Chance Rather than Trends in the Unusual 2017 California Wet Season

Tuesday, December 11, 2018 - 08:00
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The period between November 1, 2016 and April, 30 2017 ended a severe, multiyear drought across a large part of the western U.S. During this period (hereafter the `2017 wet season'), a dipolar stationary wave pattern in the upper atmosphere reversed and directed storms toward--rather than away from--California in particular. While this sudden, strong hydrometeorological reversal benefited the state in terms of ending the drought, the severity of the season caused numerous infrastructural issues across the state, including a rapid increase in the number of potholes, flooding, and overspill from the Lake Oroville dam. The Lake Oroville event in particular caused damage to the main spillway and massive erosion to the emergency spillway, which had never been used in the dam's lifetime. The spillway event was clearly unexpected by the dam operators, which might lead a person to reasonably assume that the event was associated with unusual weather in the 2017 wet season.

In this presentation, we explore what aspects of the hydrometorology in the 2017 wet season might have contributed to this event and whether there might be any discernible secular trends contributing to an increased likelihood of such events. We employ an event-focused analysis of precipitation in observations to examine the statistics of precipitation events in terms of their intensity, duration, and frequency. We focus primarily on events affecting the Feather River Watershed, since this is the watershed that drains in to Lake Oroville. The primary goal of this study is to understand the relative importance of chance versus trends on the unusual 2017 wet season. Our analysis shows suggests it is plausible that the 2017 wet season could have happened purely by chance, though there is also an indication a long-term increase in the temporal clustering of weather events may be increasing the probability of such unusual wet winters.

 

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