I started commuting about 3 weeks before it got hot. I've only missed two days in my first two months of riding. Now I dream about how I'm going to evolve in this thing when the temperatures fall 20 or 30 degrees and I can really cut loose.
I live in the dry Mohave Desert. With what I can put in a top tube bag or similar tiny compartment, how do I layer for autumn commutes? I commute on a road bike and want to keep it very light and compact. I'm a complete amateur rider.
Thank you in advance for the advice.

How far is your commute?

Good question and I'm fairly sure you'll get plenty of good responses. It's my understanding wool will be your friend.

Howdy!
My commute is only 6.7 miles each way with a 450 feet elevation gain going to work but I have 3 MUPs that converge at my place and just as soon as it isn't 108 degrees everyday, I'm going to use a longer MUP of about 10 miles each way. I can't hardly wait for this.

Wool has a fantastic way of retaining loft which in turn maintains dead air space and will retain 90% of its insulating power when wet but it won't compress worth a hoot when it is time to stow and it never rains here. Most importantly, it will rarely be below 40 degrees on my morning commute. I'm thinking my approach will be more to block the wind I will generate rather than use thick clothing but I'm not really sure.
I am an experienced backpacker but dressing for the bike is an entirely different game I've discovered.

Typically, how cold does it get there?

Edit: OP answered my question while I was typing it...

I understand wanting to keep your load as light as possible, but you may have to consider adding additional cargo carrying capacity. A handle bar bag, rack trunk, or saddle bag should all be big enough to carry/store additional layers in. Or wear a backpack or mess bag.

I think the old saying is "if you're warm before you start riding, you're overdressed."

Good points.
The thing about Las Vegas in winter is that it will so seldom get below 40 on a mornings commute that its not worth tooling up for anything less. I can drive 3 days in the winter when it does get colder. The same thinking holds true for rain. I expect a real commuter would not shy away from either but when it seldom happens it may not be worth considering.
I probably have answered my own question. Right now I'm thinking nylon long handles under mountain bike shorts and light fleece under ultralight wind breaker. Maybe a do rag under my helmet and my REI ultralight daypack to bring this stuff home at night.
I was really wondering if you guys knew of any technical ultralight clothes especially suited for bicycling or designed specifically for bicycling.
I really don't want any luggage on this bike as I plan on putting a touring bike together in the spring.

For variable temperatures, arm and leg-warmer tubes are used by racing folk, easy to stow but can slide down some arm and leg shapes. I prefer full tights style leggings.
Lightweight core insulation: sleeveless gillet windproof or thin insulation.
Windbreaker: Featherweight, tight-weave nylon or polyester, light colour with some reflective bits.
Extra insulation: neck tube.

I alternate between insulating my legs and my upper body. Both ways work for me.

This sounds on target. I see you are in England. Do you know of a common store where I might look at these articles of clothing? I wear mountain bike shorts to work and it sounds like these articles could fit in the pockets which would be ideal.
I thank you for the advice.

For 40 degrees, I'll go with lightweight tights, long sleeve jersey, full-finger gloves, and tights over shorts. Performance Bike is the easy and inexpensive way to go.

On the way down to 40, arm and knee warmers take me down to 50, maybe 45 if it's sunny and not windy.

You'll figure out your own warming scheme with a little practice, and then the temperature is the first thing you'll check before dressing in the morning.

Hah. The temperature is the first thing I've checked before dressing for 35 years.
I'll look at Performance. Heck, I'm having fun just setting up this clothing routine.

Keep a clothing log of what you wear in what temperatures. Then you can make adjustments as needed. I find that Sporthill makes about the best cold weather cycling pant out there. Look at the Infuzions or the Symmetry's. Those are comfortable from about 55f down to about 25f. Windproof and coldproof. Loose-fitting. Not your typical tight fitting cycling tight. They're designed for runners and cross-country skiing. But are great for cold weather cycling. Amazon has good prices on them right now.

If I lived in Nevada, I probably wouldn't wear clothes.

If you insist, though, you could probably get away with gloves, hat, socks, shorts and a long sleeve shirt or two.

I don't switch from shorts to pants until it's cooler than 30 degrees. You lose relatively little of your body heat through your legs.

On colder days dress so you're chilly the first 2 miles. Otherwise you'll sweat to death before you arrive.

Even in sub zero temps I don't wear anything heavier than a wind breaker over my long sleeved t-shirt, and that shirt is soaked with sweat when I get to work.

I'm looking at some of their Zone 2 stuff. I think the Zone 3 would be a little warm here in the desert.

I'm thinking a pair of thin poly long underwear or tights under a pair of shorts at the most. And you guys are right about the long sleeve T (nylon or poly) and a paper tin wind breaker should do it in Vegas.
I can't wait for it to cool down just 15 degrees. I have so many plans.