[pct-l] Sierra Snow Predictions

[Bighummel@aol.com]

In 1977 I knew, as I was planning my hike in the winter prior, that the 
snowfall in the Sierras was way below normal.  I didn't have the benefit of 
the internet data, but knew from empirical knowledge that if the winter snow 
pack was light then the spring meltoff was likely to be early and so getting 
through the Sierras could be accomplished earlier than normal.   Therefor, I 
planned to start around April 1.

Letting my scientific, anal, curiosity go wild, I wondered if the historical 
snow records for the Sierras would show any winter season correlation to 
later spring conditions.  So, I downloaded historical data from the Bighorn 
Plateau sensor for 1972 to 1999 and compared data from late February 
(normally the wettest month) to data from late April / early May (normally 
the last monthly records kept in the historical data, i.e., late May / early 
June data isn't available).  

I chose the Bighorn Plateau sensor because it is at a high elevation 
(11,350') and near Forrester Pass, the primary obstacle of concern.  I was 
looking to see if the snow was significantly above or below normal in Feb/Mar 
does that correlate to heavy or light conditions in April/May?
I used the snow depth numbers, instead of the water content, as that is what 
us hikers are more concerned with.  I was pleased to find that my hunch in 
1977 actually has some logic to it and historical record.  The data falls 
into three general categories: 1)"light" years, 2) "normal"
years and 3) "heavy" years.   Also where I found a significant amount more 
snow in April/May than in Feb/Mar, I call them "late" years and a 
significantly smaller amount in April/May than in Feb/Mar, I call them "early 
meltoff" years.

Here is what I found as some rather general, highly unreliable, rules of 
thumb that you can count on (ha!):

1) "Light" Years:  If there is significantly less than average snow (I chose 
less than 40") in Feb/Mar then expect around 14 inches of snow at April/May 
except when there is a "late" storm, then expect around 26 inches.  6 of 9 
"early meltoffs" occurred in "light' years.

2) "Heavy" Years: If there is significantly more than average snow (I chose 
more than 75") in Feb/Mar then expect around 84 inches (!) of snow at 
April/May.

3) "Normal" Years: If there is a rather normal snow accumulation in Feb/Mar 
(I chose 40-75 inches) then expect a "normal" accumulation in April/May of 
around 51 inches.

4) "Late" Years occur in light, normal and heavy years and don't seem to show 
any correlation.  I counted about 8 or 9 years out of the 28 looked at that 
were "late" accumulations.

Note: Late storms can and do hit after May 1 in the Sierras.  This data does 
not include information on the frequency of these.

Now, what do you want to do if you see one of these conditions?  

In a "Light" year you can leave early (I'm not going to define "early" here 
as it will just start an argument that wont benefit anyone, define "early" in 
your own terms) and know that you are likely to get an early meltoff and find 
low amounts of snow and ice at high elevations in the Sierras by the time you 
get there.  Of the 13 early years, only two had significantly more than 40 
inches in April/May.

In a "Heavy" year plan to leave later and hike a schedule more like Jardine 
recommends or consider hiking North to South.  

In a "Normal" year start at a normal start date, gauge the Sierra snow pack 
as you approach and try to hit it right or take your chances.

"Late" and "early meltoff" years will not be apparent until you are just 
about ready to start so watch the snow survey and weather reports in March 
and April for indications.

I will try to continue this analysis to see if there are correlations between 
this data and northern California and the Pacific Northwest.  That should be 
interesting.  Is there a correlation between light years in the Sierra and 
heavy years in the Pacific NW, and vice versa?

By the way, I lucked out.  1977 was the lowest amount of snow in April/May in 
the 28 years.  So when I say you should leave on April 1, don't listen to me.

IMHO and anal contemplation, 

Greg "Strider" Hummel
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