Over the next four days, Powdiction
will bring you the inaugural Anti-Seasonal Outlook, Seasonal Outlook: A
Four Part Series. Each day, a different aspect of seasonal outlooks
will be discussed hopefully in a coherent manner. I know you are thinking we
may have gone all Peter Jackson with splitting this up and dragging it out.
However, we hope this will inform our readers about the current state of the
climate (spoiler: not good), what could change it, how it’s forecasted, and the
difficulties in doing so.
The Sierra Nevada mountain range is
in extreme to exceptional drought, which are the two highest categories.
Essentially all of California and Nevada are drought stricken with it being the
worst in recorded history in some areas of California. This drought started way
back in 2011, after we had an epic powder year. Then the drought progressively worsened
each year.
Some areas in the Sierra had above
average precipitation over the summer, but it barely made a dent into the
drought. That is due to the low monthly precipitation averages during the summer
(June-August), which makes it easier to be above normal. For example, Tahoe
City averages 0.30” of precipitation in August, and this August Tahoe City
received 0.89” of precipitation, which is nearly three times the average. Furthermore,
Tahoe City averages 5.55” of precipitation in December. So 300% of average
precipitation in August is only about 16% of normal December precipitation.
Most precipitation in December falls
as snow, which acts as a water reservoir through the spring and into the
summer. Compared to summer thunderstorms, which are usually short, intense
rainfall events, snowmelt is preferable due to the increased duration and the
gradual nature of the water moving into the soil, plants, lakes, rivers etc.
One caveat this year is the rain event we had associated with a tropical
disturbance moving along the Sierra August 4th and 5th
that brought mostly steady rain. Bottom line, we need a good winter to bust
this drought as the below graphic eloquently illustrates.
The Sierra depends heavily on cold
season precipitation. The majority of the precipitation falls November through
March. For example, Tahoe City and Truckee receive more than 70% of their
annual precipitation in these five months. So what’s on tap for this winter?
For the past 29 months, the El Niño-Southern
Oscillation (ENSO) has been neutral. This represents the 3rd longest
ENSO neutral period since 1950. And examining ENSO’s history, each instance
where ENSO has been neutral for more than 13 months, an El Niño
has followed.
Well there has been a forecast of El
Niño
for the past several months. While the Southern Oscillation Index (SOI) has
been borderline El Niño since July, the sea surface temperature (SST) anomaly has
been stuck in neutral. The SST anomaly is much closer to an El Niño
than La Niña currently and has been since the spring.
The reason I am discussing ENSO is
it can be a strong influence on the weather and climate, and for us to bust the
drought; we need a change in the atmosphere. Ridges control the weather and
they tend to establish over areas of drought. That does not bode well for us.
So a change in ENSO or other atmospheric-oceanic circulations needs to occur to
break this rut.
In the subsequent posts, we will
discuss ENSO, some of the other atmospheric-oceanic circulations, seasonal
forecast guidance, the complexity of seasonal forecasting, and how it all
affects the Sierra and Tahoe. Stay tuned for Part 2 of Powdiction’s inaugural Anti-Seasonal
Outlook, Seasonal Outlook: A Four Part Series!
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