Monday, October 20, 2014

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Anti-Seasonal Outlook, Seasonal Outlook: Part I

            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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