Get ready for a weather phenomenon that will leave you in awe and wondering! A resilient ridge is causing a unique weather pattern in California, with a prolonged tule fog episode bringing a cold and damp experience to the Central Valley. But here's the twist: this foggy wonderland contrasts sharply with the anomalously warm and dry conditions elsewhere in the state. It's a tale of two weather worlds!
The recent autumn has been a wet one for Southern California, with some areas experiencing record-breaking rainfall. In fact, it was the wettest on record in parts of Central California, with a northeast-oriented swath of patchy rain stretching from the Central Coast to the San Joaquin Valley and over the Southern Sierra. Most areas in SoCal saw one of their wettest autumns in decades.
But here's where it gets controversial: despite the heavy rain, it wasn't a particularly cool or cold period. Temperatures were warmer than average across California and even more so in the interior West. So, autumn 2025 was a wet yet mild to warm season for much of the state.
The persistent ridging has strengthened, leading to a quieter and warmer pattern over the past two weeks. However, the Central Valley and occasionally parts of the SF Bay Area have been an exception, experiencing highly persistent radiation fog known as "tule fog." This dense fog has been a reminder of decades past when such events were more common and prolonged.
And this is the part most people miss: while the Central Valley is shrouded in fog, the broader West and most of California outside the fog layer have been experiencing an anomalously warm and dry airmass. There's a remarkable contrast between the cold, damp conditions under the fog and the warm temperatures just a couple of thousand feet higher in elevation.
The relative warmth has led to early-season "snow drought" conditions across nearly the entire West, except for a few basins in Arizona. While it's not bone-dry, the lack of snowpack is mainly due to the warm temperatures. And this trend is expected to continue, even as the Pacific Northwest faces flooding rainfall.
The next week is likely to be a repeat of the last two in California, with persistent ridging and warm temperatures above the inversion layer. The Central Valley will continue to experience cold and damp conditions due to the tule fog. Meanwhile, the Pacific Northwest is preparing for a potential widespread flood event as atmospheric rivers bring wet and warm conditions.
As we move into mid-late December, there's some uncertainty in the North Pacific flow pattern. The ensembles suggest continued ridging over California, with a higher chance of dry conditions in southern and central parts. There's a slight possibility of some precipitation sneaking through, especially in NorCal, but it's not a sure thing.
One striking feature is the high confidence in very warm conditions across nearly the entire West, including California. This means Sierra snowpack accumulation is unlikely over the next few weeks, and the Central Valley may slowly see reduced fog coverage as the overall dry and warm airmass takes its time removing moisture.
The winter seasonal outlook continues to lean towards warmer and drier conditions, especially in SoCal and the Southwest. Despite the feedback about a promised wet winter, it's important to remember that winter predictions are for Dec-Feb, and so far, it looks like a drier and warmer December for most of California.
The latest ECMWF seasonal outlook suggests a tilt towards drier and warmer conditions in southern and central CA and the lower Colorado basin this winter, with La Niña and a persistent V-shaped warming pattern in the Pacific. The warm ocean temperatures along the CA coast may continue to influence storm patterns, potentially bringing warmer, wetter, and more unstable storms.
Looking ahead, La Niña conditions are expected to fade soon, and an El Niño transition is possible by early summer 2026. This could be supported by the strong westerly wind burst in the West Pacific, allowing for SST warming in the tropics. However, ENSO predictions are tricky, and models can be sensitive to initial conditions, so we'll have to wait and see.
Join me on Wednesday morning for a live virtual event discussing hurricane trends in a warming climate. We'll explore the science behind rapidly intensifying tropical cyclones and their practical implications. It's a rare topic for the West Coast, but an important one in the global context. See you there!
