"We're dealing with a very, very new area of disaster," Heggie said.
The rain could also make cleanup efforts tougher, he said.
"It's difficult to work in any type of rainy, wet situation," Heggie said. "We're trying to get rid of water, and now we're going to be adding more water."
Others weren't too worried about the forecast. Tom Fayram, the deputy public works director for Santa Barbara County, told a crowd at a community meeting Tuesday night that the anticipated rain does not pose a significant threat.
Workers have made good progress clearing debris out of creek channels that run down the steep hills above Montecito, opening the channels so rainwater can run down the creek beds and not into the streets, he said.
"This rain will give us the first test, but we believe the creek systems can handle this next rain," he said.
"We don't know the full extent of the capacity of our drainage system," he said. "But we will find out, we will find its weaknesses and we will fix it."
Meanwhile, officials said three people are still missing after last week's mudslide, about 1,400 are without power, and a stretch of the 101 Freeway remains closed between Santa Barbara and Carpinteria -- even after crews have spent a week trying to clear the muddy, debris-filled river created by the deadly flows.
About 20 crews are trying to restore power but are hampered by "significant mudflow and debris blocking roads," Southern California Edison spokesman Steve Conroy wrote in an email. "Dozens of poles, wires and other equipment need to be replaced. There is progress, but it will continue to be slow due to current conditions."
Authorities have not determined the cause of the Thomas fire or the extent that human involvement affected the mudslides, said Cal Fire spokeswoman Blanca Mercado. But those affected by the damage are already trying to assign blame.