For drivers needing to take the Grapevine, the California Highway Patrol is recommending people take highway 14 to the 58 to avoid any mountain passes when they are shut down.
Another route to get around any potential Grapevine closures is the scenic route via the 101.
The CHP said it's expecting snow in the region beginning Wednesday. They said they expect Wednesday to be the busiest travel day on the Grapevine in a decade. They're also expecting snow and possibly ice.
Monday, they posted a Grapevine detour map on their Facebook page.
"We're going to keep the Grapevine open as much as we can, as long as it's safe," said CHP Officer Rich Anthes. "If it becomes unsafe, then we'll hold traffic, let Caltrans do their magic and clean things up, and then we'll open it back up and get the travelers through."
It could be windy too, and if it is, Anthes advises anyone traveling in a high profile vehicle to take an alternate route, or maybe postpone their trip.
Rain is expected to hit Southern California beginning Wednesday morning with possible snow to hit the Grapevine later in the day.
Before passing over the Grapevine, the CHP recommends stocking your car with food, water, and warm clothes-and to fill up your gas tank.
Millions of commuters are expected to drive through the Cajon Pass over the next few days and they could be at the mercy of the elements.
Traffic on the southbound side of the 15 freeway was stopped Tuesday morning so Caltrans could start the process of de-icing the freeway. There was no ice as of Tuesday, but that was expected to change.
"There's a really good chance that we're going to get snow down to even the 2,000 foot level," said Terri Kasinga with Caltrans. The de-icing agent needs to be in place before it starts to rain.
There's no guarantee interstate 15 - the so-called the Road to Vegas - will be open Thanksgiving Day.
"If we cannot get a handle on snow conditions, and traffic starts slipping and sliding and we see people getting stuck, that's when we start talking with the California Highway Patrol and saying 'Hey, we may have to close the road here,'" said Kasinga.
This would not be the first time the pass has closed. A similar scene played out over a decade ago, when a cold storm rolled in the week before Christmas.
People were slipping and sliding on the road and the snow plows couldn't keep up.
"They better be prepared on the Cajon Pass. If it snows, they're going to be sliding," said one commuter.
One commuter on his way to Las Vegas said the road were already crowded in some places.
"Roads are pretty full already. It's getting pretty crazy," George Abich said.