1

March.
THough xNYC is the odd ball here. When it's fine, the others are likely miserable, and vice versa. (The exception being October.)

2

I would move that back to Mid April...The higher areas Grand Canyon ,Bryce & Mesa Verde even then,could yet have winter driving conditions...A Spring storm could hold one up for a day or so... So keep the route and attractions flexible...carracar

3

carracar, I was thinking about doing a trip down in that area in November and December and possibly a little later in the year.

Does it snow much? I have winter tires I could put on the car but unnecessarily driving on them through the heat wears 'em down sooner. Might be worth it for piece of mind and flexibility though.

4

Nobody else has replied to bc_steve, so I'll make a stab.

Likelihood, as well as number of places prone to be affected, of complications from snow increase the later you get into fall (as you might guess). As for winter tires (assuming you have good all-season m/s tires anyway) ... largely depends on how much time you have. Major snowstorms aren't all that frequent most years, and are likely to only last a day or two - if you can calmly wait them out, you'll very likely be OK (they are mostly problems for people on a fairly tight schedule). Southwestern snow tends to be powdery, and (while sometimes seriously slippery) is much less apt to cause the icy messes that often tie the west-slope Cascades and Sierras in knots. Since southwestern snowstorms (east of the Sierra) tend to be discrete (rather than semi-continuous) events, roads are typically cleared reasonably efficiently after the storm exits. There are very few avalanche-prone highways on the Utah/Arizona/New Mexico parts of the Colorado Plateau (let alone farther south). On the other hand, powder snow blows easily, so there are significant visibility problems during and just after some major storms.

In earlier November, the main snow risk is at high elevation sites like Bryce or Utah 12 north of Boulder. The risk area expands later. Even in midwinter, fairly modest snow risk at the bottom of Zion. Much less risk in southern Arizona (maybe enough snow to cause a brief headache once in 5-10 years or more in Tucson, once a century or more in Phoenix).

Assuming you are driving south from B.C., probably your likeliest problem will be getting across the Siskiyous on the Oregon / California border, although any such problem can usually be bypassed via the "coastal" US101, accessed from I-5 using US199 (or, if really bad, OR38) in Oregon and CA20 in California.

5

I started my trip to Grand Canyon/Utah begin of April. Bryce Canyon might be excessively cold and/or under the snow.

6

Late spring is ideal. The desert should be blooming April/May. I prefer cool/cold weather to hot weather, but everyone is different. Roads should be good unless it is actively snowing. The snowplows are dependable and timely. Street tires should be OK, but as insurance buy 25 pounds of cat litter, put it in your trunk. It works great on ice if you happen to slide off the road.
I have visited all your planned sites, and you cannot see all of them on day trips unless you are just planning to play tag/bag and scratch the site off after visiting for a few hours. Each site can take 3-5 days to get the feel of the places. Do not rush through or it will seem more like a TV show, and you will miss lots of incredable scenery, and rock art, and cave dwellings. Another stop is Chako Canyon (misspelled) and catch it on the equinox. It is a moving experience.

7

Thank you all for your replys.

So I think we will start in late April and do the main trip in may. Long time to wait, but better than bad weather!