A few of the ski resorts reported 1-2 inches of snow above 7000 feet from the snow showers that moved through yesterday afternoon.  The snow levels rose above 8000 feet pretty quickly during the evening.  The moisture stream shifted to our North overnight.  

You can see the clouds and moisture streaming in to our North this morning on the satellite.

We have a break today into tonight, but the winds should really start to crank.  The winds are expected to drop down into the valleys as well.  We could see gusts over 70 mph in the valleys by tonight, and over 100 mph on the ridge tops.  For today they winds should be blowing in excess of 70 mph on the upper mountains, so expecting a lot of lift closures.

A cold front approaches tomorrow and ahead of it the moisture from the atmospheric river pushes South back into our area around midday.  The area of low pressure that has been helping to pull up the moisture from down near Hawaii is forecast to track across Central CA again Thursday night.  That was the forecast a few days ago, it trended well to our North the last 2 days, and now is back to tracking over us.  That will help to enhance the precip amounts and the spillover of precip to the East side of the Tahoe basin.

You can see the setup on the GFS for Thursday night.

The heavy precip is expected to move out by early Friday morning with only light snow showers for Friday and much colder air moving in.  Here is a look at the latest total precip forecast on the GFS and the Canadian models.

Total precip amounts are similar on the GFS, Canadian, and European models.  They show 1.5 - 2.5 inches of total liquid around the Tahoe basin, and 3+ inches along the Sierra crest.

So the question turns to snow levels.  There is good agreement that the snow levels are around 9000 feet Thursday afternoon.  You will see on the resort pages that the mountains that have terrain above 9000 feet I have several inches of snow for Thursday afternoon, and just rain for the rest.

Thursday night we have the combination of the AR, a cold front, and the low moving through which will create very heavy precip and falling snow levels.  The air behind the cold front is very cold which means that snow:water ratios will quickly climb behind the front.  The challenge for the snowfall forecast is how fast and how much cold air moves in Thursday night.  It makes a big difference in snowfall with this much precip.  A slight adjustment of the timing of the snow level drops and the duration of temperatures at each elevation can add or subtract several inches.

The GFS and European models have trended slower with the colder air this morning.  The GFS has snow levels still near 8000 feet at 10 p.m. where before it was closer to 7000 feet.  Then by 4 a.m. snow levels are down below 5000 feet.  So they fall fast sometime between 10 p.m. and 4 a.m.  But the heaviest precip begins to end by 4 a.m.  

Take a look at the CNRFC freezing level forecast for 4 p.m. tomorrow.  This is when I expect snow levels around 9000 ft.

Then by 10 p.m. the freezing levels have only come down about 1000 feet.  So snow levels may only drop to 8000 feet.

Then by 4 a.m. freezing levels drop another 2500 ft. and it is all snow down below lake level.

Now if you remember with the last storm the freezing levels showed that snow levels should be around 8500 ft., but we had accumulating snows falling as low as 6500 feet in parts of the basin.  If we see very heavy precip Thursday night it could knock down the winds and the heavy snowfall up high could drag the snow levels lower earlier than expected.  Also the cold air could move in faster.

Looking at the NWS Reno snow level forecast they have snow levels only near to just above 8000 ft. Thursday afternoon.  Then beginning to drop around 7 p.m. Thursday night and are below 7000 ft. by 10 p.m., and below lake level around midnight.

That is faster than the latest GFS and European runs.  The other question is how much precip falls, and how much falls before and after the snow levels fall.  On top of that, after it is snowing at each elevation what is the air temperature and snow ratios as the the temperatures should fall into the 20's behind the front.

To add to the complication this morning the latest 12z NAM model is showing the faster snow level drop comparable to the NWS forecast above, and it has about an inch more of liquid with 4+ inches along the crest.  Here ithe NAM in animation for accumlating precip Thursday and Thursday night.

If you think that putting together a snowfall forecast for each mountain and elevation in these scenarios you're crazy.  For this morning's updated snowfall forecast I am going to assume the slower snow level fall for Thursday night.  As you will see that chops about 10 inches off the high end snowfall forecast from the last 2 days.  But the forecast is still the same at 9000 feet.

I think that it is better to assume the slower snow level drop and lower precip totals, and then be surprised if they fall faster and we get more precip.  I think for this morning given all the new data this is a reasonable forecast.  We have seen storms like this drop snow levels faster than expected and we have see snow levels fall slower than expected.  Let's hope for the former.  If snow levels fall to 7000 ft. by 10 p.m. we could add several inches back in.

Much colder air is in place for the weekend with highs only in the 20's through Monday.  High pressure will begin to build off the coast helping to keep us dry through Tuesday, and temperatures should warm a little to near 40 at lake level by Tuesday.  Plenty of cold air for snowmaking through the period.

The milder air Tuesday should be short lived.  Here is a look at the higher pressure building off the coast by Tuesday.

Then the forecast models are all in good agreement that we see a cold front move down from the North next Wednesday and again Friday.  You can see the cold trough digging in by Christmas Eve.

The pattern looks to continue to be cold.  This is like what we saw the end of November into December before these mild AR systems.  

We are seeing a big pattern change over the Pacific with the EPO (eastern pacific oscillation) and the WPO (western pacific oscillation) both flipping from negative to positive over the next week.  So instead of high pressure across the Northern latitudes and low pressure underneath helping to drive these AR event, we will have low pressure to the North and high pressure to the South.  So the warm AR events should be done for a while. 

The question with these cold troughs trying to dig down from the North is how far off the coast will they dig.  The cold fronts Wednesday and Friday next week look to be moisture starved as the troughs drop down over land and don't pick up moisture off the Pacific.  So only light amounts of snow look possible with each.

Looking further out we may see the trough dig further West and get some stronger storms by the last week of the month.

Stay tuned...BA

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Geography Key:

East

Mt. Rose, Heavenly, Diamond Peak

Central

Northstar

West

Tahoe Donner, Homewood, Bear Valley, Dodge Ridge

Crest

Boreal, Donner Ski Ranch, Sugar Bowl, Squaw, Alpine, Sierra, Kirkwood

*Boreal and Donner Ski Ranch are forecast with the "West" mountains in higher snow levels events.