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Tahoe Daily Snow

Monday Morning Analysis...


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Yesterday afternoon I posted a full analysis of every storm and the long-range that you can click back to on the link at the bottom of this post.  This morning I wanted to mainly focus on the Wed-Thu storm analysis.

Let's talk about tonight's storm briefly to start.  Not much change to the forecast.  We already have wind gusts on the mountain tops to around 60 mph and some warmer air is working in ahead of the storm.  The latest model runs consolidate the storm so that most of the precipitation starts falling this evening and is done by Tuesday morning.  Snow levels start around 8000 feet this evening before falling to 7000 feet after midnight. 

The GFS is slightly wetter this morning and the European model slightly drier, but still a few tenths wetter than the GFS.  The NAM model is leaning towards the Euro instead of the GFS today.  The Canadian run is drier than the GFS.  Also, the latest GFS run puts more precip Southwest of the lake near Kirkwood.  So taking the model average here is the final forecast on this weak storm.

final final

After a break Tuesday a much wetter storm moves in on Wednesday.  This is where I want to spend our time today.  There are several moving pieces with this storm making for a tricky forecast. 

The European model has consistently been the wetter model all week.  That was until this morning.  It had over 5 inches of liquid West of the lake along the crest yesterday and has backed off a little this morning, but the GFS model that was only showing half the precip the Euro was showing 2 days ago is now showing close to 5 inches along the crest on the new 12z run!

The good news is that now the two models are less than an inch apart and the GFS has now been consistently wetter the last 2 days.  I wanted to just lay out the most recent total precipitation forecast from 6 different forecast models, zoomed into our area, listed from wettest to driest.  You have to remember it includes tonight's storm, so subtract out about 3 tenths on the East side of the lake and 6 tenths on the West side. The gray color starts at 4.5 inches of liquid and increase by .5 inches for each shade.

The GFS model, showing close to 5 inches SW of the lake near Sierra, Kirkwood, Bear Valley, and Dodge Ridge.


The NAM model showing the 5 inch line close to the areas NW of the lake as well.


The European model now maxing out only around 4 inches near the crest.


The Canadian model showing up to 4 inches along the crest.


The WPC model, which is the WPC creating a forecast off of all the models after several runs including ensemble runs.  That helps to alleviate run to run errors.  This model also starts at 4.5 inches on the darkest red shade, not gray.  So 3.5 - 4 inches West of the lake for the high end.


The European EPS ensemble mean runs taking and average of 50 ensemble runs.  This model is only averaging off it's own ensemble runs not the other models.  It is maxing out around 3 inches along the crest.


Also, to note is that the models are bringing over 2 inches of liquid to the East side of the lake.

So the 4 operational models, the GFS, Euro, Canadian, and NAM are all showing up to 4 or more inches of total liquid along the crest from Wednesday through Thursday night, a 48 hour total.  The bottom 2 models are drier because they are taking several model runs, of several models, including the ensemble runs, but they are still bringing 3+ inches of liquid to the crest.

So the question is the timing of the heaviest precip rates and the snow levels.  I want to break this storm into 2 parts, the warmer part and the colder part.

Part 1:

We have the warmer subtropical moisture moving in first on Wednesday with snow levels starting up around 9000 feet before falling to 8000 feet later in the day.  The GFS is faster in bringing in more moisture during the day on Wednesday.  

Wednesday night snow levels may sit around 8000 feet most of the night before falling to 7000 feet during the early morning hours as colder air starts to move in.  The GFS is bringing in the heaviest precip Wednesday night on the latest run, and has 85% of all the precip for the storm falling Wednesday and Wednesday night.  

The European model is later with the start on Wednesday and brings in heavier precip Wednesday night, but has the heaviest precip falling Thursday with the cold front moving through.  It only has 42% of its total precip falling through Wednesday night.  All this is doing is lowering the total precipitation average between the two models during the warmer part of the storm.  

Here is a look at the snowfall forecast for Part 1.  Don't expect to see much snow on Thursday morning unless you are above 7500 feet.  This is a rain event for the lower elevations, with 1-2+ inches of rain for lake level.

part 1

You can see the highest amounts along the crest of 15-20 inches above 8000 feet, and steep drop-off below that.

Part 2:

The 2nd half of this storm is a combination of the cold front moving through early Thursday morning and the center of low pressure moving through Thursday night.  That could keep the rain/snow showers going as much colder air moves in.  

Like I mentioned above the European model brings in the heaviest precip on Thursday while the GFS is down to just light showers.  Snow levels fall to lake level Thursday morning.  So how much precip falls Thursday into Thursday night is important because it is snowing at all elevations and the snow ratios are increasing on the mountains.

The issue is that the European model is drier than the GFS overall but bringing in 3x the amount of precip during the colder part of the storm.  That is dragging the model average way up for snowfall amounts on the 2nd half.  Looking at only the GFS you have a forecast for 1-3 inches of snow at 8k on the East side of the lake, and 3-9 inches on the West side.

Throw in the European model and as you see below the forecast for Part 2 is where most of the snow falls, and we could have 7-10 inches on the East side, and 11-17 inches on the West side.

part 2

I think that it is really important to understand the breakdown of the forecast when you have important forecast model discrepancies that can affect the snowfall forecast in big ways.  It's good to see the 12 hour break down and 2 part break down as well so that no one is waking up on Thursday expecting to see a big dump of snow out the window.

Even if you only get 3-4 inches of liquid at 8k feet along the crest, and you assume a low snow ratio average for the entire storm of 8:1, you still get 24-32 inches of heavy wet snow.  Here is the updated total snowfall forecast by Friday morning, averaging the GFS/Euro and adding in the fluctuating snow levels.

updated forecast by friday

We have better consistency and better agreement on the forecast model runs, which is increasing the confidence in the total precipitation we could see from this storm.  The question will be the snowfall based on the timing of the cold air versus timing of the heaviest precipitation, and the duration of the precipitation into Thursday night when snow ratios really jump in the cold air.


The models are still trying to figure out the Sun-Mon time period with the next storm.  If you have been reading all week you know I have been leaning towards a drier pattern next week.  We will have to see if a warm storm pushes far enough South on Sunday-Monday before we may get the drier pattern.

Stay tuned...BA


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