At a Glance
- One of the most potent snowstorms in months looms for the Sierra.
- This California storm will have both plentiful cold air and moisture to produce feet of Sierra snow.
- While some flooding is possible in lower elevations, the flood threat is not particularly high.
- This is as good a drought-relief scenario for an individual storm as you can get in California.
It's not often an upcoming storm is a positive thing. California will be pummeled by Winter Storm Quinn producing feet of Sierra snow this week, but all things considered, the flood threat is relatively low.
We've written about the paltry status of the Sierra snowpack this winter.
As of Feb. 26, the news was still rather bleak. Statewide water content in the snowpack was less than one-quarter of average for late February, according to the California Department of Water Resources.
The contrast in season-to-date snow from one year ago is nothing short of incredible. January 2017 was the snowiest month on record at Mammoth Mountain, with just over 20 feet, or 245.5 inches.
After the state's record-wettest water year one year ago, California's reservoir levels remain near average for this time of year. Lake Oroville is an exception, held lower as a precaution after last winter's spillway damage.
A Change in Fortune
Spring and summer snowmelt of Sierra snowpack is a crucial element to California's water supply, recharging reservoirs during the state's dry summer and early fall.
So the concern is that if the Sierra snowpack remains low the rest of the wet season, snowmelt recharge in reservoirs would also be low when water use soars during California's hot, dry summer.
One system earlier this week produced high-elevation snow, modest low-elevation rain, even a freak blanket of hail in the state's capital city.
Now, a more vigorous plunge of the jet stream is diving out of the Gulf of Alaska toward the West Coast and will then lumber across California and the Great Basin through Saturday.
Typically, a system diving southward into California wouldn't have a lot of moisture with it, but that's not necessarily the case this time.
"This upcoming storm could be one of the more substantial snowstorms in recent years, as we'll see a combination of both low snow levels and good moisture tap," the National Weather Service office in Reno, Nevada, wrote in its Tuesday morning forecast discussion.
"In recent years, it's been one or the other," NWS-Reno wrote, referring to storms that were either very cold, but not moist, or those that were very warm and wet with high snow levels, such as what happened in November.
"If you have been patiently waiting to read about snow totals in feet instead of inches, then this storm will be your best chance for that this season," it added.
This may end up being a Goldilocks storm: not too warm and wet such as a classic atmospheric river event to trigger widespread flooding, yet not too cold and dry to produce minimal snow. Instead, it may be just right to put a sizable dent in the state's drought concerns while keeping flooding concerns to a relative minimum.
Forecast
Here's the timing:
- Thursday: Periods of rain, wind in lower elevations of northern California. Heavy snow and high winds in the Sierra, Siskiyous and coastal ranges. Some rain and mountain snow arrive in parts of Southern California, increasing into Thursday night. Snow levels lower in northern California as precipitation continues.
- Friday: Showers, perhaps some thunderstorms, in much of the state, with low snow levels. A second wave of more solid valley rain and mountain snow spreads through northern California through Friday night.
- Weekend: Numerous lingering showers with low snow levels statewide, diminishing Saturday night. Drying out Sunday.
(MAPS: 7-Day U.S. Rain/Snow Forecast)
Snow
Snowfall through the weekend will be measured in multiple feet in the Sierra and will also be heavy in the Siskiyous and some of the coastal ranges.
The NWS has posted a blizzard warning in the Sierra for Thursday into early Friday. This prolonged stretch of snow, coupled with high winds, will likely make travel dangerous, if not impossible, over the Sierra at times.
Snowfall rates of 2 to 4 inches per hour are possible in the Sierra Thursday and Thursday night, according to NWS-Reno, along with Sierra ridgetop gusts over 100 mph.
"We strongly advise that people prepare for hazardous winter weather conditions, potential highway closures and very long travel delays," NWS-Reno wrote in its Wednesday morning forecast discussion.
Snow from this system will spread into the Intermountain West this weekend, then eventually the Plains and Midwest by early next week. A major snowstorm could be on tap for parts of the upper Midwest. For more on that story, click the link below.
(MORE DETAILS: Winter Storm Quinn May Become a Major Northern Plains, Upper Midwest Snowstorm)
Rain
Periods of moderate to locally heavy rain may occur both ahead of the initial cold front and in heavier showers and thundershowers, particularly in northern California.
This may be a concern in the recent burn areas of both northern and southern California.
However, given this storm lacks the classic, deep tap of moisture from the subtropics, we are not anticipating widespread heavy rain and flooding in lower elevations.
"This (rainfall forecast) puts the higher-elevation burn areas at the low end of flash flood/debris flow risk," the NWS office in Oxnard, California, wrote in its Wednesday morning forecast discussion.
That said, NWS-Oxnard also stressed "people in and near to recent burn areas cannot let their guard down" with this storm.
"This storm will be great for the snowpack, but one storm alone – even a strong Sierra storm – won't constitute a miracle March," said Dr. Daniel McEvoy, regional climatologist at the Western Regional Climate Center's Desert Research Institute.
McEvoy said in late January that wet periods in late winter and spring are common after extended dry periods, as happened in California from the second week of January into February.
"I will say that a snowy March is fairly common during La Niña years," McEvoy added. "The spring 2011-12 season ended as a drought year, but was saved from being one of the worst (driest) winters on record by a wet and snowy March."
With California's ground water supply tapped during the five-year drought not nearly replenished, all eyes will be on the rest of this spring to see if another miracle March can stop the state from sliding into drought once again.
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. Follow him on Facebook and Twitter.
