A large and complicated rescue operation is underway in Houston as floodwaters continue to rise, fed by unrelenting rain.

So far, there’s no end in sight.

As a tropical storm, Harvey is expected to produce 6 to 12 more inches of rain through Friday over the upper Texas coast, where some areas — including the Houston metropolitan area — may see accumulations of up to 50 inches.

If you’re outside the affected area, here are options to help. (If you’re in Texas and displaced by the storm, here’s how to get help.)

Local organizations

The Hurricane Harvey Relief Fund of Houston’s mayor, Sylvester Turner, which is administered by the Greater Houston Community Foundation.

Houston Food Bank and the Food Bank of Corpus Christi are asking for donations.

The South Texas Blood and Tissue Center is reporting a critical shortage, and has extended hours at all of its San Antonio-area donor rooms. To donate, call 210-731-5590 or visit their website for more information.

Carter BloodCare covers hospitals in North, Central and East Texas. To donate, call 877-571-1000 or text DONATE4LIFE to 444-999.

To help animals suffering from the disaster, visit the Houston Humane Society or the San Antonio Humane Society. The Houston Society for the Prevention of Cruelty to Animals has set up an animal emergency response hotline (713-861-3010) and is accepting donations on its website.

The Texas Diaper Bank in San Antonio is asking for diapers and wipes, which can be dropped off in person or mailed to 5415 Bandera Road, Suite 504, San Antonio, Tex., 78238.

The United Way of Greater Houston flood relief fund will be used to help with immediate needs as well as long-term services like minor home repair. Visit their website to donate or text UWFLOOD to 41444.

The L.G.B.T.Q. Disaster Relief Fund will be used to help people “rebuild their lives through counseling, case management, direct assistance with shelf stable food, furniture, housing and more.” It is managed by The Montrose Center, Houston’s longtime community center for the area’s gay, lesbian, bisexual and transgender population.

For more options, the Federal Emergency Management Agency recommends checking with the National Voluntary Organizations Active in Disaster for a list of trusted disaster-relief organizations in Texas.

National organizations

The American Red Cross is accepting donations on its website. You can also text HARVEY to 90999 to donate $10.

AmeriCares takes medicine and supplies to survivors.

Catholic Charities provides food, clothing, shelter and support services to those from all religious backgrounds.

Donations to the Salvation Army can be made online, by calling 1-800-SAL-ARMY (1-800-725-2769) or texting STORM to 51555.

AABB, which coordinates a task force to manage blood collection efforts during disasters, put out a call on Sunday for blood donations in the aftermath of Harvey. Most in demand: those with type O-positive blood.

Those interested in donating blood may contact the following organizations:

• AABB: 301-907-6977• America’s Blood Centers • American Red Cross: 1-800-RED CROSS (1-800-733-2767)• Armed Services Blood Program: 703-681-5979

Online-only organizations

Airbnb is waiving service fees for those affected by the disaster and checking in between Aug. 23 and Sept. 25, and can guide users in creating a listing where their home is offered to victims free.

GoFundMe has created a page with all of its Harvey-related campaigns, including one started by the country singer Chris Young, who donated $100,000, and another created by the president and chief executive of the Rockport-Fulton Chamber of Commerce.

YouCaring has a fund-raising page set up by J. J. Watt of the Houston Texans with a goal of $3 million. By 4 p.m. Tuesday it had raised more than $2 million.

GlobalGiving’s Hurricane Harvey Relief Fund supports local organizations by helping to “meet survivors’ immediate needs for food, fuel, clean water, hygiene products and shelter.” It will also assist with longer-term recovery efforts.

How to avoid being scammed

Before giving money to an organization, do your research.

Charity Navigator, which identifies worthy charities, has a handy list of organizations that are responding in the aftermath of the storm.

For advice on avoiding fraudsters, check out tips from the Federal Trade Commission.

“Be wary of charities that spring up too suddenly in response to current events and natural disasters,” the F.T.C. website says. “Even if they are legitimate, they probably don’t have the infrastructure to get the donations to the affected area or people.”

GoFundMe, the source of many new fund-raisers that popped up after Harvey, offers a way for donors and campaign organizers to communicate directly.

Bobby Whithorne, a spokesman for GoFundMe, said in an email that if a specific campaign is raising questions, “report the campaign directly to GoFundMe by clicking ‘Report Campaign’ on the GoFundMe campaign page or, report your concerns to the state Consumer Protection Hotline.”