Summer is always slow because so many Americans are on vacation, but the Red Cross says this month's severe weather has made the situation much worse.
All those summer storms and the power outages that followed led to the cancellation of many blood drives.
What's more, doctors say the extreme heat is keeping many regular donors at home this summer.
"It really puts our community in a very difficult situation," said Red Cross medical director Dayand Borge. "Patients don't get vacations. They're in the hospital and they need blood."
The shortage means hospitals may soon have cancel some elective surgeries or even postpone more serious procedures like liver transplants because they require so much blood.