Pacific Storm Parade Returns Wednesday, Will Add to One of California's Wettest Winters in Years

Jon Erdman
Published: February 13, 2017

    

Another series of Pacific storms will plow into the West starting Wednesday, with more rain and mountain snow for California, in an area that has already seen one of its wettest water years on record.

Despite a break in the barrage of flooding rain and heavy snow since Friday in storm-fatigued northern California, concerns over the possible failure of Lake Oroville's emergency spillway prompted evacuations of roughly 200,000 downstream residents Sunday.


Eastern Pacific Satellite and Jet Stream

(MORE: Latest News on the Oroville Dam Spillway Emergency)

Flood warnings persist not only along the affected Feather River in northern California, but also for the Humboldt River in northern Nevada, while overland flooding over the past week has been widespread in parts of southern Idaho.

(INTERACTIVE: Latest Storm Reports, Shelters Open)

The active Pacific jet stream will guide three separate storms into California starting Wednesday.

Some of these storms will have a deeper tap of moisture, or atmospheric river, capable of producing at least a period of heavy rain, mountain snow and elevated snow levels.

(MORE: Atmospheric Rivers: A Blessing and a Curse)

Here is a rough timeline of each storm and potential impact.

  • Wednesday and Thursday: Wettest in Pacific Northwest, northern California; Snow levels moderately high, then falling
  • Friday and Saturday: Heavier rain threat for Southern California, Desert Southwest
  • Sunday through Tuesday: Colder, lower snow levels


Rain, Snow Outlook Next 5 Days

(MAPS: 7-Day Rain/Snow Forecast)

Taken together, much of California and the Pacific Northwest will pick up another 1 to 3 inches of rainfall through the Presidents Day holiday weekend.

Coastal ranges and foothills of the Sierra and Southern California mountains below snow level will pick up much heavier totals, raising the threat of additional flooding and debris flows with each successive storm.

A foot or more of snow is likely above snow level in the Sierra, Siskiyous and parts of the Rockies and Cascades.

(FORECASTS: San Francisco | L.A. | Sacramento)

A Record Wet Season?

A persistently wet pattern earlier this season was hailed as one that could finally put a sizable dent in California's multi-year drought, though deep groundwater supply still needs to be replenished.

Now, it has gone too far and may shatter all-time records in the state.

Since Oct. 1, it's been by far the wettest "water year-to-date" in California's northern Sierra, according to data from the California Department of Water Resources.

Northern Sierra precipitation water year-to-date from Oct. 1 through Feb. 12, 2017 (in blue trace, with latest value circled). Previous water year is shown in brown. Average is in light blue shading. The two wettest water years on record, both during strong El Niños, are shown in green (1982-1983) and purple (1997-1998).

The same can be said in the central Sierra, and the southern Sierra is also close to its record pace. California's "water year" runs from October through September, with the wettest months generally from November through March.

Northern Sierra precipitation has been more than double the average in the 2016-17 wet season, and, more impressively, it's at least 20 inches ahead of the pace of the two previous record wet seasons, 1997-98 and 1982-83.

Both of those standing record wet seasons were during strong El Niños, but 2016-17 featured a weak La Niña, which just recently dissipated.

During the record-tying strong El Niño in 2015-16, the northern Sierra picked up 10 fewer inches of precipitation through the entire water year than they've picked up just through Sunday this water year.

(MORE: El Niño Myths)

The six-month period ending Sunday was the wettest such period on record in Sacramento (24.49 inches) and Pocatello, Idaho (12.81 inches), according to the Southeast Regional Climate Center.

Examining statewide data, it has been the second-wettest water year-to-date on record dating to the late 19th century, according to Paul Iñiguez, science and operations office at the National Weather Service in Phoenix. Only the 1968-69 water year started out wetter.

According to the graph above, the northern Sierra typically picks up about 40 percent (or roughly 20 inches) of its water year after Feb. 12 through the end of May, when California's dry season typically is in place. An average end to the wet season would likely flirt with the 1982-83 wet season record in the northern Sierra.

Reservoir Impact

In addition to Lake Oroville, Shasta Lake, San Luis Reservoir, Don Pedro Reservoir, Lake McClure and Castaic Lake are all at least 90 percent of capacity, as of Sunday – well above the historical average for this time of year.

(MORE: California Reservoir Status)

In the Sierra, this is in large part due to a pair of atmospheric river events – one just last week, and a wetter event in early January – that produced rain over higher elevations, rather than snow.

This rain-on-snow last week lead to marked inflows of water both from precipitation and melting of the impressive snowpack from feet of heavy snow since January. If that wasn't enough, the water content left in the Sierra snowpack also remains well above the mid-February average.

Estimated water content of the western snowpack on Feb. 13, 2017, in percent of normal for the date. The river basins near Lake Tahoe, highlighted by the box, were at least double the average.

Therefore, it's not just a heavy rain and snow event that is of concern. Any rapid warmup over the next month or so could unleash rapid snowmelt, putting stress on nearly full reservoirs.

It seems almost inconceivable the state had its driest year on record just four years ago.

Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7.


 

MORE: California Flooding


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