The large storm system now making its way through Oklahoma, Texas, New Mexico and other parts of the Southwest is particularly hard to predict.
National Weather Service meteorologist Tom Bradshaw says that's because a couple of degrees here or there with the temperature will determine whether regions see rain, sleet or snow.
Bradshaw says the storm system is "slow moving" and "sort of bringing its energy out in pieces."
A winter storm warning has been issued for chunks of North Texas until midday Monday. Parts of Oklahoma are also under a winter storm warning.
More than 300 flights have been cancelled at Dallas-Fort Worth International Airport, representing about one-third of the scheduled departures, and a spokeswoman says deicing equipment is ready.
After the storm plows through the Southwest, meteorologists expect the Arctic mass to head south and east, threatening plans for Tuesday and Wednesday as people hit the roads and airports for some of the busiest travel days of the year.
At least eight people have died in weather-related accidents since the storm began in California.