I understand the time frame thing as I deal w them myself. The road will likely be wet and gritty and narrow anywhere there's still a lot of standing snow and Ca got a lot of snow this year. Speaking for myself, I would still go with my fingers crossed, hoping for the best . What made me backtrack from saying it could be done is the idea of you running into black ice in the morning. That could lead to you going down very hard.
I was thinking about this quite a bit last night. As an alternative, I give you this option to consider:
Leave from San Diego in March on the Southern Tier. It passes within a 100 miles± of the Grand Canyon. Go up to the Grand Canyon. If you're up for it you can take the hike down to the bottom, it's very cool. ACA has a route map called the "Grand Canyon Connecter". Take that route north to Zion NP (my favorite of the National Parks). The scenery between the 2 is like riding thru an old western movie. From Zion you can go directly to Bryce Canyon and join the WE or you can go in to Cedar City and join the WE there. There is a 10600' climb out of Cedar City that has Cedar Breaks at the top which is definitely worth seeing but you can check road conditions to see if its free of snow when you get there.
The advantage of the ride as described above as that you can leave when you want to. You will get to see the GC and Zion without the intense crowds that are there a little later in the year. And you will get the very best of southern Utah, which is the best of the WE. That is Bryce, Grand Staircase, Capital Reef. It's 2 or 300 miles of almost unbroken eye candy.
Alternatively, if you leave from SF and take the WE the whole way, my opinion was there isn't anything great on that whole first map from the Pacific to Fallon Nv. Once I got to the Great Basin area I thought it was fantastic, like riding on another planet, so I'm sorry I can't be more encouraging in coming up with a way for you to ride it.
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