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Dangerous, Multipronged Storm Strafing Central/Northern California

By: Bob Henson 5:57 PM GMT on February 20, 2017

What may become known as the President’s Day Storm of 2017 barreled into the San Francisco Bay area on Monday morning. The core of the storm was an atmospheric river roughly 75 to 100 miles wide, pointing a firehose of moisture toward vulnerable foothills and urban areas. With soils already sodden, the risk of flash flooding was high, and the tail end of the storm promised to bring ferocious winds into the area that could knock out power for many thousands of residents, in some cases for prolonged periods. Toward the Sierra, the storm will deliver yet another onslaught of torrential rain and several feet of mountain snow, as the region’s water infrastructure groans under its fiercest assault in a number of years.

Figure 1. An atmospheric river of moisture extended from Hawaii to the San Francisco Bay area in California at 3 am PST Monday, February 20, 2017, as seen in this satellite-derived measurement of total precipitable water (TPW)--the total amount of water that would fall on the ground if one were to condense out all of the water vapor in the atmosphere. Image credit: University of Wisconsin SSEC.

Heavy rain: Location is everything
Monday’s atmospheric river (AR) extends back in a fetch almost directly from Hawaii, as shown in Figure 1, making it a classic “Pineapple Express.” The AR was carrying at least 1.25” to 1.5” of precipitable water, or the amount of moisture in the column of air above a particular spot. That’s an amount seen on average once every 5 to 10 years in the area, noted the Sacramento office of the National Weather Service. Unlike many big storms, this one won’t arrive with a strong surface low; instead, a relatively weak low will be moving into Oregon on Tuesday, well north of the fairly linear AR.

Oriented from west-southwest to east-northeast, the AR will be translating north and south across the southern Bay Area on Monday. The exact timing and duration of those north-south shifts are difficult to predict, but they will largely control where the heaviest rains fall. Since Sunday, models have tamped down the peak storm totals somewhat, and pushed the most likely location of the heaviest rains southward into the South Bay region, roughly from San Mateo to Santa Cruz. Localized amounts could be enormous--potentially 10” or more on the west side of the Santa Cruz Mountains.

The cities of San Francisco, Oakland, and Sacramento will see moderate to heavy rain throughout Monday, most likely totaling 2” to 3”. These amounts could be lower or higher depending on how much time the AR spends on the northern edge of its north-south range. Pockets of urban flash flooding are a given, but the bigger impact in these cities could be the high winds expected to rip through the area later on Monday, toward the tail end of the storm (see Figure 3). With the soils so wet, these winds could easily knock large trees onto power lines. If these outages are especially widespread, it may take days for repairs to be completed.

Figure 2. Precipitation forecast from the 12Z Monday run of the 3-km NAM model for the 24-hour period from 4 am PST Monday to 4 am PST Tuesday, February 20-21, 2017. Image credit: tropicaltidbits.com.

Figure 3. Potential top wind gusts predicted for Monday night, February 20, 2017. Widespread, extended power outages are possible. Image credit: NWS/Sacramento.

Oroville and other dams and levees
Precipitation totals of 5” or more are also possible across parts of the Feather River watershed feeding into Lake Oroville. This is bound to push lake levels toward a new spike even as water continues pouring from the lake’s damaged main spillway. Fortunately, the lake level is down some 50 feet from its overflow state last Sunday, February 12, when the emergency spillway at Oroville Dam showed signs of failure that prompted the evacuation of more than 180,000 people. By Sunday, the amount of water in the reservoir had dropped to 79% of capacity. This greatly reduces the odds that the impending storm will bring the lake back near capacity. Meanwhile, repair work has proceeded along the emergency spillway (see embedded video at bottom).

Other levees and dams across central and northern California will need to be watched over the next few days, as new trouble spots could easily emerge. Overall, the region’s saturated soils will be raising the flood risk to very concerning levels. “We may see flooding in locations which haven`t been impacted in many years. We are strongly advising all residents in interior Northern California to be prepared for flooding,” warned the National Weather Service’s Sacramento office in a flood warning for urban areas and small streams that covers 24 counties through 4 pm PST Thursday. More wet weather is possible toward the end of the week and into early next week, although these storms look much less potent than Monday’s.

Climate change and atmospheric rivers
California’s patterns of rain and drought are notoriously variable, and climate change promises no help in alleviating this whiplash. Recent work has shown that warming temperatures associated with human-produced climate change are worsening the impacts of drought when it occurs in California. Climate change may also intensify the atmospheric rivers (ARs) that bring the U.S. West Coast its most intense multiday rains.

In many parts of the world, the most extreme rainfall events are getting heavier, as greenhouse-warmed oceans pump more moisture into the atmosphere. Up to now, this effect has been difficult to detect in the Southwest, and especially in California, when juxtaposed against the high natural variability of the state’s hydrological regime (evident in Figure 4 below). The 2014 U.S. National Assessment showed that the amount of precipitation falling from 1958 to 2012 on the wettest 1% of days climbed by 71% in the Northeast and New England, and by 37% in the Midwest, but by only 5% in the assessment’s Southwest region, which includes California.

Figure 4. Averages of contributions to water-year total precipitation on Pineapple Express days, plus the following day of each event, at 202 cooperative weather stations in central and northern California, 1951–2008. The graph shows that Pineapple Express events contribute as little as 0 or as much as 54% (0.54) of water-year precipitation for any given year. Image credit: Michael D. Dettinger, Fred Martin Ralph, Tapash Das, Paul J. Neiman and Daniel R. Cayan, “Atmospheric Rivers, Floods and the Water Resources of California,” Water 2011, v. 3, pp. 445-478.

There remains a good deal of uncertainty over whether total precipitation will tend to increase or decrease over California and the Southwest as the century unfolds. It’s entirely possible that intense periods of multiyear drought--made more destructive by higher temperatures--will alternate with some very wet winters. Regardless of whether average precipitation goes up or down, there is evidence that the strongest ARs events will dump more rain and exert more of an impact over time. The 2014 USNA noted: “An increase in winter flood hazard risk in rivers is projected due to increases in flows of atmospheric moisture into California’s coastal ranges and the Sierra Nevada.” A 2015 study in the Journal of Hydrometeorology shows that West Coast rainfall on the most extreme AR days could increase by as much as 39% by the end of this century, mainly because of more water vapor within the ARs.

A plausible worst-case scenario: 1861-62
Even without any help from our changing climate, we know that California can already get intense multiday rain spells and mammoth multi-week totals unlike anything in living memory. As we noted in Sunday’s post, most of the top 1-, 2-, and 3-day rainfall totals in the cities of central California occurred in several landmark events before 1900. The great concern among water managers and hydrologic scientists is the recurrence of a winter like the one of 1861-62, which produced nearly 30” of rain in San Francisco over a month’s time and more than 100” in the town of Sonora in just two months.

A major study called ARkStorm evaluated the potentially catastrophic effects if a winter similar to 1861-62 were to occur today. The study found that direct damages and losses to economic activity would total a mind-boggling $725 billion. Paleoclimate data indicate that we could expect such an event about once every 300 years, even before taking climate change into account. An ultra-mega-flood is indeed the other “big one” that Californians must take seriously. For more on ARkStorm, see these posts from Jeff Masters and Christopher Burt.

We’ll be back on Tuesday afternoon with a post reviewing the impacts of the southern California storm on Friday as well as the currently unfolding storm over central and northern California.

Bob Henson

Figure 5. Downtown Sacramento at the height of the flood in January 1862. Sacramento recorded 23.68” of rain during the two-month period of December-January 1861-62, compared to its annual average of 19.87”. Massive runoff from the mountains during the warm storms filled the Sacramento and San Joaquin Valleys almost from the foothills of the Sierra on the east to the hills on the west side of the Great Valley, and a giant lake, estimated to be 250-300 miles long and 20 miles wide, covered the present-day locations of cities and farmland across much of the San Joaquin Valley. Image credit: Bancroft Library Collection, University of California, Berkeley.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.