mev

I'll make a counterpoint as more than a few months makes it more likely to show up in Oregon in November or later, at which point likely hood of having wet conditions approaching Western Oregon significantly increases. From the maps, it looks like the OP is leaving from mid-Arizona which is already cooler than lower areas near Phoenix and Tucson.

You can view average high temperatures using the NOAA maps here https://www.ncdc.noaa.gov/climateatlas/ From this, it suggests that St George Utah is a reasonable proxy for one of the warmer parts of the trip. Looking at St George climate average here https://weatherspark.com/m/2380/9/Av...-United-States suggests St George starts September with high temperatures averaging 97F with 90th percentile in the low 100s- and ends September with high temperatures of 87F and 90th percentile in upper 90s. St George at start of September seems foolhardy to me. St George around end of September seems more doable, particularly if one looks for places to cool off along the way and/or avoid hottest times of the day and takes sufficient water.

So I think waiting a few weeks makes some sense. I also think it makes some sense on the early (hot) parts of the route to try plotting things out to find some places where ideally you can end the day at mid-day indoors with AC - and also get out on the road early at sunrise to take advantage or relatively cooler temperatures.

I also think waiting a lot longer than that, starts to run into problems later in the cooler parts of the route, where as you get closer to the coast the likelihood of cold/wet conditions increases if you end up arriving too late...

So waiting a few weeks is probably good. On the other side of the equation is looking at weather averages in Portland: https://weatherspark.com/m/757/10/Av...-United-States where the probability of precipitation goes from ~25% at start of October to to finish October around 48%.

Also for what it is worth, following is a climate comparison between Ridgecrest and St George - so they aren't actually that different in their temperature profiles - https://weatherspark.com/compare/y/1...d-Saint-George