After five years dominated by a grinding, near-statewide drought, it’s a bit disorienting to find ourselves looking at a dramatically wet pattern enveloping much of California as 2017 gets under way. One major slug of heavy rain and mountain snow moved through the heart of the state from Tuesday into Wednesday. As of 4 am PST (12Z) Wednesday morning, central California had racked up some impressive 24-hour rainfall amounts
, including widespread 3” - 5” amounts and a few 5” - 7” totals. In Monterey County, Anderson Peak was inundated with 7.25”. Large swaths of the mountainous West were under winter storm warnings on Wednesday, from northern California and southern Oregon to central Colorado, with 2 to 4 additional feet of snow possible
above 7000-8000 feet in the Sierra (where an avalanche warning was in effect
). Winter weather advisories for lighter snow have been hoisted along the storm’s projected track through Kansas City and St. Louis.
An even more intense round of heavy precipitation is on track to strike California this weekend, setting the stage for a potential major flood threat in some areas. Already, on Wednesday morning, a flash flood warning was posted
for the California coast near Big Sur in the steep mountains where the Soberanes Fire
burned last summer, and the Big Sur River
was already at moderate stage.Figure 1.
Parts of the Sierra Nevada may rack up more than 20” in precipitation (including rain and the moisture within snow) over the seven-day period from 00Z January 4 to 11 (4:00 pm PST Jan. 3 to 10), based on this projection from the 0Z Wednesday run of the GFS model. Image credit: www.tropicaltidbits.comFirehose of moisture for a thirsty state
Computer guidance, including multiple runs of the ECMWF and GFS models, are in strong agreement that a powerful, long-duration atmospheric river (AR) will set up across a southwest-to-northeast swath of California over the weekend, although there are differences on timing and location from run to run and model to model. See my post from last August for background on ARs
, which deliver beneficial rains and snows to California and other areas but can also lead to rampaging floods. This weekend’s AR is projected to pull moisture from the midlatitudes and subtropics and channel it into a relatively narrow swath that will push directly into coastal mountains and on into the Sierra Nevada. The heaviest coastal rains this weekend will probably strike somewhere between Santa Barbara and the north CA coast, although it’s too soon to nail down the exact location of the AR, as noted on Tuesday evening by Daniel Swain
(California Weather Blog).
“Since this system is expected to be slow moving, the associated atmospheric river may stall over some portion of northern or central California on Sunday or Monday--or even waver back northward temporarily,” Swain wrote. “If and when this occurs (as has been suggested by recent runs of both the ECMWF and GFS), there may be a 100-200 mile wide band of even higher precipitation totals. It’s impossible to say at this time where any stalling or frontal waves might occur, but that has the potential to be a serious situation locally.”
Some model runs have been generating mind-boggling amounts of snowfall in the Sierra over the next few days, on the order of 8 to 10 feet (and much more in some cases). There’s no question that enormous amounts of moisture are likely to fall over these mountains, and the very highest elevations will be slammed with massive amounts of snow. However, any model projection of snowfall needs to be taken with several grains of salt, as the actual on-the-ground snow totals will hinge on the precise location of the AR, how much moisture it hauls inland, and how much warmth accompanies that moisture. The lowest snow levels in the Sierra could jump from as low as 2000 feet on Friday to as high as 10,000 feet by Sunday, according to Swain.Figure 2.
Very large amounts of integrated water vapor (IVT) will be pushed by an atmospheric river into the heart of California on Sunday, January 8, based on calculations from the 00Z Wednesday GFS model run. IVT measures the amount of water vapor, in kilograms, carried per second in the air flowing above and across an imaginary meter-wide threshold that is oriented perpendicular to the atmospheric river. Arrow lengths denote the intensity of the moisture transport. Image credit: NOAA/ESRL Physical Sciences Division
Jared Gaston, 25, hesitated before jumping flooded Fairview Street to meet a friend during rains on Thursday, Dec. 22, 2016, in Garden Grove, California. Image credit: Ken Steinhardt/The Orange County Register via AP.A water year that’s already in the black moisture-wise (but not snow-wise)
The Pacific jet stream took aim at the U.S. West Coast for much of the autumn and early winter, which led to the wettest fall on record in Washington
and one of the 20 wettest for both Oregon and California. Now the jet has shifted bodily into California, as is typical in midwinter, carrying rich slugs of deep Pacific moisture into the state. Compared to the paltry moisture totals racked up during most of this decade, California has already seen generous amounts for the 2016-17 water year, which began on October 1.
What hasn’t changed much since the multiyear drought began in 2011 is the year-to-year prevalence of milder-than-average temperatures over most of the state, including the Sierra Nevada. The extreme heat of recent years has intensified the impact of drought in concrete ways--by baking the already-dry landscape during midsummer, for example, and by eroding the Sierra’s winter snowpack prematurely. But unusual warmth can also torque the hydrologic system during wet periods, such as the water year now under way.Figure 4.
Frank Gehrke, chief of the California Cooperative Snow Surveys Program for the Department of Water Resources, crosses a snow-covered meadow as he conducts the first snow survey of the season at Phillips Station Tuesday, Jan. 3, 2017, near Echo Summit, CA. Although the scene looks wintry enough, the survey showed the snowpack was only at 53% of normal for this site at this time of year. Image credit: AP Photo/Rich Pedroncelli.Figure 5.
The amount of water held in snowpack across California’s three Sierra regions was only about a quarter of its way to the typical April 1 total as of January 3, 2016. Collectively, the three regions had about 70% of the amount of snow water content observed on a typical January 3. Image credit: California Department of Water Resources
.Through January 2
, South Lake Tahoe had received 162% of its average precipitation for the water year to date (including rain and melted snow). However, as of January 3, the three Sierra regions held only about 70% of the typical amount of moisture found within snowpack (snow water equivalent) for the time of year, and only 53% of normal at the traditional measuring site of Phillips Station, as reported in the season’s first official monthly snow survey
. This discrepancy is due to mild Pacific storms with high snow levels as well as to extensive melting in between storms, especially during a fairly dry November. While midwinter snowmelt does help replenish reservoirs and aquifers at lower elevation, it’s also important that a hefty amount of snow remain in the Sierra till at least early spring, in order to help protect the region’s ecosystems from early-summer heat and to sustain a healthy flow through rivers and creeks as far into the hot season as possible.
If the air in this weekend’s atmospheric river is as warm and moist as models suggest, the Sierra could end up with an even bigger gap between how much moisture falls and how much snowpack remains. Even so, water is water, and a series of big storms like this could make a major contribution to quenching California’s fierce multiyear drought. Let’s hope that residents and visitors take this weekend’s potential flood threat seriously--as weird as it is to be on guard against water’s power after craving moisture for so long.
Jeff Masters will be posting his roundup of the top ten weather and climate events of 2016 later today. I’ll be back on Friday with an update on the impending atmospheric river in California, as well as the potential for a significant weekend snowstorm across parts of the U.S. South.