Powder Forecast – Friday, December 12th, 2025
Ted Schlaepfer CCM —- Mammoth Mountain WeatherGuy
Snowfall forecasts are valid at the Sesame snow course (Main Lodge) for the prior 24 hours as reported in the 6-7 AM morning snow report.
**Snowfall forecast confidence ranges from very low (1) to very high (5)
Sat 12/13 = 0”
Sun 12/14 = 0”
Mon 12/15 = 0”
Tue 12/16 = 0”
Wed 12/17 = 0”
Thu 12/18 = 0 “
Fri 12/19 = 0 – 1”
Sat – Mon 12/20 – 12/22 = 2 – 6”
December Snowfall = 0”
December Forecast = 50 – 70”
Detailed 4-day Snowfall Forecast:
Sat 12/13 through Tue 12/16 – No snowfall expected all days.
Forecast Summary:
Short Term (Days 1 – 4):
The current infrared satellite image (below) this afternoon shows high pressure centered over CA, with storm systems riding up and over the ridge into British Columbia. That ridge is producing mild, benign weather, with temperatures at the Main lodge in the mid-50s.
The ridge will weaken a bit this weekend, then spread eastward before restrengthening again early next week (image below). The result will be only slightly cooler temperatures under continued fair skies.
Long Range (Days 5+):
The longer-range guidance is continuing to trend more positively heading into the Christmas time frame, as a significant shift in the northern hemisphere teleconnection pattern unfolds. This could eventually lead to a good shot at a significant snowfall pattern toward the latter part of the month.
The latest 12z run of the ECMWF flattens the ridge around the middle of next week (image below) as a strong short-wave trough moves southward into NorCal. It looks like it will stay too far north for any meaningful precipitation in Mammoth, with any precipitation likely rain, as snow levels will remain high. A better bet is a round of gusty winds on Tuesday and Wednesday.
That model moves another low into the PacNW and CA toward the end of the week and over the following weekend (image below). This time, the southern branch of the jet stream (purple arrow) is a bit farther south, and Mammoth has a better chance of precipitation.
The 12z ECMWF run has about 2 inches of liquid (image below), and the ECM EPS (two images below) has about 1 inch. Unfortunately, current guidance indicates the jet stream isn’t far enough southward to bring enough cold air, and snow levels are forecast to be in the 8500-9500 foot range in the subtropical flow. It is still about a week out; hopefully, the forecast will turn more favorable for snowfall in the lower part of the mountain, as it may be mostly confined to the Top.
The longer-range forecast says the teleconnection pattern will become more favorable for storminess to move southward into Mammoth and even southern CA as we head into the last part of the month. The Stratospheric Polar Vortex (SPV) is forecast to strengthen (image below) starting next week and continuing through the rest of December, then remaining in strong ranges through at least the first part of January.
The MJO has weakened over the past few days and moved back into the unit circle on the RMM index plot. The guidance (image below) says it will re-emerge in phase 7/8 next week, where it will linger for most of the rest of the month. That combo of a strong SPV and an MJO in phase 7/8 is favorable for strengthening the Bermuda and Aleutian ridges, with a trough and a breakthrough jet stream in the eastern Pacific impacting the West Coast. See the November 25th post for more information about this relationship and the type (h) pattern.
That pattern is what the longer-range GEFS (image below) and ECM EPS (two images below) are showing for the 11-15 day period. This is a snowy pattern for Mammoth, especially if there is strong southwest flow and the jet stream (purple arrow) is south of the central Sierra as currently depicted.
The ECM EPS is showing around 5” of liquid for the period (image below), and the Canadian ensemble is similar (two images below), with more liquid on the western slope of the Sierra. The GFS ensemble is the driest (three images below), but still has around 4” of liquid.
Some recent deterministic runs showed over 10” of liquid for Mammoth, and the latest ECMWF shows around 7” (image below). Hopefully, something like this happens, and the mountain receives many feet of snowfall as a big Christmas present.
The fantasy range forecast says the storm door will remain open for CA and Mammoth through the end of the month and into January (image below). It says the train will then keep going into the second week of January too (image below). Looks like good times ahead. WG
