Atmospheric River Impacting California

January 28, 2021

This event has the potential to trigger a bloom of desert annuals in the southwestern United States, perhaps followed by an outbreak of Painted Ladies by early March.  This depends on whether enough of the heavy rainfall predicted for the coastal regions of southern California on Friday, January 29 crosses the San Jacinto Mountains to substantially water the deserts to the east.

From the U.S. National Weather Service on January 28:

An ongoing heavy precipitation event continues to unfold across central and southern California this afternoon as an atmospheric river swings through the region. Up to 12 inches of rain has already fallen along coastal areas, with more than 4 feet of snow across the central Sierra Nevada. The focus for heavy rain going into tonight will shift to coastal sections of southern California. Up to 3 inches of rain is expected to fall here, with locally higher amounts. Flash flooding and debris flows will be possible near recent burn scars, as soil cannot retain much rain in a short amount of time. Flash Flood Watches are currently in effect.  Meanwhile, heavy snow is expected to continue through Friday morning across the central and southern Sierra Nevada. A widespread additional 1 to 3 feet of snow can be expected, making travel extremely dangerous and impossible at times. 

If not for this atmospheric river, 2021 would probably not have been a potential year for an extensive Painted Lady outbreak.  The Pacific Southern Oscillation is currently in a strong La Niña phase, which brings sinking air and dry conditions to the southwestern deserts:

La Niña—the cool phase of the El Niño-Southern Oscillation climate pattern—was firmly in place across the tropical Pacific in December 2020. Forecasters estimate a 95% chance La Niña will last through Northern Hemisphere winter. La Niña can influence seasonal climate in the United States. Conditions so far have not looked especially La Niña-like, but winter is far from over.

https://www.climate.gov/enso

The atmospheric river itself is impressive - appearing on satellite photos as a curving loop of clouds extending from east of Hawaii across the Pacific into central California, then continuing northeastward into eastern Washington, southeastern British Columbia, and southern Alberta.  The leading edge of the loop is forecast to move eastward over the next 24 to 48 hours, which will bring the potential for rainfall to southern California and Arizona.  The loop itself is propelled by circulation around a strong low-pressure system that has been spinning to the west of the Oregon/Washington coast for the past three days. 

Events such as this one suggest the potential for Painted Lady outbreaks to be triggered even during years which would otherwise not be favorable for them to occur.  Will there actually be a large outbreak this year?  Observers in California and the Southwest, please keep watching for Painted Ladies, and let us know what you see!
 

--Royce Bitzer, January 28, 2021