I'd plan on adding 2 to 3 days to your expectations, depending on weather and traffic.
The Siskiyou pass is much easier to negotiate than Snoqualmie. I've done both in snow and cold. For safety, maneuverability and frequency of help, I'll take the Siskiyous any day. I-5 also has pull-outs constantly which I-90 doesn't have, and less wind.
There is an alternative route to the Siskiyou pass, Hwy 101 an older 2-lane scenic route. HOWEVER, these massive, several-days-long snow and ice storms have covered 101 with snow, layers of ice, wind, and many areas of complete impassibility.
I-5 is the main commercial truck route from British Columbia into Mexico. It is
always the best route in snow and ice because it is the widest route and the one most plowed and de-iced or chopped up by the trucks' chains. Today, Monday, alternative routes are basically not an option. I-5 will be your safest best. It also has constant State Trooper patrols which will help people in trouble, and a very healthy number of gas stations and restaurants.
BUY AS MUCH GAS AS YOU CAN IN SOUTHERN CALIF. Up here in western Washington many gas stations have simply run out of gas because the tanker trucks can't get to the stations in the unbelievably icy and snowed-in conditions. Since I-5 is the main commerical route for the entire west coast, it's likely that gas stations on I-5 will be supplied, since they'll be better plowed than in the cities and towns. In the past 3 days, the snow has just been falling faster than plow crews in the cities can keep up with. They're working on 12-hour shifts, 24 hours a day in western WA and western OR.
Northern California is likewise buried. A friend living in Weed, CA near Mt. Shasta and of course at elevation, reported a temp of 10 degrees during the day yesterday which he says is unusually cold. Make sure your antifreeze is good for 0 temp or -10.
When you get to snow -- pull over and chain up. Drive slowly. Be awake, relaxed and thinking. Drive carefully and fully awake. If you go into a skid DO NOT BRAKE DO NOT BRAKE DO NOT BRAKE. Steer INTO the direction of the skid. Try just a bit to steer in a better direction. Stop steering if the car won't change direction. DON'T BRAKE. Try steering again, just a bit. Don't brake. Eventually the tires will find a spot where they can get a bit of traction and you can SLOWLY bit by bit steer into a better direction.
I send you all strong energy for safety and happiness.

Don't be afraid, or its cousin worried, because fear clouds thinking and wisdom. Just be careful and wide awake. I wish you well!