Where Was the Most Extreme Precipitation in the West Yesterday?

New alert available through Scripps sends updates of major events

A new service delivers the location of the most extreme precipitation levels during storms, such as the atmospheric river-fueled event that struck Northern California last week.

Weather-watchers can sign up to receive an automated "R-Cat Extreme Precipitation Alert" e-mail from the Center for Western Weather and Water Extremes (CW3E) at Scripps Institution of Oceanography at the University of California San Diego.

It uses a scale developed by CW3E Director Marty Ralph and U.S. Geological Survey scientist and center member Mike Dettinger.

The maximum three-day precipitation during the weekend event was 521 mm (20.51 inches) at a location called Strawberry Valley on the western slopes of the Sierra Nevada Mountains, about 1,161 meters (3,807 feet) above sea level, near Interstate 80.

That made this an “R-Cat 4” extreme precipitation event on the CW3E’s scale.  This is the top magnitude possible and is very rare.  “R-Cat” stands for “Rainfall Category.”

The landfall of a very strong long-duration atmospheric river (AR) followed by a second AR in California over the last few days produced extreme precipitation over much of Central and Northern California.  The extreme precipitation was identified and reported in real time by a new tool developed by Scripps climate scientist David Pierce and Ralph that automatically monitors rain gauges across the Western U.S. and sends an email alert to anyone signed up for the service when extreme precipitation events occur.  The service is free and is intended to provide information to interested individuals in a timely manner. 

To subscribe to the alert via email, e-mail a message with subject "subscribe" to <rcatalert@cirrus.ucsd.edu>

The alerts use a simple new method to identify extreme events.  The categorization method is based on three-day observed precipitation totals (rain and/or the liquid equivalent of snow that fell).

R-Cat 1:  200-299 mm (roughly 8-12 inches) / 3 days

R-Cat 2:  300-399 mm (roughly 12-16 inches) / 3 days

R-Cat 3:  400-499 mm (roughly 16-20 inches) / 3 days

R-Cat 4:  more than 500 mm (more than roughly 20 inches) / 3 days

The number of R-Cat 3 or 4 events annually roughly matches the average number of major hurricanes that occur annually in the Atlantic and the number of the most extreme tornadoes that occur.

In light of the events of last weekend, Ralph said it is useful to note that, in the western United States between 1948 and 2010, 44 of the 48 occasions when R-Cat 3 or R-Cat 4 conditions were reached coincided with the arrival of an atmospheric river storm.

Related Image Gallery: Views of the January 2017 atmospheric river event


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