Southern California’s Strongest Storm In Almost Six Years Departs, Next System Targets NorCal

Moderate to heavy rain is sliding into the Southwest, as a powerful Pacific storm has pushed into California. This major storm was one of the strongest winter storms to impact southwestern California in almost six years. A second storm will arrive late Sunday, bringing with it a dose of heavy rainfall for the other end of the Golden State.

(Latest News: 4 Dead in Southern California Storms)

Current Flood Alerts

(MORE: 8 Reasons Why Rain Is a Big Deal in Southern California)

Flash flood or flood watches remain in effect for parts of Southern California, including San Diego. Flash flood watches are also in effect for Southern Nevada, not including Las Vegas, and much of western and central Arizona including Phoenix. Flooding may occur in these areas through the weekend.

Rainfall totals on Friday exceeded 6 inches in many spots across southern California. The highest rainfall total was 10.44 inches in El Deseo north of Santa Barbara. Numerous locations broke daily rainfall records. A list of selected rainfall reports is below the forecast.

(INTERACTIVE: Latest Storm Reports, Shelters Open)

Flood watches are also posted in parts of central and into much of northern California and western Nevada, including Sacramento and San Francisco, California and Reno, Nevada and Lake Tahoe for the next storm which will begin to impact the region Sunday evening.

(MORE: Latest News on Oroville Dam)

Current Radar, Temperatures, Conditions

The parade of strong low pressure systems will continue into early next week due to the active Pacific jet stream that will guide multiple systems into the West. A second major storm will roll into northern California late Sunday.

Current Satellite and Moisture

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)

Storm Timeline

Here is a rough timeline of each storm:

• Saturday: Heavy rain threat moves inland into Arizona and southern Nevada, including the Vegas Valley, Phoenix and Tucson.
• Saturday night into Sunday: Ahead of the next system, light to moderate rain is possible from central California into the Pacific Northwest, with snow in the higher elevations.
• Sunday night through Wednesday: A second strong storm will bring a potential prolonged northern California heavy rain/mountain snow, with snow levels lowering Tuesday/Wednesday; colder/lower snow levels in the Pacific Northwest.

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

Forecast Next 5 Days

The numbers for each day correspond to the percent chance of precipitation.

Impacts

Additional Rainfall

Saturday:

• Amounts on Saturday should be substantially lower than on they were on Friday.
• Additional rainfall in Southern California will generally be less than an inch, however any additional rainfall may cause problems.
• 1 to 2 inches of rain can be expected in parts of central Arizona through Sunday morning.
• Thunderstorms are possible early Saturday in parts of southeastern California and southern Arizona, potentially producing brief intense heavy rainfall, hail and strong wind gusts.

Early Next Week:

• Through Wednesday, most low elevations of northern California are likely to total at least 3 inches of rainfall.
• The foothills of the northern Sierras may see as much as a foot of rain below snow level.

(MAPS: 7-Day Rain/Snow Forecast)

Flooding/Debris Flows

• Brief heavy rainfall could cause rapid runoff with urban flooding possible into Saturday afternoon.
• Rivers will rise again over much of the state into next week. Reservoirs, including Lake Oroville, will need to release water prior to each event to make room for storm water inflows.
• Debris flows over wildfire burn-scarred areas can still occur and could be damaging to any homes, structures or roads nearby. Remember, debris flows can occur with relatively low rainfall rates.
• Mud/rockslides are possible even in areas not recently charred by wildfires.

Rain, Snow Outlook Next 7 Days

Locally heavier rain totals are likely on west and south-facing slopes of mountains and coastal ranges below snow level.

Snow

• Feet of additional snow will fall in the Sierra through the middle of next week. This may, at times, force road closures, including Interstate 80 over Donner Summit. Snow is likely above 5,500 feet in the Sierras on Saturday.
• Over a foot of snow is also expected through the middle of next week in the Cascades, Siskiyous and mountains of Idaho and western Wyoming.

High Surf

• Dangerous and damaging surf will persist through Sunday along the Southern California coast, particularly along west-facing beaches.
• Rip currents, large breaking waves, beach erosion and minor coastal flooding are possible. Damage may occur to piers and jetties.
• Stay out of the water this weekend in Southern California.

Storm Reports

February 16

The first round of this latest Pacific storm parade soaked portions of California on Thursday as a band of moisture moved southward through the Golden State.

Up to one-half inch of rain fell in much of the Bay Area. Parts of Marin and Napa Counties had picked up one-half to one inch of rain, while the wettest locations in Sonoma County had already exceeded one inch of rain. As of 4 p.m. PST Thursday, 0.37 inches of rain had fallen at San Francisco International Airport.

This first teaser rainfall event also brought rain to southern California. Much of the Santa Lucia Range received 0.20 to 0.40 inches of rain on Thursday. Lower elevations including Santa Barbara, Oxnard and Los Angeles received less than 0.05 inches.

February 17

A strong atmospheric river event set up in southern California, dumping heavy rainfall on Santa Barbara, Ventura and Los Angeles counties.

Multiple roads were closed in Santa Maria late Friday morning, due to flooding. Severeal reports of flash flooding, mudslides, urban flooding, and debris flows came in from Southern California. Nearly 9 inches of rain was dropped onto the mountains just north of Santa Barbara, while in downtown Los Angeles rainfall totals came up just short of their daily rainfall record.

As the low pressure system approached the San Francisco area, it rapidly strengthened and underwent “Bombogenesis.”

The low may have also clinched the record for lowest pressure in the month of February at San Francisco Airport, a record that was last set in 1998.

These record low pressures caused winds to ramp up during the afternoon hours.

Winds gusted across to over 50 mph across much of central and southern California, knocking over many trees and power lines. Some gusts exceeded 80 mph.

Selected Wind Gusts as of Friday Evening, all in California: 

• 108 mph was measured near Grapevine
• 84 mph was recorded in Burns Canyon
• 75 mph at Palomar Mountain Lookout
• 72 mph in Hesperia and San Clemente
• 67 mph in Laguna Beach
• 65-75 mph in San Bernardino and San Diego counties (widespread)

In San Diego and Orange Counties, a squall line developed on Friday afternoon. Strong winds pushed over numerous trees. A wind gust was reported on the Long Beach Pier.

Record daily rainfall was reported across southern California. Long Beach, Santa Barbara, Sandberg, Santa Maria and Lancaster all received more rainfall than any other February 17th. Most sites doubled or trippled their old records.

Selected rainfall totals as of Saturday 5:00 a.m. PST in California: 

Bolded values are locations where daily record rainfall occurred.

• 10.44 inches in El Deseo
• 9.91 inches at San Marcos Pass
• 9.89 inches at Old Man Mountain
• 9.80 inches at Upper Matilija Canyon
• 9.15 inches in Montecito Hills
• 7.68 inches at Red Mountain
• 7.02 inches in Los Prietos
• 6.57 inches in Westlake Village
• 6.22 inches in Opids Camp
• 5.50 inches at Crestline Ridge
• 4.84 inches at Malibu Canyon
• 4.84 inches in Canoga Park
• 4.52 inches in Oxnard
• 4.16 inches at Santa Barbara​
• 3.98 inches in Rocky Butte
• 3.92 inches in Black Mountain
• 3.86 inches in Sandberg
• 2.94 inches in Laguna Hills
• 2.91 inches in Cambria
• 2.77 inches in Long Beach
• 2.74 inches in Pasadena
• 2.05 inches in Downtown Los Angeles
• 1.98 inches at Santa Maria Airport
• 1.09 inches at San Diego International Airport
• 1.05 inches in Downtown San Francisco
• 1.16 inches in Oakland
• 0.73 inches in Fresno

All this rainfall in a relatively short period of time, allowed rivers and washed to fill up rapidly. The Ventura River has reported some local flooding and the Conejo Creek in Camarillo was seen water spill over into farmland.

The Pacific Coast Highway east of Santa Barbara was closed at times on Friday afternoon due to mudslides and flooding. Interstate 110 was also closed near Slauson, California on Friday evening.

The Santa Barbara Airport also closed due to flooding on runways Friday evening into Saturday morning.

Numerous swift water rescues were performed across Southern California.

In the Sierras, Ponderosa receieved 24 inches and Mt Baldy Ski Resort saw 20-24 inches of new snow in 48 hours ending Saturday morning and Squaw Valley received 12 inches in 24 hours ending Saturday morning. New snow at Alpine Meadows put them over 500″ for the winter.

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).  (California Department of Water Resources)

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, in order to encompass one entire wet season, peaking 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 Feb. 12 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 October-through-February period on record dating to the late 19th century, according to Paul Iñiguez, science and operations officer at the National Weather Service in Phoenix.

With the upcoming parade of storms mentioned earlier, 2016-2017 will likely bypass 1968-69 as the wettest such period.

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

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

On Feb. 16, Lake Oroville fell below 90 percent capacity for the first time since the Oroville Emergency Spillway became endangered.

These reservoirs will surely rise again early next week with incoming rainfall.

(MORE: California Reservoir Status)

The Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite captured this natural-color image of sediment-filled waterways in California’s Sacramento Valley on February 11, 2017 (second image). For comparison, the first image shows the same area on November 9, 2016, before the wet weather arrived.  (NASA Earth Observatory)

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

This rain-on-snow in early February 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.

(IMAGES: How Northern California Went From Parched to Flooded in a Few Months)

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.  (USDA/NRCS National Water and Climate Center)

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.