Our climate changes from hot and dry to cold and dry, so even an 18*F morning start won’t be as painful as in a high-humidity climate. And at 5K+ feet, we have huge and quickly changing differences between early morning lows and late afternoon highs — 30* is common, I’ve seen over 40*, and temps warm up very quickly as the sun rises — so you have to be prepared to jettison and carry outer layer tops and gloves as morning temps quickly rise — even my gofast bikes have seat bags big enough to carry rolled up gloves and outer garments.
It’s about the start of November, here in river-level North Valley ABQ, NM (av el 5075 feet per random website), when morning temps start dropping to 40* and lower, that I start pulling out the winter hats, first the PI and Walz, and then as morning temps drop toward freezing, the Raphas with better ear coverage. I have OR thin fleece skull caps to wear under those other caps when temps get toward my low-temp limit, about 20*F +/- 2 degrees depending on wind, sun, and humidity. I do have a balaclava but don’t like having to continually adjust the fit over my mouth.
Shoes: I have toe covers for my Shimano mesh-front SPDs, but I also have a couple of other pairs of SPD shoes made without mesh; all are roomy enough for a thin liner under a thicker merino sock and, for temps below say 25*F or for longer rides much below 32*F I have a pair of Lake SPD boots that, unfortunately, are a pain to put on; but they are warm.
Fingers are the biggest problem, and I’ve evolved a layer system, starting with the wonderful and apparently indestructible DeFeet knit wool gloves at about 40* and down to mid 30s, depending on sun and wind and length of ride, and then a lovely pair of soft lined leather gloves, Bundeswehr surplus courtesy of Verusteleka; these I buy 2 sizes large, both to keep ~1 cm air gaps between end of gloves and beginning of my fingers, and also to accommodate silk liners or/and the DeFeets. With both, down to low 20s. For ultimate cold, which I’ve not yet experienced, I do have a very nice pair of fleece-lined leather mittens into which fit a pair of thick ragg wool mittens.
I do have PI lobsters, but those are good only down to about freezing; their big benefit is that they are warm down to say 30* without having to hassle with putting on multiple layers.
Tops: Wool jerseys, ss +/- arm warmers and neck gaiters, then ls wool, then ls with neck gaiters or scarfs, the Ibex wool vest, then full-zip heavier ls wool over jersey, then an even heavier wool full zip, all intermixed with layers of different weights of gilets and shells, finally with a nice very lightly lined Leatt shell to which I had pit zipps added.
Bikes: My bikes remain the same, only I choose one of the 2 with fenders (IGH custom Riv Road clone or road bike for dirt) for messy weather.
The big hassle with winter riding is all the layers that you have to put on and take off; and here again, hands are the big problem, since to check your phone or find your keys or blow your nose you have to remove one or more glove layers.
BTW, ISO/WTB/WTT black or gray or burgundy (preferred) or “black with flag” Carradice Barley, for the Roadeo — the Carradice Zip Roll is very good for summer but I had a hard time cramming a wadded wind shell, rolled up leather gloves, and a small pouch of personal stuff into it on top of the rather (by necessity) bulky repair kit. Still, the Zip Roll is a keeper, for sure.
… A few weeks ago I pulled down the bin of gloves and caps and dug out several pairs of gloves of varying thicknesses. My go-to combo is thin merino liners underneath buckskin gloves. I find this works well to a little below freezing, which covers 90% of my needs. I also pull out the thick wool socks which I wear under my 2-layer Blundstones.