December in the Sacramento Valley is a mixed bag by nature, and CIM's race-day weather has reflected that over the decades — some years cool and clear, others damp, and a handful memorable for the valley's signature winter fog. This guide covers the patterns worth knowing, in general terms; typical average temperatures for this race are on the data card above.

What a typical December morning in the valley looks like

Early December in the Central Valley tends to bring cold, still mornings that warm up modestly as the sun climbs — good conditions for distance running in the sense that overheating is rarely the concern. The bigger variable year to year is moisture: Northern California's wet season is well underway by early December, so a dry, crisp morning and a damp, misty one are both well within the range of normal for this race.

Tule fog: the valley's signature winter weather

The Sacramento Valley is famous for tule fog — a dense, low-lying radiation fog that forms in the Central Valley on calm, clear nights, typically after rain has left the ground moist. It's been a recognizable feature of Northern California winters for generations, even though how often and how thickly it appears varies considerably from year to year. That fog forms because the Central Valley sits in a basin enclosed by the Sierra Nevada and the Coast Ranges, which traps cool, moist air near the ground during calm winter weather — the same geography that gives the valley its reputation for both fertile farmland and stubborn winter fog. When it does show up on race morning, it tends to thin out as the sun climbs rather than lingering all day, and the cool, still air that comes with it is generally more of a help than a hindrance to distance runners, even if the pre-dawn shuttle commute can feel a little eerie.

Why conditions vary so much from year to year

Part of what makes CIM's weather hard to predict from historical averages alone is that early December sits at a genuine transition point in the valley's seasonal pattern — late enough that winter storm systems are a real possibility, but early enough that a stretch of calm, dry high pressure (the same setup that produces tule fog) is just as plausible. Runners who've done the race multiple times often describe wildly different conditions from one year to the next for exactly this reason: there isn't a dominant weather pattern this time of year so much as a genuine toss-up between a few different ones.

Rain is the bigger wildcard than cold

For most runners, a bigger practical concern than temperature or fog is rain. December sits inside California's rainy season, and a wet race morning is a real possibility in any given year, even though plenty of editions have run dry. It's worth planning for both outcomes rather than assuming the valley's reputation for mild winters means precipitation is off the table.

Weather and the early shuttle, together

CIM's weather doesn't just affect the run — it affects the hours before it. Because the point-to-point course requires an early bus to the start, runners are often standing outside in the coldest part of the morning well before sunrise, whatever the day's eventual high turns out to be. A morning that ends up mild and pleasant by the finish can still start with a long, cold wait at a bus pickup point or the start corral, so it's worth dressing for that wait as much as for the race itself.

How to dress for race morning

• Plan for a cold wait before the start — the pre-race period outdoors near Folsom Dam can feel considerably colder than the race itself once you're moving.
• Bring inexpensive throwaway layers you can shed in the early miles once your body temperature comes up.
• Pack for the possibility of rain even if the forecast looks dry a few days out; Central Valley weather can shift close to race day.
• Don't over-dress for the fog. It usually signals cool, calm air rather than the kind of cold that requires heavy layers once you're running.

Daylight is also limited

Early December is close to the shortest days of the year, which means the start happens in low light rather than full daylight, and the bus rides beforehand happen in the dark. It's not a weather factor in the strictest sense, but it shapes the same practical decisions — reflective or high-visibility layers for the pre-dawn wait aren't just a formality, since visibility around the bus pickup points and start area is genuinely lower than it would be for a race starting later in the morning or later in the year.