I have ridden across Nevada many times - the most recent this past June.
An early May start eastbound is iffy, at best.
There are three places that may present challenges.
1. The California Sierras
2. The basins and ranges of Nevada & Utah
3. The Colorado Rockies
It all depends on two things as you mention -
How cold and snowy the winter of 2019-20 is and how late it lasts.
Back in the late 1990s, I had snow Memorial Day weekend - in Honey Lake Valley - 4000 ft.
This past summer we had cold, wind, & rain all across Nevada & Utah - in early June.
It was 35F in Milford - lowest point on the Western Express in western Utah.
It is unlikely that the stretch in the Sierras will have melted out in early May -
So you will be riding thru walls of snow - Caltrans keeps Hwy 88 open all year.
Plus, none of the campgrounds will be open, yet.
In Nevada, bad weather will make the ride more challenging.
Also, when it is stormy you usually have east winds - i.e. headwinds.
The Western Express in Utah has some significant high elevations which will be snowed-in.
Colorado will be iffy, too, in mid to late May.
In the Rockies, we always have May snows and sometimes June snows, as well.
And there will be all the snow from winter that is waiting to melt out.
Snowy winter? Dry winter?? Who knows?
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If you are riding X-USA in the summer in any ways it is best to do east-to-west.
You mention that it will be hot be the time you reach Nevada.
If you leave in late May from Calif., it will be brutal in the East by mid-summer.
If you stay on your schedule riding east-to-west,
you will escape most of the summer heat in the East,
you will hit the Rockies in wildflower season - still cool with snow patches -
and Utah/Nevada will be fine, esp. if you start early in the morning.
(Nice weather in the Great Basin tends to have westerly winds.)
I'm not a big fan of the ACA WX route thru Sacramento and along I-80.
If you are interested, I can offer options through the Delta country.
The back roads of the Delta are sweet - windy, but sweet.
Any chance you want to walk-in camp on the cliffs overlooking the Golden Gate Bridge?
Pix -
Snow on the Nevada border, Memorial Day weekend
Snow banks at Sonora Pass, Memorial Day weekend
Stormy weather in central Nevada, mid-June