oroville Dam's emergency spillway before more rains pummel the area and place the structure under even greater stress.

Engineers have been trying to lower the water level in Lake Oroville, which lies behind American's tallest dam. But more rains are forecast for Thursday.

Nearly 190,000 people remain out of their homes for fear of catastrophic flooding. Airbnb, the home and apartment rental service, has waived all fees for people who live in the affected area, and is offering homeowners a way to offer shelter for free.

More from NBC News:

Jerry Sandusky's Son Jeff Charged With Sex Offenses Against Minors

Michael Flynn Resigns, Hillary Clinton Delivers Wry Response

Judge Denies Tribes' Emergency Bid to Halt Dakota Pipeline Construction

Many people have fled their homes with little more than the clothes on their back, photographs and other items of sentimental value.

The emergency spillway developed a hole Sunday, raising the risk of collapse. The dam's primary spillway developed a 200-foot-long, 20-foot-deep hole last week.

On Monday, California Gov. Jerry Brown wrote to President Donald Trump requesting emergency federal assistance for three counties in the northern part of the state.