After several years of deep drought over much of California, last year’s deluge produced some relief and recovery for the state’s strained water supplies. November 2017 was much wetter than normal, at least across northern portions of California, inspiring hope that the state’s water resources would be able to continue their recovery. However, this moisture failed to reach southern portions of the state and near-record dry conditions dominated the state in December, enabling some of the most intense and destructive wildfires in state history to sweep across portions of the state, including the catastrophic Thomas Fire. December registered as the second-driest statewide, and February ranked 3rd driest in history, portending another potentially disappointing wet season for the state (Figure 1). A near-average January in between acted to prevent drought of 2014 proportions, but by early March, the state was facing the prospect of yet another top-10 driest winter.

Dec 2017 and Feb 2018 statewide precipitation rankings.

Figure 1. Statewide precipitation rankings for the months of December 2017 (left) and February 2018 (right), indicating the 2nd and 3rd driest Dec. and Feb., respectively, for California. Images courtesy of https://www.ncdc.noaa.gov/climate-information/analyses/monthly-us-climate-reports.

One popular metric for assessing California precipitation is the Northern Sierra 8-station Index (Figure 2), which shows the average precipitation across 8 mountain sites in the northern portion of the state. The current year, along with some other historical high and low annual precipitation benchmarks, are plotted. Around March 1st, precipitation accumulation was just over half of average, portending a potentially poor outcome for the water year. However, since early March, large upward jumps have occurred as a sequence of atmospheric river events have impacted the state, allowing precipitation to reach 84% of normal as of April 11th.

north sierra 8stn ppt index

Figure 2. The Northern Sierra 8-Station Precipitation Index, which offers a simple metric of accumulated precipitation for the water year in northern and central California. Image courtesy of https://cdec.water.ca.gov/cgi-progs/products/PLOT_ESI.pdf.

As the large jumps in the precipitation accumulation plot suggest, March precipitation was essentially driven by three major events: one near the beginning of the month, one near the middle, and a final event towards the later days of the month. Another jump occurred in early April associated with one of the most moisture-laden airmasses on record (see this blog post and embedded Tweet for details). We used our state-of-the-science storm analysis tool, MetStorm, to take a look at the two most recent major events.

The first of these events was the last in the sequence of March storms, which had mixed results for Sierra Mountain snowpack. The mid-March storm was relatively cold, and cranked out prodigious snowfall totals all throughout the Sierra. The second storm, however, was notably warmer, bringing snow levels above 10,000′ for many locations. While this wasn’t good news for the state’s already low snowpack, the heavy rains did at least make a significant contribution towards refilling the state’s reservoirs, which are now mostly above historical averages and approaching storage capacity (Figure 3). One notable exception to this is Lake Oroville, which is being deliberately held at a lower stage while repairs are made to the dam’s spillway following last year’s near brush with catastrophe.

CA reservoir levels

Figure 3. California reservoir levels compared to their capacity (blue) and historical average (red), current as of 12 April 2018. Image courtesy of https://cdec.water.ca.gov/cgi-progs/products/rescond.pdf.

The impressive rainfall totals from this final March storm (20th to 23rd) are shown below in Figure 4a. Our preliminary MetStorm analysis shows that virtually the entire Sierra range received around 4″ or more of precipitation over the 3-day period, with high-elevation locations frequently exceeding 7″. One isolated report even suggests that totals in excess of 20″ may have been observed! While larger Sierra storms have certainly been observed before, the wet season is often winding down for California by late March, and the storm was therefore somewhat unusual for its timing.

MetStorm ppt 2panel

Figure 4a (left): MetStorm preliminary storm total precipitation for March 20-23, 2018. Figure 4b (right): MetStorm preliminary storm total precipitation for April 5-8, 2018.

Even more anomalous was last week’s storm, which produced up to 12″ in the Northern California mountains in the 3-day period April 5-8, 2018 (Figure 4b). The atmospheric river that was responsible carried the highest moisture content on record for any November-April storm hitting the California coast. Somewhat fortunately, as explained by Bob Henson and Jeff Masters at their Wunderblog, the storm lacked the powerful dynamics to produce even heavier precipitation that may have accompanied a similar storm in, say, January or February, so the worst of the flooding and mudslide potential was avoided. The storm still produced widespread rainfall of 4-8″ in the coastal ranges of Northern California and the Sierra near and north of Tahoe, which are impressive totals for April. While overall these storms were not exceptionally rare for the west coast, they did produce rainfall of notable rarity in some locations (Figure 5). Both storms produced maximum 24-hour recurrence interval values in excess of 1000 years at isolated locations (note that these are preliminary, and quality-control may find these reports in error), with scattered pockets of 5-50 year recurrence intervals, indicative of how intense these late-season rains were for many.

CA storms ARIs

Figure 5. Maximum 24-hour Average Recurrence Interval (ARI) values in years for March 20-23, 2018 (left) and April 5-8, 2018 (right).

While some minor flooding occurred in association with these recent events, by and large they provided a welcome boost to the state’s water supplies. California now looks to be in good shape for the upcoming growing season, and a relapse into significant drought appears to have been avoided for this year.

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