Author: cw3e-web

Odds of Reaching 100% of Normal Precipitation for Water Year 2017 (March Update)

March 8, 2017

Contribution from Dr. M.D. Dettinger, USGS

The odds shown here are the odds of precipitation in the rest of the water year (after February 2017) totaling a large enough amount to bring the water-year total to equal or exceed the percentage of normal listed. “All Yrs” odds based on monthly divisional precipitation totals from water year 1896-2015. Numbers in parenthesis are the corresponding odds if precipitation through February had been precisely normal (1981-2010 baseline).

Click here for a pdf file of this information.

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 in the long-term historical record 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.

[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)

Current Winter Setting a New California-Wide Record Precipitation Accumulation

March 7, 2017

Fueled by a string of strong atmospheric rivers (ARs), California’s current winter-to-date accumulated precipitation has hit a new record high level, eclipsing the previous record set during the strong El Niño winter of 1982-83.

The winter began with an unusual early season AR, which contributed 6% of normal annual California-wide precipitation over the period Oct 14-17. Strong AR activity continued in Jan and Feb 2017, with exceptionally strong precipitation Jan 8-10, which produced 14% of normal statewide annual precipitation in just three days and reached R-cat 4 intensity. (R-cat levels measure intense precipitation events; a fuller description of R-cat levels and this event can be found here). The AR during Feb 7-9 produced 9.5% of total annual California precipitation. Together, the latter two AR events produced nearly a quarter of an entire normal year’s precipitation in just 6 days, with each event including extreme intensity AR landfalls in the state.

The figure below shows the water year (Oct 1st – the following Sep 30th) that holds the record for maximum precipitation in California accumulated since the beginning of October for each day of winter. The current water year, 2017, broke the old record in early February and has continued to be the record-holder up to the current date (first week of March). Currently, 1982-82 holds the record for the maximum state-wide accumulated precipitation at the end of May in observations that go back to 1948. The accumulation so far this year is above the pace of 1982-83, but 1982-83 received a significant amount of precipitation in March and early May.

This figure shows California statewide accumulated precipitation estimated from 96 stations distributed across the state, but similar results are seen in the “Eight Station Index”, which uses eight stations in the Sierra Nevada selected for their importance to the state’s water supply. The eight station index is likewise currently at new record levels of accumulated winter precipitation, superseding the previous record-holding winter of 1982-83.

The southern portion of the state, including the greater Los Angeles region and San Diego county, are unusually wet so far this winter but not at record breaking levels. For instance, the Los Angeles region received substantially more precipitation in 2005, which led to widespread flooding, infrastructure damage, and several deaths.

The record-breaking precipitation has led to high values of snow cover, as shown by the yellow line (winter of 2016-2017) below. In the central and southern Sierra Nevada, current values are almost twice what is seen at the typical peak of snow accumulation on April 1st, and significantly above the high values seen during the El Niño winter of 1997-98 (dashed blue line). Snow is an important component of California’s water supply, since it holds the precipitation from intense winter storms, releasing the water more slowly via snow melt.

Contact: David Pierce and Marty Ralph