The FedEx big rig overturned on the center divider around 6:10 a.m., snarling traffic for drivers as two westbound and three eastbound lanes were shut down, according to Caltrans. An estimated reopening time was not available.
The California Highway Patrol says the driver was on the eastbound side of the freeway when he spun out, crashing and sparking a massive fire that spread to both trailers. The driver and passenger are expected to be okay.
Officials say the incident serves as a reminder to use caution while driving in the rain.
"You can hydroplane at slow speeds so just be careful out there," said CHP Sgt. Bryon Robinson.
San Bernardino County firefighters were working to extract all the singed packages to put out the flames.
The storm is expected to gain strength throughout the day, potentially impacting the afternoon commute.
Most areas saw consistent light rain in the early morning hours, but areas like Anaheim were hit with heavy downpours overnight, which was expected to be the case for most of the region after a brief lull later in the morning.
SOCAL STORM: Heavy rain falling over region Tuesday, Wednesday
Roadway flooding, minor debris and mud flows and hazardous driving conditions are expected through Wednesday, according to The National Weather Service.
Major flooding from the storm has yet to be reported.
Water was seen pooling up along major streets in San Bernardino, though it seemed to be under control. The Inland Empire will see 1-2 inches of rain.