I'm moving from Seattle to Denver this fall, and since I have a pretty open schedule, I thought I'd ride there via the TransAmerican Trail. I wouldn't be able to start my tour until mid-August and I'm worried about the weather at that time of year. When is it too late to bike through the Rockies before the conditions becomes too treacherous?

Mid-September, you may get a short storm you might need to wait a day or two to continue. Past the first week of October, storms could be more serious.

I think you are fine if you are moving right along, and as long as you are a little flexible and have budget to stay in a hotel if the weather is bad.

Consider Sierra-Cascades to Western Express, too - a little further south. You miss Yellowstone, which tends to get winter before the rest of the rockies, but gain Utah and get a nice chunk of Colorado.

valygrl needs to add a couple of weeks to her time frame but she's correct on the weather otherwise. Mid August to mid October are actually good times to travel the Rockies. Other parts of the trip, not so much, especially if you are traveling through Nevada, Utah and western Colorado.

September is my favorite time of year in the Central Rockies. The summer monsoon moisture has moved out, and you have a chance at seeing the short aspen leaf season. Yes, there's sometimes the first snow of the year in September, but not always and it's a short one and quickly melts.

For one frame of reference, the average nightly low in September in Wisdom, MT, which is on the TransAm on the eastern side of the Bitteroot mountains, is 27 degrees. (It was a little below 40 when I was there on July 1, 2011.) Yellowstone will likely be sub-freezing as well. Nice if waking up in that sort of cold is your sort of thing.

This is great to hear. I'll bring a warm coat. I should be okay with my current tires then? No need to swap out for fatter tires for potential snow? Or would that be a good idea just in case?

MaryLou -

Since you are a newbie, a little info might help.
Although such a trip would be pretty easy for those posting already -
If you have little to no touring experience, more caution might be advised.

A few things to remember - -
No matter how experienced you are - you get fewer miles in the fall.
That's simple - - the days are shorter. So don't expect summer mileage.

A coat may not be needed - but layers would be more useful. Rain jacket/windbreaker.
Light polypro, heavier polypro, ski gloves, fleece headband, 1 pair wool blend socks.
Some kind of long pants - convertible or hiking lightweight nylon. Lightweight thermax tights.

If you are so inclined - an Aug 15 departure is ideal for the Northern Tier out of Anacortes.
(You can also access via Arlington, but Anacortes is just so nice.)
The Northern Tier has some brutal climbs right off the bat - so its not for wusses.

But if you do the NT, you can ride to Glacier, then down to Yellowstone.
You should get to Glacier in two weeks, max - then to YNP in another week - US 89 east side.
That would give you two more weeks of genrally goodweather to get to Denver by Sept 21.

Three options are available in Colorado once you get to Walden.
Cache la Poudre Canyon to Fort Collins is an incredible downhill ride, but you have the Front Range cities.
Trail Ridge Road in Rocky Mtn NP is stunning, but iffy late in the season.
US 40 over Berthoud Pass is the most direct, but has a good deal of traffic.

Have a safe trip.