Drivers are warned to be cautious of slick roads as weather forecasters predict rain will arrive in the Modesto area tonight.
National Weather Service forecasters expect the wet weather to make its way to the Sierra foothills later tonight with a possibility of snow at the highest elevations.
Law enforcement officials say the first rain that falls after several months of dry weather loosens oil and grime from the road surface, resulting in slippery conditions for the valley and the foothills.
Oil residue builds up on the road during the hot summer months and forms a slick, white, foamy mixture with the rainfall, said Officer Tom Wills, spokesman for the California Highway Patrol in the Sonora area.
The oil residue isn't the only material on the road that gets slippery when wet. Wills recalled his time working as an officer in the Tracy area. Trucks hauling tomatoes leave behind residue that coats the pavement, he said.
"It's slick," Wills said of the tomato slime combined with rain. "It's just like having diesel fuel on the roadway."
He said drivers should check the quality of their tire tread and slow down, so they can reduce the chance of hydroplaning.
Weather service forecasters predict a 30 percent chance of thunderstorms tonight in the Modesto area, with a chance of showers Thursday. There is another chance of rain Friday, but the high temperature should be in the upper 70s.
Forecasters predict the chance of rain to last through Friday in the foothills, too.
Fall officially starts Friday, but Wills said snowfall hazards should always be taken seriously, no matter what the time of year. Even with a slight chance of snow predicted for the higher elevations, drivers should be prepared with tire chains and other safety equipment for the possibility of heavier snowfall, Wills said.
"If accidents do happen, everyone should be buckled up," Wills said. "You don't know how fast others are driving."
On the Net: For road conditions on California Highways: www.dot.ca.gov/cgi-bin/roads.cgi
Bee staff writer Rosalio Ahumada can be reached at email@example.com or 578-2394.