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Brian Crow

Heavy Precipitation in the Northern Sierras Leads to Oroville Dam Crisis

By | Extreme General Storm Precipitation | No Comments

A dominant story in news headlines over the past few days has been the crisis at California’s Lake Oroville dam, the tallest earthen dam in the United States. Buoyed by weeks of heavy precipitation throughout much of central and northern California, the reservoir for the first time in its 49-year history exceeded its capacity and began to overflow into the emergency spillway (Figures 1 & 2). The temporary evacuation of some 200,000 people occurred on Sunday when engineers became alarmed by the rate of erosion at the lip of the emergency spillway. This came just days after standard releases from the dam began to cause serious erosion of the main spillway, necessitating the reduction of flows while engineers inspected the damage. While the erosion of the main spillway has apparently stabilized as of this writing, the emergency spillway remains potentially unstable if needed again before proper repairs can be made.

Figure 1. An aerial view of the severely eroded main spillway of Lake Oroville on Saturday, 11 Feb 2017. Water can also be seen running over the emergency spillway above and to the left of the main spillway. Image credit: Wikimedia Commons.

Figure 2. Lake Oroville water storage levels in 2016-2017 as compared to other historically wet and dry years. Image source: CA Dept. of Water Resources.

As indicated by the water storage graph in Figure 2, water levels have spiked sharply multiple times already this water year (water years run from October to September). These spikes are the result of a series of powerful low pressure systems that have produced copious amounts of precipitation over the Lake Oroville catchment area, particularly in the mountains. Since mid-December, there have been at least four major storm systems that produced widespread storm-total liquid precipitation totals in excess of 10″ (Figure 3). Above 6,000-8,000 feet in elevation, much of this moisture remains locked up as snow and will only gradually contribute to the flow of water into Lake Oroville. However, as evidenced by the time series below, taken at a point at just 3,300′ elevation, the extremely heavy precipitation has also occurred at lower, warmer altitudes, where almost all of it becomes immediate runoff.

Figure 3. A time series of analyzed daily precipitation values at a point in the mountains upstream of Lake Oroville. Image source: PRISM Explorer.

Based on this time series, MetStat decided to analyze four recent weeklong periods of precipitation in the Oroville drainage area: 10-16 December 2016, 5-11 January, 19-25 January, and 4-10 February. Gridded precipitation data was acquired from NOAA’s Multi-sensor Precipitation Estimates (MPE) and compared to precipitation frequency grids from NOAA’s Atlas 14 Vol. 6 to produce the Average Recurrence Interval (ARI) product. The ARI shows the approximate rarity of an event expressed in terms of the average amount of time that would be expected to pass between events of identical magnitude. In other words, an ARI value of 200 years would indicate a storm that would only occur once every 200 years on average. The images below show the storm total precipitation for each event on the left and the 7-day ARI on the right.

Note that while no individual storm is incredibly rare individually, the combined effects of a 1-2 year storm (storm 1), a 10-25 year storm (storm 2), a common but heavy storm (storm 3), and another 10-25 year storm (storm 4) all in succession has led to some truly staggering rainfall totals. This is illustrated most starkly in the 45-day precipitation totals (from 28 Dec to 10 Feb) and 45-day ARI analysis, which conveys the rarity of seeing this much precipitation in the course of a month and a half (Figure 5).

Figure 5. 45-day total precipitation (left) and and 45-day average recurrence interval (right) for the Lake Oroville drainage area.

The precipitation during this period was definitely impressive: over 20″ of liquid has fallen on all of the hilly and mountainous terrain in the region, and a substantial area upwards of 60″ of liquid is analyzed in the heart of the mountains! Such huge precipitation values are consistent with average return intervals of anywhere from 10 to 250 years in the Oroville region, underscoring the infrequent nature of such a persistently wet pattern in the Northern Sierras. It is worth reiterating that much of that high-elevation precipitation remains in the form of snow, but this highlights the issue at hand: Lake Oroville will be dealing with enormous amounts of water for months to come. In addition to gradual snowmelt over the next several months, the wet pattern looks to continue for the immediate future. The Weather Prediction Center is forecasting another 4-10″ of precipitation to fall over much of the basin in the 7 days to come (Figure 6; see also the WPC website).

Figure 6. A Google Earth view of the WPC’s 7-day precipitation forecast, valid from 15-21 Feb 2017.

With a near-record snowpack building in the Northern Sierra Mountains and another 6-12 weeks of California’s wet season still to come, the crisis at the Lake Oroville dam will likely be ongoing for weeks. Officials are confident that the spillways can be reinforced enough to survive the season, but another extended stormy period could still be a burden and potential threat to water infrastructure all throughout the Sierra Nevada. This event emphasizes the importance of the hydrometeorological expertise that MetStat provides to help design, engineer, and operate safe water infrastructure.

Data Sources:
HDSC Precipitation Frequency Grids
AHPS Precipitation Analysis

Images:
Aerial view of spillways
Lake Oroville storage
PRISM precipitation time series

Coastal Washington and Oregon Deluge Sets Up Region for Wettest October on Record

By | Extreme General Storm Precipitation | No Comments

Earlier this month (October 2016), parts of the coastal Pacific Northwest faced nearly a week of heavy rains and high winds as a series of storms crashed ashore. An intense Pacific jet stream, amplified by interaction with the remnants of Typhoon Songda, carried copious amounts of moisture into the coastal mountains of Washington and Oregon, with the most intense rainfall occurring October 13th-17th. Forecasts of widespread hurricane-force wind gusts failed to materialize, but localized wind gusts were nonetheless impressive: peak wind gusts were 70 mph near Honey Lake, CA, 89 mph near Incline Village, NV, 94 mph near Megler, WA, and 103 mph near Oceanside, OR. In addition, two rare tornadoes managed to touch down in the coastal Oregon cities of Manzanita and Oceanside, with substantial structure and tree damage reported in Manzanita.

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Noteworthy though the winds may have been, the widespread heavy rainfall was the most impactful aspect of these storms. Our preliminary MetStorm analysis of regional rainfall shows just how widespread the heavy rainfall was, particularly in mountainous areas. Many high-elevation locations recorded in excess of 6″ of rain during this 4-day period, with our analysis indicating a local maximum of up to 20″ in the Olympic Mountains of northwestern Washington.

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Even in lower-lying areas between mountain ranges, including such cities as Portland, OR and Seattle, WA, rainfall was substantial enough to push several areas towards their wettest Octobers on record. The rainy season is usually just ramping up through October in the Northwest, so several-inch lowland rainfall events in October are not particularly common, although they wouldn’t be out of place later in the season. The majority of the area received rainfall consistent with what one would expect to occur at least once a year. However, isolated pockets in the mountains and in the Puget Sound region saw much rarer heavy rains, consistent with recurrence intervals in the 10- to 1000-year range. This is particularly impressive when considering that the heaviest rains in these areas generally tend to occur between November and April.

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MetStorm’s mass curve analysis illustrates how this multi-day event unfolded. The point of heaviest rainfall in our analysis is isolated, and two fields from that location are plotted below: hourly rainfall values throughout the event (shaded), and cumulative rainfall since the beginning of the event. Several distinct low pressure systems, including one associated with the remnants of what had been Super Typhoon Songda in the west Pacific Ocean several days earlier, brought distinct waves of rainfall to the region. While hourly values never exceeded 1″, the persistence of moderate to heavy rain over such a lengthy period added up–to over 20″ in this case.

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Flooding fortunately proved to be minimal with this event, but many areas in the Northwest are now dealing with very saturated ground heading into what forecasters expect could be a wetter-than-average winter. If rainfall events of this magnitude continue to occur this winter, more severe flooding and mudslides are certainly a possibility.

Please note that the maps presented here are preliminary and will be updated when new data become available. If you are interested in this product, or any other product from our MetStorm Precipitation Analysis tool, please email us or send us a message though our contacts page here.

-MetStat Team