A sudden downpour was the culprit last month, when an inch of rain fell on the foothills during a one-hour period, filling catch basins and backyard pools with mud, Gregg said.
Although Monday’s storm, which started at about 4 a.m., didn’t drop rain at a high rate, it may have left communities vulnerable to mudflows from now-saturated hillsides, he said.
“It didn’t have the intensity, but it does have the soil all wet now,” he said.
Rain on Monday did not shift soil, as far as officials could tell, or threaten homes that crews had lined with sandbags and concrete barriers in preparation for runoff from the exposed hillsides, said Steve Zurn, Glendale’s director of public works.
“There was nothing noticeable in the basins,” Zurn said.
Authorities have paid close attention to the foothills since the 160,000-acre Station fire, which tore through the Angeles National Forest and surrounding lands, clearing plants and trees that help to absorb water and hold soil in place.
With the foothills scarred and topsoil burned to a level that may have reduced its absorbency, authorities have warned of a higher risk of mudflows and strong runoff.
With Monday’s soaking, officials said they were concerned of what another storm would bring later this week. “It appeared that especially the burn areas were effectively absorbing the water and holding it,” Zurn said. “You begin to worry if the rain continues, because that’s when you get saturation problems.”
Rain would likely have caused mud and debris flows if it fell at a high rate, perhaps dropping a half inch of water per hour, or a total of at least 2.1 inches over a 12-hour period, according to the United States Geological Survey.
Showers are forecast for as soon as Wednesday evening, but they will likely begin Thursday, said Jamie Stern, a spokeswoman for the National Weather Service.
“We don’t expect it to be particularly harsh,” Stern said. “It’s going to be less of an intense storm compared to the one we just had.”
That storm is expected to end Friday, although more rain is expected for Saturday.