Jonathan Erdman
A weather pattern change is bringing much-needed rain and mountain snow to parts of the drought-stricken West, including California, this weekend.
So far this month, the jet stream has often taken a huge detour northward over the eastern Pacific Ocean, effectively blocking Pacific storms from reaching the West Coast.
Because of this block, most locations in California other than the northern Sierra have yet to pick up any rain this month. Even typically wetter locations in the Pacific Northwest, such as Seattle and Portland, Oregon, have had one of the driest starts to April on record.
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But this has now changed due to a sharp change in the jet stream.
The jet stream is taking a sharp southward plunge over the eastern Pacific Ocean and into the West, returning the Pacific storm track and, thus, rain and mountain snow to California, Oregon and Washington.
Rain and mountain snow has already arrived in the Pacific Northwest and California and will slowly drop southward as the weekend comes to a close.
Sunday night into Monday, some rain and high-elevation snow may spread into Southern California, including the L.A. Basin and San Diego, as well as the Desert Southwest.
Sunday's ForecastMost of California should pick up less than an inch of rain from this event. However, some coastal ranges and Sierra foothills at elevations warm enough for rain could pick up somewhat higher totals.
The threat of flash flooding and landslides is relatively low. However, debris flows are possible in a few areas recently scarred by wildfires, especially if brief, heavier showers or thunderstorms occur Sunday or Monday.
Rain and Snow ForecastDrought Denter?
Unfortunately, this won't help the western drought much.
The West is in its most widespread drought in over 17 years, according to the latest Drought Monitor analysis. Parts of California, Oregon and the Southwest are categorized in the two most serious categories of drought.
Drought Monitor analysis as of April 20, 2021. Areas of more serious drought are depicted in the darker red and brown contours.Droughts develop over a period of months.
This drought began to spread in earnest last spring, shifted into high gear after a weak summer monsoon in the Southwest, then worsened and spread into winter and spring 2021.
For the second year in a row, California is enduring a dry rainy season, the period between late fall and early spring during which they receive the majority of their precipitation.
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So this precipitation over a few days, while welcomed, won't make much of a dent.
Even a foot of snow in California's Sierra – where snowpack is critical for replenishing reservoirs during spring and summer snowmelt – can't possibly make up for a snowpack only 30% of the average for this time of year.
It will at least dampen parched ground a bit.
The National Weather Service in Monterey, California, noted moisture contained in vegetation near the Northern California coast was at record dry levels for this time of year, more typical of July than April.
But we're running out of time before California's dry season kicks in, usually by May.
What's more, California's rainy season appears to be shrinking.
According to a January 2021 study, the start of the state's rainy season has been trending later in the fall since the 1960s, driven by a drier trend in November.
This drier late-fall trend overlapping the time of year when Santa Ana and Diablo winds are most frequent and vegetation is at its driest is an increasingly volatile recipe for wildfires.
The study also found California's rainy season is becoming more tightly squeezed into a stretch from December through March, offering less time for the state to pick up the precipitation vital to its water supply.
The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.
The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.
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