1-5-2018 Mammoth Weather Guy Powder & Weather Forecast

Powder Forecast –Friday January 5th, 2018

Ted Schlaepfer CCM  —- Mammoth Mountain WeatherGuy

Forecast Summary:

Snow develops overnight and become moderate/heavy during the morning Saturday before tapering off late PM/evening.  Sunday will be dry, then snow/rain develops Monday, becoming heavy overnight into Tuesday when snow levels come down to town with a couple feet likely at Main. 

Snow ends Tuesday night followed by dry weather Wednesday through next Sunday and into the MLK Holiday.  

A major storm pattern is then possible over the following week and into the last week of January that could lead to multiple feet of snowfall.  Storminess could then continue into the end of the month, although there is certainly a chance for a break in storminess too.

Next update Tuesday 1/9

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 1/6 = 3 – 4” (H2O = 0.45” – 0.60”) **4
Sun 1/7 = 2 – 3” (H2O = 0.35” – 0.45”) **4
Mon 1/8 = 0”

Tue 1/9 = 14 – 16” (H2O = 2.25” – 2.75”) **3
Wed 1/10 = 10 – 12” (H2O = 1.00” – 1.25”) **3
Storm Total = 27 – 28” (H2O = 3.25” – 4.00”) **3

Thu 1/11 = 0”
Fri 1/12 = 0”
Sat – Mon 1/13 – 1/15 = 0”

January Snowfall = 1”
January Forecast = 100 – 150”

Detailed 5-day Snowfall Forecast:

Sat 1/6 — Snow becomes moderate/heavy during the early morning, tapering off during the afternoon, ending evening.  Accumulations 5 – 7”, ~9” up top by Sunday AM.

Sun 1/7 — No snowfall expected.

Mon 1/8 — Snow/rain develops during the day, becomes heavy overnight with snow levels lowering to town by Tuesday AM.  Accumulations 15 – 18”, ~24” up top by Tuesday AM.

Tue 1/9 — Heavy snow during the morning, decreasing PM, tapering off overnight.  Accumulations 10 – 12”, ~15” up top by Wednesday AM.

Forecast Discussion:

Short Term:

Satellite imagery (below) shows a frontal system off the CA coast that is currently moving ashore with rain from about the Bay Area northward.   That system appears to be tapping subtropical moisture from a quasi-stationary low pressure system far west off the Baja peninsula as it slowly moves south and eastward.

   The latest 12Z operational ECM model (image below) pushes it eastward into the N. Sierra tomorrow morning with the subtropical flow moving into the Central Sierra (south of the purple arrow).   It looks like there will be enough dynamic cooling associated with the upper level low pressure system to push snow levels down to 8K, and possibly even the town, during the morning hours Saturday.   I suspected diurnal warming may push levels back to 8K or slightly higher by the afternoon.

   The latest GFS (image below) and ECM trended wetter since the last update with the ECM model showing close to an inch liquid QPF  (Quantitative Precipitation Forecast).  With snow levels starting at 8.5K tonight when the snow starts before falling late tonight, snow ratios will be low as this is Sierra cement type snow.  Current forecast favors about 6” at Main and close to 9” up top with 3-4” at Canyon.

   Dry weather will then return for a day before a bigger storm takes aim at CA Monday before moving ashore strongly Tuesday (image below).   Looks like Mammoth will be in the perfect position to take the brunt of the storm as it will be taking a more southerly track that will also produce the first rainfall of the season for SoCal including snow for Big Bear.

   Latest model QPF is around 3-4” for the storm event as it will also be tapping subtropical moisture from the same quasi-stationary low pressure system off Baja that will inject moisture into Saturday’s storm.   Moist upslope southwest flow begins Monday morning before increasing in strength during the day ahead of the frontal passage Tuesday morning.

   The GFS model is suggesting 2”+ liquid for the Sierra crest (image below) Monday under this strong orographic flow that can turn out wetter if it aligns perfectly with the San Joaquin River drainage system on the west side that creates a convergence zone near Mammoth Pass.  This could be such a scenario.

  However, due to the subtropical nature of the air-mass and strong warm air advection ahead of the cold front, snow levels will probably be in the 8500-9000 foot range Monday, probably rising to 9000 feet Monday evening as the moisture plume is technically an Atmospheric River.   That means rain at Canyon and probably a period of rain at Main too before snow levels come down early Tuesday when the front arrives.   Should be all Sierra cement snow up top.

   Snow levels fall fast Tuesday as the front and upper level low pressure system moves ashore.  Snow levels may get down to near 5000-5500 by Tuesday night when the snowfall is expected to taper off.   Looks like another inch or so (image below) of QPF per the GFS is suggest by the models during the day Tuesday under the lower snow levels for about a foot of additional snowfall.   ECM model is similar.   Overall,  probably about 2 feet at Main, one foot at Canyon, and 3-4 feet up top.

Long Range:

   One of the main themes during the December updates during the dry spell was patience and that our time would come.  Longer range guidance is now suggesting we will indeed be rewarded for our patience.   Even though dry weather returns by Thursday next week (image below), it appears it will be short-lived to just over the MLK Holiday weekend, although there is a chance snow could return by Monday night.

    The latest 12Z ECM model (image below) continues to trend toward what the longer range 10-15 day ensembles have now been favoring for the last few runs or so.  Those ensembles favor a deep upper level trough to develop along the West Coast (two images below) resulting in a breakthrough of the westerly jet stream into California and a large opening of the storm door.

   The GFS operational model is showing a similar pattern with the operational model bringing the first big upper level low ashore by the middle part of the following week (image below).  Past analogs for similar strong westerly jet breakthrough events favor an accompanying strong or extreme AR with initially high snow levels before lowering as the jet stream digs southward and 500-mb geo-potential heights lower.

   That model than digs the upper level trough (image below) heading into the following weekend which would likely mean cyclo genesis and potentially a big Sierra snowstorm for a double punch of heavy precipitation.   The lower 500-mb geo-potential heights associated with the polar jet (blue arrow) will provide plenty of cold air to keep snow levels way below town.

   Longer range GEFS ensemble mean (image below) keeps the favorable storm pattern going into the last full week of January as the pattern appears poised to flip second half of January with the East Coast warming, just as the MJO analogs suggested will happen.

   The CFS model is still stubbornly dry for this whole period (why it is often referred to as the Can’t Forecast S#!t model) while the ECM weekly climate model trended wetter with yesterday’s update.  The monthly version that goes out 6 months was released this morning and it also trended wetter for January and it also shows a big February.

    Overall, confidence is certainly building fast for a major storm pattern just after mid-month that could last up to two weeks. That means there could be 6-9 feet at Main over the period and the monthly forecast total was bumped up significantly.  It is still a ways out, but past major storm patterns showed a similar strong signal in the 10-12 day range.   They just don’t sneak up on you nowadays, it is the weak storms that are tough to forecast in the extended range.  Get ready. WG