Ok, it's time for that post I do once a year where I go through all the data from the season.  Some of you may fall asleep, but for the snow and weather nerds there is a lot to digest here.  I'm a big numbers nerd so this is one of my favorite posts of the year.

The data I am using is not all official.  I am pulling my numbers from snowfall reports from the ski resorts, Truckee reporting stations, and the UC Berkeley's Central Sierra Snow Lab (CSSL).  I am using the official numbers reported by those sites, but then I'm putting my own analysis on the numbers and making my own graphs to try and interpret the numbers or find trends.  That is all my own work and I'm not an official source, so please use this analysis and graphs as just as a fun analysis and not as an official source.

I will start with the April recap first and then get into the season recap.  First let's take a look at the fun numbers which are the snowfall numbers.  Here is the graph using the CSSL snowfall which is measured at 6,883' in Soda Springs, CA.  The red line is the average April snowfall from 1970-2018.

You can see we had a below average snowfall month in April.  That means we only had one above average snowfall month all season, March.  We still have May which averages 10 inches of snow.  We may have picked up an inch on Monday, so we would need at least 9 more inches this month, with no significant storms in the 2 week window.  Here is a look at the current snowpack numbers.

For total precipitation we did better in April with above aver total precipitation thanks to that pineapple express storm where we had heavy rains and 10,000 ft. snow levels.

Below are the ski resort numbers for the season.  For the total precip I take the Oct-Apr average from the CSSL since 1970 and compare that to the total precip reported Oct-Apr this season.  For the snowfall I take the totals reported by each ski resort and average that, and then divide that by the average of the stated seasonal snowfall averages at each ski resort.  That gives us this for the season...

You can see that high snow levels or lack of storms was the story this season.  We saw strong warm storms in November, then hardly any storms Dec-Feb.  Then we had strong cold storms in March and finally strong warm storms in April.  So the total precip numbers for the season are much better than the snowfall numbers at the ski resorts.  These are the upper mountain numbers, so the lower mountains had even less snowfall.

I had a ski resort season forecast of 85-92% of average last fall based on my analog calculator I use.  Some ski resorts like Northstar, Squaw, and Mt. Rose hit 92% of average snowfall.  The North Lake resorts as a whole were at 82% of average snowfall, but the South Lake resorts were only at 67% of average.

Here is a look at the CSSL snowfall chart since 1879 through last season.  The average since 1879 is 406 inches.

Let's take a look at the total snowfall numbers at the CSSL since 1950 including this season so far through April.  

When you use 1950 as the starting point the average snowfall drops to 393 inches.  Using that average we are running at 68% of average snowfall as of the end of April.  That is at 6883', likely around 1000' lower than the average reporting elevation of the ski resorts that ended at 76% of average.

March was definitely a Miracle month in the midst of the below average snowfall season, with much above average snowfall, and the storms were cold so it snowed a lot even down to lake level.  Going into the 3rd week of February we were dipping down to the lowest snowfall season on record below 1977, but thanks to March we shot out of the bottom 10 seasons since 1970.

For the total precipitation we are above average through April at the CSSL with 102% of average YTD since October.  Here is the graph going back to 1970.

Thanks to all the precipitation on top of being above average the last 2 seasons, we are staying out of a drought in the Northern and Central Sierra.

So what about the lower levels?  I have been tracking the Truckee snowfall and temperatures as well.  I was using the WRCC (western regional climate center) data through 2008 which was using the old Truckee ranger station located on the hill behind town.  Since the move the data has not been consistent and I started using the Truckee airport data.  The airport is lower than town which can make it a bit colder for temps, and in more of a rain shadow and more affected by higher snow levels meaning less snowfall on average than in downtown. 

For the 2009 to current numbers used in the data below I made a slight adjustment to the data to match the variance that was running in the overlapping years from 1998-2008 between the airport and ranger station.  That raises the snowfall slightly in the recent years to more closely match what likely fell at the ranger station based on the overlapping years, and warms the temperatures slightly to match more closely as well.  Overall there wasn't a significant difference.

Here is the snowfall graph, showing a slight declining trend in snowfall since 1934.

The official average going back to 1900 is 202 inches, pushing that forward to this graph I created starting in 1934 I have the average as 190 inches.

Here is the temperature graph...

There is a lot that goes into temperature.  Outside of the overall temperatures in the region, there are things that control temperatures around the lake.  Snow on the ground versus no snow can keep daytime temperatures cooler by reflecting the sun, and night temperatures colder by radiational cooling.  Cloudy days can keep us cooler but clear nights colder allowing radiational cooling.  The winds can be calm allowing for inversions which make it colder in the lower elevations, or mix the air erasing inversions.

So having a lot of snowstorms can both bring snow and colder air that make it colder, but storms can also keep us warmer with cloud cover and by erasing inversions.  So the overall trend of temperatures up or down or flat like the graph seems to show, is hard to pinpoint to one thing.  There are so many micro climates in Tahoe with the tall mountains and low valleys that it's hard to come up with trends if they aren't consistent across all areas and elevations.  

However, I have been trying to go through the data and find trends in the snowfall since snowfall is my obsession and I'm always wanting it to snow more, and to understand the snowfall trends.  Again, these are not official graphs I'm showing you, these are graphs I've been working on to try and understand the snowfall trends from the data I have and I'm just sharing my musings with you.

Here is a look at total precipitation and total snowfall at the CSSL since 1970.  I put the total precip next to the total snowfall for each season.  The red and orange lines are the average of each since 1970.

I could hypothesize that on average the total snowfall compared to average should be close to the total snowfall.  But there are a lot variables, especially the number of storms pulling in subtropical moisture.  Some years there is a correlation to above average precip and snowfall, or below average precip and snowfall.  But not all seasons.

In high AR event years like this one we have a lot of warm wet storms created quite a variance between total precip and total snowfall versus the average.  We saw a lot of high elevation storms with more rain than average at 7k, and lower snow:water ratios where there was snow.

In seasons like 2010-11 we saw a lot of cold storms from the Gulf of Alaska causing more snowfall at 7k vs average than total precip vs the average.  Lots of cold storms also brought high snow:water ratios.

Trying to dig a little deeper, I took the % of average for both snowfall and total precip from the CSSL data.  The negative correlation is when there is more total precip as a % of average versus snowfall as a % of average, and a positive number when there is more snowfall as % of average vs total precip as a % of average.  Putting those negatives and postives into a graph here is what that looks like since 1970.

You can see when I insert the trend line we are trending down, which means more seasons where we have total precip as a % of average beating the snowfall as a % of average.  There has a been a big dip the last 6 years.  I mentioned earlier that the average snowfall since 1879 is 406 inches and since 1950 falls to 393 inches, but it falls to 385 inches since 1970, and 359 inches in the last 10 years.  So the drop-off of the average snowfall seems to be picking up.

We have seen a few seasons over 500" the last 10 years, and also some of the lowest snowfall seasons on record, so the extremes from high to low snowfall may have increased a bit the last 10 years as well.  

That is all from the data from the CSSL.  It's a bit harder to do a comparison to all the micro climates around the lake.  I did a similar postive/negative correlation graph for Truckee snowfall at 5900' versus the CSSL snowfall at 6900' to try and see if there is a trend by elevation.  I was going to do a 7k vs 8k graph as well, but I don't have enough consistent data for that elevation outside of remote sensors or the ski resorts.  Not the same as a scientist measuring at the ranger station or the snow lab.

Here is the graph I came up with.  Taking a look at the snow line (blue) first, that line is a positive number when the snowfall is higher as a % of average in Truckee versus up at the snow lab.  In theory with everything running on average, the average snowfall should be close to the same at both elevations versus the averages for those elevations.  The variances up and down from positive to negative is from Truckee running ahead or behind versus average, or in my theory, snow levels running higher or lower than average in a season.

If you look at the trend line for the snowfall it gradually declines meaning snow levels have been trending higher over the last 10 years on average, causing less snowfall versus average at 5.9k in Truckee vs 6.9k at the Snow Lab. 

I then put in the temperature departures from average for each season in Truckee.  One of my hypothesis before adding in temperatures, was that in colder seasons we could have lower than average snow levels and higher snow ratios at the lower elevations.  That would cause higher snowfall vs average in Truckee versus the Snow Lab.  It would work the opposite for warm seasons.  If that were the case every time, everytime the temperatures are below the "0" line the snowfall should be above the line, and vice versa.

You can see that is the case 11 seasons, or 55% of the time.  So there is some correlation but there are so many other variables in snow levels and snow ratios that you can't just look at the average temperature for the season.  Those cold and warm temperatures could have been created by periods not during storms.  I just thought it would be interesting to try it. 

What we can see, is that the snowfall as a % average is trending down at the lower elevations versus the higher elevations, even when the overall average temperatures at the lower elevations don't seem to be trending up at the same rate.  Almost like we are seeing warmer storms coming in with higher snow levels, but between storms the temperatures are sitting close to average.  This is just one comparison between two points in the Tahoe basin.  We could do the same for several other points and come up with different results.  The micro climates are what make forecasting so much fun here.

So there is all the data I have for now if you are still awake, and some of my musings on snowfall trends.  Feel free to put your own graphs or data into the comments.  I'd love to read them.  Let's hope for more of the 500" seasons in our future.

I'm excited to blog on some summer topics soon as well.  I'm planning to post on something at least once a week.  Could be about weather, outdoors, or maybe just a another musing.  Something to hold you over until the snow flies again next season.  We are only 4 months away from what could be the first snow of the 2018-19 season!

Stay tuned...BA