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Mammoth Mountain Weather Forecast & Discussion
“Daily zone-based weather updates for Mammoth Mountain, Mammoth Lakes, Crowley, and Bishop – plus Snowman’s forecast insights and ECMWF model discussions.”
Mammoth Mountain Weather Discussion
9-16-25 – Higher heights remain in the area today with Tropical Storm Mario well to our south, along with a very weak cutoff low near the SoCal coast. As we move towards mid-week, the remnants and energy from Mario merge into the weak cutoff low, and by Thursday, the COL has strengthened into a moderate strength system that lifts towards the north.
Currently, most of the energy will travel up the coast and into Northern California. The Eastern Sierra could experience some light rain at some point, but it appears to be a setup for overdevelopment rather than moderate thunderstorms.
Since we are dealing with a cutoff low, just a slight change in direction could lead to a significant change in the current forecast. I will update this discussion on Wednesday if any significant changes are observed in the morning model runs.
Here is the latest NAM MSLP (12Z). Friday looks to be the most productive day, rainfall-wise. However, confidence remains low in this outlook.
The QPF from this wave of moisture is inconsistent across all the forecast models I examine. The end time on each image below is when the maximum rainfall was recorded.
Have a great day and enjoy the Eastern Sierra Weather!
— Steve Taylor, the Mammoth Snowman (WX Links)
Images provided by WeatherBell.com
September Seasonal Model Update – Looking Toward Winter 2025–26
Today’s ECMWF seasonal model update keeps the weak La Niña in play a bit longer. Instead of fading quickly, the latest run holds it through at least January, with neutral conditions not expected until mid to late February.
Here’s what the maps are showing:
Nov–Dec–Jan: Greens light up across northern California and into the Sierra. That often points to more storm action and a better shot at early season snowpack.
Jan–Feb–Mar: More green but a shade lighter, that’s a good sign of a full seasons worth of snowfall. Of course this is way off in Fatasy land but green is always the trend we want to see.
Take a look at the ECMWF maps below — green shades highlight wetter than normal trends, while brown shades mark drier signals.
September runs are always a key checkpoint. If the Sierra stays painted green again in October’s update, confidence grows for a solid early and regular season. For now, it’s encouraging to see the signal lean our way.
Snowman
❓ Top Mammoth Weather Questions (from the Snowman)
When does Mammoth usually get its first snowfall?
Light snow can fall in late September, but consistent snowfall usually begins in late October or early November. Opening Day typically lands between November 10 and Thanksgiving, depending on storm activity and snowmaking.
How does the ENSO pattern affect Mammoth’s late fall and winter weather?
ENSO (El Niño/La Niña) plays a big role in Sierra storm patterns. El Niño winters often bring warmer, wetter storms. La Niña typically favors colder, backloaded seasons with more snow from February through April. Neutral years are the wildcards with high variability.
What is 500mb height and why does it matter?
500mb height maps show atmospheric pressure around 18,000 feet — a critical layer for tracking storm tracks, ridging, and jet stream placement. Watching 500mb patterns helps forecast Mammoth’s incoming storms and long-range trends.
What are the PNA and PDO, and how do they affect Mammoth weather?
The PNA (Pacific-North American Pattern) influences western U.S. ridging and troughing. A negative PNA favors cold, stormy setups. The PDO (Pacific Decadal Oscillation) affects sea surface temperatures in the North Pacific — a negative PDO often aligns with enhanced storm delivery into California during winter.
👉 Want more answers? Check out the full Mammoth Weather FAQ – 50 Expert Answers to learn more about storms, snow, winds, and long-range patterns from a trusted local source.
Author Bio: Steve Taylor, The Mammoth Snowman. Over the last 40+ years, Steve has spent countless hours studying and learning about Mammoth Mountain Weather and Snow Conditions.
Dr. George, Don Marcelin, Howard Sheckter, and Ted Schlaepfer were his weather mentors at that time. Steve used to hang out with Howard in the Weather War room in the early ’90s, getting first-hand knowledge of Mammoth Weather from the (The Dweeb) himself.
Steve has spent countless hours reading, studying, and watching weather discussions from the NWS and multiple Private Weather Forecasters. He is a long-time member of WeatherBell, Weather West, and AccuWeather Pro. Disclaimer: Steve is a hobbyist forecaster with over 35 years of experience reporting recreational weather and travel reports for the Mammoth Lakes area.