Put the best sleeping bag you have in the car, a blanket to put inside the sleeping bag, and buy a few handwarmers to add in the sleeping bag if needed. Have boots, pants and cold weather gear to put on if you have to walk away from a breakdown in heavy snow, zero temperatures and at night (tennis shoes, blue jeans, and a jacket, won’t cut it).
Take a few items from home like a jar of peanut butter and a couple of cans of something, and some bottled water, and a small ice chest (ice chest and a handwarmer means no frozen water and food).
Remember your phone charger, and always, always, always, always, top off your fuel tank before getting into mountains, and remember that you will be in temperatures much colder in mountains than you will hear about on the weather reports.
The distance of your trip, coupled with the fact that you have to either cross the Rockies *and* the Sierras (or go around them) mean that you have no time for unplanned diversions or delays. IMO, you have a pretty tight schedule there.
That said:
You can run into snow anywhere from the Plains westward. You can run into snow on I-40 through ABQ and Flagstaff, you could take US-50 and have clear sailing, you could take I-80 and flip a coin. It is very difficult to tell these things as far out as five or six days. Storm predictions along the Rockies are hard to nail down even only three days out.
The best advice I can give you is avoid going across the Divide on I-80 through Wyoming. It is deservedly notorious for foul weather in winter... having driven it many times (sometimes with a loaded trailer behind me), it can be more “entertaining” than one might like about eight months out of the year.
The WX along US-50 is usually pretty dry. That’s why that area (from the CA/NV border east to Pueblo, CO) is high desert. They simply don’t get much precip along that area of the country. But there are huge stretches where there is no help for 10’s to 100+ miles if you break down on US-50. And there are many areas along US-50 with no cell coverage as well.
Do you have tire chains? Know how to use them? Only about 25mph maximum using chains. But then, being from PA you know all of this? Like--- Remove them when encountering dry pavement. Stop after putting them on and driving a small distance, to tighten, or re-tighten them, particularly until you get good at putting them on. And off. then back on. then off again. oh, happy trails! Up and down mountains in the West can be so much fun in the Winter, but there can be breaks in the weather when even if the road isn't exactly completely clear, then at least they can be traveled without having to chain up. But you will HAVE TO have them in the vehicle in Colorado.
Depending upon the weather, if you need to get to Phoenix from Denver, perhaps forget about the mountains of Southern Colorado(?) and just go South on I-25 to I-40 then West, or go through Albuquerque, all the way to I-10, and then West. The pass at the Colorado/New Mexico border can be snowed in too, even as it drops some elevation from Denver & Colorado Springs, to Albuquerque. The North wind just lo-ooves Raton Pass in the Winter.
When there is snow and ice at Gallup (not exactly on your route, but West of Albuquerque on I-40) many times it is frozen over, all the way to past (West of) Flagstaff. From Flagstaff to Phoenix on I-17 there are some fairly steep grades both down, then back uphill, before dropping down once again.
Keep an eye on the weather. If you don't know or enjoy snow driving, maybe don't. Even if you do know snow (you're from PA?) remember to budget your minimum needed driving time to DOUBLE normal, at least, while always thinking...could I live through the night spun out into the ditch on the side of the road right here?Prepare accordingly.
Use chains to get OUT of snow, instead of INTO snow, even though one can often enough do about 40 mph on level packed snow without chains. With chains, 25mph. 15-20mph many times better. No chains, on hard ice (not packed snow) 15mph MAX. period! 5-10mph is more like it!
In the Western States, rock is used more than salt. Beware rocks flying from trucks (tires). Truckers get cracked and pinged windshields from other trucks! How well are you going to make out???
Truckers usually figure 50mph when planning trips in the summertime, even when they can legally go much faster. A guy needs to stop and take a leak every once in a while. It takes some minutes, just to do that.
Do you use mile markers? It can be a quick cheat for old-fashioned paper map navigation. Mile numbers on the Interstates usually (almost always but with a few exceptions system wide) start in the West, and South, getting bigger going FROM those origins, smaller going those directions. For example, going across Kansas, from East to West on 1-70, the mile marker numbers will get smaller.
Stop at a truck stop (most any PILOT would do) and buy yourself a Motor Carrier's Atlas. $14.95-$15.95 at most any of them. For another $4.95 one can get a truck stop Pocket Guide which will give all truck stops open 24/7 for all Interstates keyed to mile markers on the various Interstates. Stops on state routes will be listed, but not under order of mm's, They will be listed as town or city, and then road number.
I don't necessarily recommend fueling or stopping at truck stops if you don't have to (one gets tired of them when driving truck) but they can be handy --- and if they are listed in the pocket guide, they'll be open 24/7 and will just about always offer gasoline along with diesel fuel. The food in truck stops is usually lousy, to be avoided, but I know how to find the good, or at least better ones. None of them were ever Pilot, if I properly recall...save in the very rare instance when there would be a real mom-and-pop type attached or nearby. Eat elsewhere, if you can.
Get some tire chains
There are times the Rocky mountain passes are closed except to vehicles with chains
I live north of Denver. We are actually have a VERY mild winter right now. I wouldn’t go west of Denver right now; for some reason the mountains seem to be getting pounded.
Find out what your talk/news radio is in each region so that you can get the scoop on road/weather conditions. There are so many people in the Denver/CO Springs area, that even when it snows, the road conditions improve very quickly unless we have a blizzard. But, this year, we have only experienced dustings a couple of time.
Since I see you are avoiding mountains now (good plan), any expertise I might have is made null...
Still wouldn’t go anywhere in the winter without CB radio, chains/cables, siped snow tires, and enough gear/food for 3 days with a dead car (including an independent means of making fire).
Most folks get in trouble up here by figuring their car can keep them warm. Rope, a few wool army blankets, and a couple tarps don’t take up much room, but you’d be amazed at what one can do with them when the chips are down.
Just sayin’...
Have a good trip.
From El Paso to Phoenix the border patrol will be checking you and everyone else out.
Cheers and enjoy!
Chains, tow strap, HEAVY sleeping bag. two changes of heavy clothes, LOTS of water and whatever protein you like with some carbs to burn, folding shovel, extra coolant, extra oil, assorted simple hand tools, VERY good knife, small sterno stove, firestarter (nice bottle of Purell, and a flint, NO Bics). I live on the edge of the Sierras and all of that stuff and more goes into all my vehicles on October 1.
Drive on down to South Padre Island in TX and eat ya some peyote buttons, then decide where the road goes.....
Road flares are handy and double as fire starter - especially if you need to light a tire off.