Posted on 10 January 2023 by Guest Author

This is a re-post from Yale Climate Connections by Bob Henson and Jeff Masters. While predicated on accepted scientific findings, this article includes conclusions of the authors and is presented to our readers as an informed perspective.

A series of Pacific storms that’s taken aim on California since late December is on track to continue into mid-January. Ferocious winds will slam much of the state on Wednesday and Thursday, accompanied by heavy rain likely to trigger widespread flash floods. After a brief break, at least two more potent systems are expected over the following week.

The driver of the relentless storminess is an exceptionally strong jet stream that extends across much of the Pacific at unusually low latitudes. Each major impulse along the jet stream has pulled tropical moisture northward and concentrated it into atmospheric rivers — the channels of moisture aloft that bring California some of its most intense rain and flood events.

On Wednesday, an approaching upper-level disturbance triggered a spectacular “bomb cyclone,” defined as a surface low that deepens by at least 24 millibars in 24 hours. With its central pressure dipping below 960 mb, the storm was one of the strongest on record over the North Pacific at latitudes below 40 degrees north. Most of its historical rivals are hurricanes rather than winter-type extratropical cyclones

A sharp warm front spinning around the bomb cyclone was on track to bring an initial surge of rain into California on Wednesday night. On its heels, a strong cold front is predicted to bring even more rain and winds, gusting well over 40 mph inland and up to 50-70 mph in some coastal and mountain areas, likely toppling many trees and power lines. This storm is expected to be the season’s strongest to date in Southern California, including the Los Angeles area, where rainfall could total 2 to 4 inches.

National Weather Service offices across California stressed the manifold dangers of the Wednesday/Thursday storm.  The NWS/Los Angeles office warned: “Wind damage to trees and large objects is likely. Many power outages are nearly certain and could be prolonged by the concurrent heavy rain.”

Figure 1. Multiple atmospheric rivers (ARs) have affected or will affect California from December 30, 2022, through at least January 11, 2023. The most intense atmospheric river for the San Francisco area (large orange dot in left image) is predicted to hit January 9-10. On the 1-5 atmospheric river scale, this is predicted to be at Level 4 (orange) in the Bay Area, with the January 4-5 atmospheric river predicted to be Level 3 (yellow) and the January 7-8 atmospheric river predicted to be at Level 2 (green). (Image credit: Center for Western Weather and Water Extremes)

As if this system wasn’t enough, forecast models have been consistent in predicting a somewhat weaker atmospheric river over the weekend, followed by another potential powerhouse next week (see Figure 1 above).

NWS/Sacramento noted: “The cumulative effect of the repeated rounds of moderate to heavy rain will lead to the potential for more widespread flooding with increasingly severe impacts.”

Some individual runs of the GFS and European models have spit out mind-boggling precipitation totals. Ensembles of these models, which are typically more reliable than any single run, haven’t been quite so hyperactive, but together they point toward an extended period of high-impact weather for much of California.

The Sierra Nevada could easily rack up three to five feet of snowfall over the next week, with even larger amounts atop mountain passes.

Figure 2. Precipitation forecast for the seven-day period starting at 8 a.m. EST Wednesday, January 4. (Image credit: NOAA/NWS/WPC)

Sierra snow survey hits the jackpot, but will the moisture peter out before spring?

The Sierra Nevada snowpack – which furnishes about 30% of the water used in California – showed a bountiful supply for the date as of Jan. 3. The Phillips Station benchmark site in the central Sierra reported 55.5 inches of snow depth and 17.5 inches of moisture. The moisture content was 177% of the long-term average for the snow season to date.

The bounty was shared well beyond Philips Station, as the statewide average snowpack was at 174% of average for the date.

Figure 3. Water levels in key California reservoirs as of January 3, 2023. (Image credit: California Department of Water Resources)

It’s been tough in recent years for the state to scare up a full winter’s worth of moisture, as California has been in the grip of a devastating 22-year drought that has cost billions of dollars and helped fuel record wildfire seasons

The trend toward “hot droughts” has made these long moisture-free periods even more worrisome, helping extend the wildfire season into months where fire was once uncommon. Thus, the rains of 2023 will bring welcome drought relief and are likely to prevent hundreds of millions of dollars of drought damages. Several reservoirs in the state have already gone above their average water levels for this point in the season (Figure 3), and additional liquid gold will fill the reservoirs in the coming weeks.

The ongoing series of storms will surely push January’s precipitation toward average for the month, and perhaps well beyond. But will the faucet keep running? In its Jan. 3 statement, the California Department of Natural Resources, which carries out the snow survey, noted: “This January’s results are similar to results in 2013 and 2022 when the January 1 snowpack was at or above average conditions, only for dry weather to set in and lead to drought conditions by the end of the water year (September 30).”

Unfortunately, the rains will bring costly flooding likely to cause hundreds of millions of dollars in damage. This whipsaw from damaging drought to damaging flooding is a particularly striking example of the exacerbation of precipitation extremes that a warming climate is likely to continue producing in an area naturally prone to weather whiplash, as documented by Daniel Swain and coauthors in a 2018 paper in Nature Climate Change, “Increasing precipitation volatility in twenty-first century California.”