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Weekly Weather Event -Week of October 18th

October 22, 2021

A series of Pacific storms are expected to impact Northern California this weekend, bringing much needed rain to the drought-stricken region. Three separate systems will be impacting Northern California, with one bringing heavy rains Friday morning, one expected on Saturday, and the final system on Sunday into Monday. The last storm is expected to bring the most amount of precipitation, as the system will be tapping into a moist atmospheric river. Up to a foot of rainfall is possible between the three storms, and up to three feet of snow could fall in the Sierra Nevada.

Atmospheric rivers are long, narrow regions of the atmosphere that transport large quantities of water vapor, approximately about as much water content as flows through the mouth of the Mississippi River. Due to these high quantities of water vapor, atmospheric rivers can bring both much needed moisture as well as flash flooding. In California, a particular concern for this set of storms is the risk of mudslides. This recent wildfire season has resulted in large burn scars, and the upcoming rain could result in debris flow and heavy erosion.

In 2019, researchers from Scripps Institution of Oceanography at the University of California San Diego created a scale to classify atmospheric rivers. It uses a scale from 1 to 5 with the categories “weak”, “moderate”, “strong”, “extreme”, and “exceptional”. Unlike the scale used for hurricanes, the atmospheric river scale takes the duration of the storm over one area into consideration. The atmospheric river event taking place is currently forecasted to be Level 4 or Level 5, indicating a mostly hazardous system bringing large amounts of rain and flooding risk.