Here Comes Winter! Storm This Week, More Activity Through November?

Overview

My friends...

Winter finally arrives in earnest this week with a more moderate storm. AI models have been trending stronger with this storm, while guidance is still split. The caveat to the stronger trend on AI models is higher snow levels. Uncertainty remains around the precipitation amounts, track, and timing of the storm, but as of now several inches of snow up to double digit totals is possible along the Sierra Crest and higher peaks, with a mix of mostly rain and some snow in the town of Mammoth Lakes.

After this, a much more favorable pattern for stronger storms is trending likelier as a Gulf of Alaska trough appears to be setting up. Uber long range modeling suggests this could stick around more often than not the entirety of November. This could be a canonical response to El Niño emerging, which would then be favored more often than not throughout winter per seasonal guidance and our dynamical understanding of strong El Niño events.

Storm This Week

After a dry stretch, another cutoff low swings in from the Pacific on Wednesday. Models have seen this one coming for quite a while.

500 mb height anomalies valid Wednesday evening, October 21, from the ECMWF IFS ensemble, the AIFS ensemble, the operational ECMWF AIFS, the ICON and WeatherNext 3, all showing a closed low off the Central California coast

WeatherNext 3 data accessed through Google Cloud Storage. © 2024-6 Google LLC, whose machine learning models were used to create the experimental data made available under the following licence terms https://storage.googleapis.com/weathernext-public/terms-of-use.pdf. This data is intended for experimental modelling only and is not intended, validated, or approved for real world use.

Unlike last week's low, this one is favored to stay offshore long enough to pull moisture in from the south before moving inland. That's good news for precipitation. The tradeoff is that it also pulls in mild subtropical air, so snow levels start out high. Over the past few runs, this storm has trended stronger, but also substantially warmer.

Precipitation looks to start Wednesday morning, with the heaviest rates Wednesday afternoon through Thursday, and lingering showers into Friday as colder air wraps in behind the low.

But How Much Snow??

Here are the precipitation percentiles through Thursday evening from the Euro ensemble (IFS) and the Euro AI ensemble (AIFS). The 10th percentile is the drier end of each ensemble, and the 90th is the wetter end:

ECMWF IFS ensemble and AIFS ensemble total precipitation percentiles through Thursday evening, October 22, with the AIFS ensemble wetter along the Sierra

The Euro AI model is showing more moisture than the Euro dynamical and GFS models. WeatherNext 3 also is showing more moisture.

WeatherNext 3 ensemble mean total precipitation through Thursday evening, October 22, with half an inch to three quarters of an inch along the Sierra near Mammoth

Personally, I'm more inclined to believe the AI models here, which are favoring a slightly wetter and warmer storm.

Here we can see the plumes from each model showing the spread of outcomes.

Total precipitation plumes at the grid point southwest of Mammoth from the ECMWF IFS and AIFS ensembles, ICON-EPS, and the operational ECMWF IFS, AIFS and ICON, through October 25, ranging from about 0.3 to 1.2 inches

Up on the higher peaks, at 11,000 feet, the AI models bring about 10 to 12 inches of snow, while the Euro is closer to 4 or 5 inches:

Snowfall plumes at 11,000 feet at the grid point southwest of Mammoth from the ECMWF IFS and AIFS ensembles and the operational ECMWF IFS, AIFS and ICON, ranging from about 3 to 12 inches

QPF and Snow Levels

Precipitation (QPF). Models currently show a range of about 0.4-1.2" of liquid equivalent along the crest near Mammoth through Friday. The AI models sit on the higher end, around 0.7-1.2", while the IFS and ICON come in closer to 0.3-0.6". Leaning on the AI models, I'm expecting around 0.7-0.8" along the crest, with considerably less spilling over to the east. This storm is bringing in a strong southerly flow component, which means Mammoth Mountain may be rainshadowed here relative to other peaks.

The bulk of it falls Wednesday afternoon through Wednesday night, with lighter showers Thursday as the low moves through.

Snow levels.

Snow level plumes at the grid point southwest of Mammoth through October 25, starting near 9,000 to 10,000 feet on Wednesday and falling to around 7,000 to 7,500 feet by Thursday night

Snow levels will be roughly 9-9.5k during the heaviest precipitation Wednesday and Wednesday night, lowering to roughly 7-7.5k by Thursday morning as the colder air wraps in behind the low. Unfortunately, most of the QPF falls before the cold air arrives. A second, weaker wave late in the weekend also looks to come in with high snow levels, around 8-9k.

Wednesday appears to mostly rain main lodge and below, with only the highest peaks seeing snow. By Thursday morning, snow levels may come crashing below main lodge but the town of Mammoth Lakes is likely to remain marginal.

Operational ECMWF AIFS 6 hour precipitation by type over the continental US ending Wednesday evening, October 21, mostly rain across California and Nevada, and ending Thursday morning, October 22, with snow and a mix along the Sierra crest

My Forecast for This Storm

Summit (11,053'): 5-9" of heavy, wet snow, with a range of 3-11". About 0.7-1.1" of liquid at around 8:1 overall. Sierra cement!

Main Lodge (8,900'): 1-5", with up to 7" possible. Rain or a rain and snow mix Wednesday, turning to snow overnight into Thursday as the snow level drops below 9k, at around 10:1. About 0.5-1.2" of liquid, much of it falling as rain.

Town of Mammoth Lakes (7,800'): 0-2", with up to 3" possible. Mostly rain, possibly ending as wet snow Thursday night. About 0.5-0.75" of liquid, less on the east side of town.

In total, I'm expecting about 5-9" of snowfall for the Sierra Crest near Mammoth, 1-5" at Main Lodge, 0-2" in town, and little to nothing along most of the 395 corridor. If the AI models are right, the upper end of these ranges is well within reach. If the warm air hangs around longer, totals below 9k will come in on the low end. Not enough to get the ski season going, but a great start! It's also still a few days out and subject to change.

A Gulf of Alaska Trough, and Our El Niño Winter Getting Started Early?

After this storm, the pattern starts to shift. By early November, both ensembles build a trough over the Gulf of Alaska and the northeast Pacific, with the jet aimed more directly at the West Coast:

500 mb height anomalies from the ECMWF IFS ensemble and the AIFS ensemble valid Friday, October 30, with a ridge over Alaska and lower heights off California, and Monday, November 2, with lower heights across the northeast Pacific

The plumes keep building precipitation into early November, with the AIFS ensemble averaging about 2 inches of liquid through November 2 and the IFS ensemble about 1.2 inches. The operational AIFS ends its run with a stronger storm right around November 1st... Something to watch!

15 day total precipitation plumes at the grid point southwest of Mammoth from the ECMWF IFS and AIFS ensembles and the operational ECMWF IFS and AIFS through November 2, with precipitation continuing to build after this week's storm

The jet stream tells the same story. Here is the 200 mb wind from the AIFS ensemble on day 15. Even in the mean of all 51 members, there's a distinct southern branch of the jet aimed straight at California:

AIFS ensemble mean 200 mb wind valid Monday, November 2, with a southern jet branch of 60 to 80 knots running from the central Pacific into Southern and Central California

And the ECMWF weeklies from today's run keep that trough parked in the Gulf of Alaska for the entire month, from November 3 through December 3, with a strong ridge over central Canada and a wetter than normal signal across California and the Sierra:

ECMWF extended ensemble 500 mb height anomaly averaged from November 3 to December 3, with a deep trough in the Gulf of Alaska and a strong ridge over central Canada

ECMWF extended ensemble 46 day precipitation anomaly through December 3, wetter than normal across California, the Sierra and the Southwest

Images: WeatherBell Analytics, ECMWF data.

This is quite a strong signal and it's been showing up consistently across all models for quite some time. And this is the canonical El Niño look, so seeing this on models is also supported by our intuitive understanding. It's the same pattern the seasonal models show for our winter:

Copernicus multi-model mean precipitation anomaly and 500 mb height anomaly for November, with lower heights off the California coast and wetter than normal conditions along the California coast and southern United States

Usually, the El Niño response is backloaded towards January and February. The seasonal models show this backloaded response still being favored, but with a wet signal now emerging for November, I'm inclined to believe we start up sooner.

Monthly precipitation anomaly at Mammoth Main Lodge from nine seasonal models, near normal in October and November and wetter than normal from December through March, peaking in January and February

This El Niño is so far beyond any previous event in history. 1997-98 was the last historically strong east-based event, and the gold standard for what an "east-based" event looks like. This year, the Indo-Pacific Warm Pool (IPWP) sits a full 15 degrees of longitude farther east than in 1997-98. That's over 1,000 miles!

ENSO Longitude Index for 2026, a 15 day running mean ending October 16, with the warm pool shifted far east of climatology to near 165°W

Image: Alex Boreham, cyclonicwx.com.

The farther east the IPWP shifts, the stronger the Subtropical Jet response should be across the eastern pacific into California. It blows the last event out of the water. I do really think the effects of El Niño will be felt profoundly starting in November, and throughout the winter. In my opinion, this is the climate event of the century!

The Season Ahead

Every single seasonal model shows a profoundly wet outlook for California. We've simply never seen this level of agreement. At Mammoth, the median of all members comes in at about 41 inches of precipitation through early May, against a normal of about 29 inches, or 143% of normal. And I believe this is underplaying the local impact in Mammoth. San Francisco shows 180%+ of average.

Season total precipitation at Mammoth Main Lodge from October 18 through early May from every member of six seasonal models, with a median of about 41 inches against a normal of about 29 inches

The odds are stacked incredibly so towards a wet and wild winter in California, both in the modeling and in our understanding of the climate. While there's always a chance it doesn't happen, I would be a fool to forecast anything different. I have almost no more doubts about the coming winter.

The one thing to watch, as this week's storm shows, is temperature. The seasonal models lean warm early in the season, and warm, juicy storms can mean rain down low and snow up high.

You can explore all of this on my Seasonal Outlooks page in the Climate Lab.

Winter is knocking on the door, my friends. My full seasonal outlook is coming soon... Until then, pray for snow! As always!