Wet Pattern This Week Is Good News for the West Coast, California

Linda Lam
Published: March 11, 2018

A wet weather pattern will emerge for the West Coast in the week ahead, which is good news for areas where rain and snow have been lacking over the past several months.

A powerful low in the Gulf of Alaska will dive southward in the week ahead. The result will be rounds of rain in the lower elevations and snow in the mountains through late week. 

(MORE: Winter Storm Central)

Ahead of that strong low, a subtropical disturbance has already brought some rain to Southern California this weekend. Downtown Los Angeles picked up about a half inch of rain on Saturday.

A brief break in the precipitation will develop Sunday and last into Monday.

Then, an active weather pattern kicks into gear by Monday night across the West Coast as the low in the Gulf of Alaska slides southward.


Current Satellite, Pressure and Moisture

An omega block will be in place over the U.S. midweek, which means an upper-level ridge of high pressure, or northward bulge of the jet stream, will be over the central U.S., with upper-level troughs of low pressure, or southward dips in the jet stream, over both the East and West coasts. 

This upper-level trough will remain in place over the West Coast and eastern Pacific through late week, which will keep cold and unsettled weather over the region.

Wet and Cool Conditions Ahead

Showers will begin to develop ahead of the first system late Monday. Widespread rain and mountain snow will be found from Washington into much of California on Tuesday. 

A second and likely stronger system will move into the West Wednesday.

(MAPS: Weekly Planner)

This midweek system will also be colder than the one this weekend and early in the week, which will allow snow to fall at lower elevations. Temperatures will trend colder-than-average from California into the Great Basin and Southwest.


Next Week's Forecast

Heavier rain may develop later in the week, which will increase the risk of flash flooding and mudslides in recent burn areas.

(MORE: Where March and April Are the Snowiest)

Travel impacts are also likely in the week ahead, including at passes across the Sierra.

In fact, feet of snow are expected to pile up in the Sierra during the week ahead, with locally heavy rainfall expected, especially in the foothills of the Sierra.


Rain and Snow Forecast

There is also the chance for a few thunderstorms, possibly with small hail, the National Weather Service in San Francisco noted in its forecast discussion on Saturday morning.

Gusty winds will also be a concern Monday night into Tuesday from southern Oregon into the Great Basin.

(FORECAST: Seattle | Sacramento, California | Reno, Nevada | San Diego)

There are some uncertainties as to the details of this pattern and exactly how strong each system will be, as well as the track of the upper-level low. Consequently, it is difficult to determine how impactful each round of precipitation will be at this time. Be sure to check back to weather.com for updates.

Some Good News For Areas Where Rain and Snow Have Been Lacking

Rain and snow have been below average for much of the West over the past few months.

In fact, a small area of extreme drought was added in California this week for first time since Feb. 2017, according to the U.S. Drought Monitor.

As of March 6, 2018, just under 50 percent of California was experiencing drought conditions. At the beginning of October, when the "water year" began, only 8 percent of the state was in drought. 

Areas in orange and red are experiencing drought conditions as of March 6, 2018.
(U.S. Drought Monitor)

Even more notable is the Sierra snowpack, which is crucial to California's water supply in the spring and summer.

A recent pattern change, including Winter Storm Quinn, brought some much-needed snow to California. Prior to the late February into early March snow, which brought up to 8 feet of snow in the highest elevations of the Sierra Nevada, California's snowpack was about one-quarter of the average for the 2017-18 winter season.

(MORE: Feet of Snow Bring Big Change to California's Sierra Nevada, But It's Not Enough)

Although the recent snowfall is helpful, it is not nearly enough to put a substantial dent in the snowfall deficit that grew throughout the winter. 

The snow water equivalent of the Sierra Nevada as of March 9 was only 36.5 percent of normal for the date, according to an experimental model developed by the University of California, Los Angeles Civil Engineering department.

The California Department of Water Resources confirms this low percent to normal, with an average of 38 percent to normal as of March 8.

It is not just California that is below average for snowfall. Areas from Oregon into Nevada, Utah, Colorado, Arizona and New Mexico are well-below average season-to-date.

According to the USDA Natural Resources Conservation Service, the snow water equivalent in portions of the Sierra, southern Oregon, Nevada, southwestern Idaho, southern Utah, Arizona and New Mexico is below 50 percent, based on the 30-year average from 1981-2010.

Consequently, the rain and snow expected in the upcoming week will be very welcome, but a lot more precipitation will be needed to make up the difference that has resulted due to a drier-than-average winter.


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