Odds of Reaching 100% of Normal Precipitation for Water Year 2016 in California (March update)

March 10, 2016

Contribution from Dr. M.D. Dettinger, USGS

The Febraury 2016 precipitation observations are now in, and have allowed for an update to the calculation of the odds of reaching 100% of normal for the water year across three key climate divisions of California and Nevada. These odds have decreased slightly across all of California and Nevada as a result of a very dry February. The odds of reaching 100% of normal Water Year precipitation in the key northern California climate division that encompasses the Sacramento River, and the State’s largest water supply reservoirs, decreased from 52% as of the end of January 2016, to 38% as of the end of February 2016. A series of Atmospheric Rivers have made landfall over Northern California and produced heavy precipitation throughout early March and will most likely cause changes in these odds when updated at the end of March.

How these probabilities were estimated:

At the end of a given month, if we know how much precipitation has fallen to date (in the water year), the amount of precipitation that will be required to close out the water year (on Sept 30) with a water-year total equal to the long-term normal is just that normal amount minus the amount received to date. Thus the odds of reaching normal by the end of the water year are just the odds of precipitation during the remaining of the year equaling or exceeding that remaining amount.

To arrive at the probabilities shown, the precipitation totals for the remaining months of the water year were tabulated (for all years, for only El Ninos, or for only La Ninas) and the number of years in which that precipitation total equaled or exceeded the amount still needed to reach normal were counted. The fraction of years that at least reached that threshold is the probability estimate. This simple calculation was performed for a full range of possible starting months (from November thru September) and for a wide range of initial (year-to-date) precipitation conditions. The calculation was also made for the probabilities of reaching 75% of normal by end of water year, 125%, and 150%, to ensure that the resulting tables of probabilities cover almost the full range of situations that will come up in the future. Contact mddettin@usgs.gov for more information.

[One key simplifying assumption goes into estimating the probabilities this way: The assumption that the amount of precipitation that will fall in the remainder of a water year does not depend on the amount that has already fallen in that water year to date. This assumption was tested for each month of the year by correlating historical year-to-date amounts with the remainder-of-the-year amounts, and the resulting correlations were never statistically significantly different from zero, except possibly when the beginning month is March, for which there is a small positive correlation between Oct-Mar and Apr-Sept precipitation historically.]

Contact: Michael Dettinger (USGS)