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

April 10, 2016

Contribution from Dr. M.D. Dettinger, USGS

March is over (and the precipitation totals are in), so here is the update to the historical odds of getting to normal (and other fractions of normal) precipitation this year. Notice that the odds from the March 1 situation (in parens; click here for March report) are included alongside the April 1 odds, so that you can see whether March wetness made much difference. In fact, the odds haven’t changed too much in most areas because, although March was wettish (except down south), those additions were balanced in these April odds by the fact that we have almost run out of time this year to get much more precipitation.

Bottom lines (both of which you probably already know): We’re not going to fill the precipitation deficits we accumulated over the past 5 years with this year’s precipitation (except, long-odds, maybe in eastern NV). In southern California, the precipitation situation is especially grim and even the odds of making it to 75% of normal this year are well less than even.


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)