More fun today as the computer models continue to make big adjustments.  This has been happening all week and making being a forecaster a lot of fun.  I saw a couple other forecaster posts this week that said they refused to make a snowfall forecast until the models were in agreement and consistent.  I guess I'm the one of the crazies trying to keep up with the daily adjustments and trying to make a snowfall forecast.  

The data I use comes from several models that run several times a day.  Those are the best tools we have.  I know some of you think I like to adjust the forecast up and down everyday just to mess with your emotions, but it is the data that is changing daily.  It messes with my sanity, and I would never try to piss off 25,000 people on a daily basis.  I do like to go out in public...  Per a reader request I have written a piece about the difference in the models we use which I will put out soon and link to it.

I have the updated Northern Sierra 8 station index amounts after the last storm.  They picked up an average of 4.9 inches of liquid even though we saw much higher amounts in places around the Tahoe Basin.  I will have the Tahoe comparison at the end of the month.  Like we thought we shot back above the wettest year.  We have close to half of the water season average, and we should hit that this week.

On a side note, does anyone know what is going on with the cdec website?

Tuesday:

The first change this morning is Tuesday.  The atmospheric river has continued to slow its arrival.  Now it looks like we won't see any precip until the afternoon.  Yesterday I showed how the AR would aim to our South.  Now it is aimed at Tahoe again, but only light precip makes in during the day Tuesday.

Snow levels start below lake level in the morning and start to rise during the day.  They drop briefly in the afternoon as the precip starts as the atmosphere moistens and causes the air to cool.  So we may see snow levels start at lake level in the afternoon.  The precip amounts on the latest model runs are so low that we may only see a dusting of snow up to an inch on the mountain tops.

Tuesday Night:

Heavier precip starts to move in Tuesday night but the stream of moisture begins to shift North.  Snow levels on the latest model runs are around 7500 feet by 10 p.m., and then up to 8500-9000 feet by 4 a.m. Wednesday.  That means the accumulating snow is only above 8000 feet Tuesday night, and only for half the night before snow levels rise even higher.

Here is the updated snowfall forecast for Tuesday night.

The precip that moves in Tuesday night is not that heavy.  The total precip on the high end for the GFS and Euro is only three to four tenths of an inch along the crest, and lesser amounts East across the Tahoe basin.

Wednesday - Wednesday Night:

The forecast models continue to show the moisture stream lifting to our North Wednesday into Wednesday night.  The GFS and European models now have basically no precip during this time period.  That is a good thing as the snow levels sit above 8500 feet.  

You can see the 6z GFS for Wednesday with the moisture moving in to our North.

Look at the total precip forecast from yesterday's 18z run of the GFS through Wednesday.

Then look at the 6z run today...

and some of you wonder how the snowfall forecast can go from 12-21 inches along the crest through Wednesday down to 1-3 inches in 24 hours???  That's how...  The forecast models have the AR to our South, right at us, to our North, arriving Monday night, arriving Tuesday afternoon, etc....  We usually don't see swings like this 1-2 days out.  I think maybe the NWS Reno putting out a Winter Storm Watch for Tuesday last night jinxed it because that is when the models starting trending drier.

The European and GFS models are similar in the light precip scenario for the first system, but the Canadian model is about twice as wet still.  Will have to see if it trends towards the other two today.

Thursday:

The good news is that the trend has been wetter for the storm on Thursday into Thursday night, so the total snowfall forecast has not adjusted down much this morning.  The scenario from 2 days ago though is completely different.

Originally it looked like the center of low pressure would move right across Central CA as cold air tried to push in from the North.  So we thought we could see mostly all snow after heavy rain and snow Tuesday and Wednesday.  The forecast models had up to 8 inches of liquid near the crest.  That is why snowfall numbers were so high.  8+ inches of liquid at an average of 10:1 ratio would give you over 80 inches of snow.

Over the last two days the trend has been for the trough to dig more off the coast Thursday and for the low to move inland further and further North.  Yesterday it was far Northern CA, and today it is up into Washington/Oregon.  What that is doing is not only lifting the aim of the AR to our North Wednesday as the trough digs off the coast, but it is also drawing up warmer air ahead of the storm. 

We will have plenty of moisture available being pushed in ahead of the cold front that will be approaching as the trough pushes into the West Coast Thursday night.  Expect heavy precip to move in Thursday morning.  The latest model runs show that the snow levels could quickly drop back near 8000 feet Thursday morning as the heavy precip moves in.  So in the snowfall forecast I have the assumption of all snow above 8000' for this storm.

Here is the updated snowfall forecast for Thursday.

The amounts aren't impressive yet as the snow ratios are low at 8000', I have them at 6:1 with heavy wet snow.  Above 9000 feet on some of the taller mountains the amounts may be more with higher snow ratios.

Thursday Night:

This will be the main event of the week as the cold front combined with the abundant moisture from the AR combine for heavy snowfall and crashing snow levels.  The 6z GFS had snow levels approaching lake level by 10 p.m. Thursday night, then crashing well below lake level by 4 a.m. Friday.  Colder air moves in and snow ratios increase Thursday night.  

My snowfall forecast assumes all snow above 7000 ft. Thursday night and snow to lake level about half of the night.

The latest model runs show up to 1.5 inches of liquid near the crest Thursday and up to 2 inches Thursday night.  The GFS has a total of 3.6 inches of liquid while the European has 2.4 inches.  My snowfall forecasts take the average and adjust the snow ratios every 12 hours.

Friday:

The heaviest precip should move out of the area by Friday morning with only scattered snow showers and colder air moving in.  We could see another inch or two of snow on the mountains.   

Here is a look at the GFS total precip forecast this morning through Friday.  

The Canadian model like I mentioned is wetter Tue-Wed, but it is coming in between the GFS and Euro Thursday with around 3 inches near the crest.  That makes me fell good about the snowfall forecast that is taking the average of the GFS and Euro.  Here is the total precip forecast on the Canadian model through Friday.

Looking at the total snowfall forecast through Friday it is not that much different than yesterday.  Even though we lowered Tuesday-Wednesday, Thursday-Friday increased from 14-20 along the crest to 25-32.  

If we average up and down snow ratios through the week near 10:1 at 8k through Friday, with a model average of 3.5 inches near the crest, then the forecast seems reasonable this morning.  Of course there will probably be some changes again tomorrow morning.  Hopefully not as big as we keep seeing.

The weekend looks cold and quiet as a cold trough is over the West.  Plenty cold to continue snowmaking.

Just and FYI that I'm putting together these forecasts starting at 5 a.m.  I know the 12z runs come out about the time I post but I have already spent 3-4 hours calculating and writing off the 6z runs.  I took a peak at the new 12z GFS just after posting this and it is drier for Thursday more in line with European model with 2.5 inches near the crest in total.  Let's see what happens over the next 24 hours.

Something that was very common the last several year but not so common this year may happen starting Monday.  Yes, a ridge may build near the West Coast bringing continued dry weather and some milder temps.

That ridge may retrograde Westward during next week allowing troughs to dig down the West Coast.  Like I mentioned the last few days, the question is will they dig down just to our East, over us, or just off the coast?  The former two scenarios are drier and the latter would give us a better chance of a storm diving down over the ocean and picking up some moisture.

The ensemble mean forecasts from the GFS, European, and Canadian models are all showing a similar pattern next week with the ridge to start and a trough trying to dig down the coast by Thursday.  Here is the GFS for next Friday.

The operational model runs show the chance for snow with a cold front next Thursday and maybe another storm for Christmas.  That is 10 days away and plenty to deal with this week.

Stay tuned, stay sane...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.