Key Highlights
- A persistent ridge of high pressure has kept much of California and the Southwest rain/snow-less and warmer than usual.
- The Central Valley experienced a remarkably long episode of tule fog and damp chill due to moisture from autumn rains trapped by subsidence, creating an extreme temperature inversion.
- Over the coming days, warm atmospheric rivers will bring widespread light to moderate warm rains to Northern California, potentially ending the persistent tule fog layer.
- The Sierra Nevada snowpack remains extremely low despite the onset of precipitation due to the warm airmass.
- A stronger storm period is expected later in December, bringing potential flooding and strong winds, though details are still uncertain.
California’s Persistent Warm Spell
For several weeks, a significant mid-level ridge of high pressure has dominated much of California and the Southwest, leading to a prolonged period of rain- and snow-less conditions. This weather pattern has kept temperatures higher than usual across these regions.
The persistent warmth has been so pronounced that it has led to some remarkably warm nighttime temperatures, particularly in inland areas at higher elevations. For instance, it has often been warmer (even at night) in the Sierra Nevada mountains a mile up compared to locations like Sacramento, making for a truly bizarre climate phenomenon.
The Central Valley’s Unique Fog Phenomenon
While much of California basked in warmth, the Central Valley faced a different kind of weather drama. A persistent episode of tule fog and stratus layers created an unusual cold and damp environment, particularly during the fall months. This phenomenon was facilitated by autumn rains that added moisture to the soil, which then evaporated into near-surface air under the influence of a highly persistent ridge.
This combination of factors led to a shallow layer of moisture trapped less than 2,000 feet deep near the surface, creating an extreme temperature inversion. This inversion has resulted in absurd temperature contrasts between the valley floor and surrounding foothills or coastal mountains, with temperatures sometimes varying by over 30-35°F within just 1,000 to 2,000 feet of elevation.
Approaching Weather Changes
Over the coming days, a series of warm atmospheric rivers are expected to bring much-needed rainfall to Northern California. These rivers will gradually move southward along the Pacific Coast, bringing an end to the persistent tule fog and stratus layers in the Central Valley.
The arrival of this moisture will likely result in a gradual warming across the region as lower levels clear out and warmer subtropical air moves in. While it may not be warm and sunny all the time, conditions are expected to feel more balmy by comparison to recent weeks.
Sierra Nevada Snowpack Concerns
Despite the onset of precipitation, the Sierra Nevada snowpack remains at extremely low levels due to a lack of recent significant precipitation and very warm temperatures above the temperature inversion. Locations below 7,000-8,000 feet are expected to see nearly all rain, while any appreciable snowfall is likely limited to higher elevations.
Some notable locations in the central and northern Sierra already have no snow on the ground at all, including the Central Sierra Snow Lab. While there is still a chance for some recovery as we move into January, the current outlook suggests that even with precipitation, the snow situation may worsen over the next 7-10 days.
Potential Storms and Flooding
By late in December, conditions are expected to shift, bringing a more stormy pattern to California. This could include stronger statewide storms, though details remain uncertain as of now. The potential for significant flooding increases as soils become saturated with the onset of heavier rainfall.
While it is still early to predict exact outcomes, there is a real possibility that some colder storms might approach closer to New Year’s Day, adding complexity to the weather forecast.
To stay informed on these developments and other related issues, follow updates from relevant weather services and join virtual office hours for in-depth discussions. The current climate pattern presents both challenges and opportunities, requiring careful monitoring as we move into the final weeks of December.
