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New Tool Helps Forecasters Measure Systems Carrying Rain

Cars drive in the rain at Mission Beach, Dec. 5, 2018.
Nicholas McVicker
Cars drive in the rain at Mission Beach, Dec. 5, 2018.

Weather forecasters are using a new system to measure the patterns that bring so much rain to Southern California.

Much of the rain that drenched San Diego is coming to the region on an atmospheric river. The pineapple express is one that famously drags moisture from the central Pacific and soaks Southern California.

Before now, meteorologists did not have a way to measure or categorize the weather systems responsible for 90 percent of the region’s rainfall.


That has changed.

Marty Ralph, a meteorologist at the Scripps Institution of Oceanography, led a team that included the National Weather Service’s Johnathan Ruiz and several other experts. They worked together to develop a tool to allow forecasters to observe and predict the intensity of the systems.

RELATED: More Snow, Rain Expected In San Diego County

Researchers considered three factors.

“The amount of water vapor in the air, strength of the low altitude winds, and the duration of the AR (atmospheric river) in a given location,” Ralph said.


The rivers are classified in one of five categories ranging from beneficial to damaging.

The scale for atmospheric rivers:

— AR Cat 1 (Weak): Primarily beneficial. For example, Feb. 23, 2017, AR hit California, lasted 24 hours at the coast, and produced modest rainfall.

— AR Cat 2 (Moderate): Mostly beneficial, but also somewhat hazardous. An atmospheric river on Nov. 19-20, 2016 hit Northern California, lasted 42 hours at the coast, and produced several inches of rain that helped replenish low reservoirs after a drought.

— AR Cat 3 (Strong): Balance of beneficial and hazardous. An atmospheric river on Oct. 14-15, 2016 lasted 36 hours at the coast, produced 5-10 inches of rain that helped refill reservoirs after a drought, but also caused some rivers to rise to just below flood stage.

— AR Cat 4 (Extreme): Mostly hazardous, but also beneficial. For example, an atmospheric river on Jan. 8-9, 2017 that persisted for 36 hours produced up to 14 inches of rain in the Sierra Nevada and caused at least a dozen rivers to reach flood stage.

— AR Cat 5 (Exceptional): Primarily hazardous. For example, a Dec. 29, 1996 to Jan. 2, 1997 atmospheric river lasted over 100 hours at the Central California coast. The associated heavy precipitation and runoff caused more than $1 billion in damages.

The systems can vary greatly and knowing how much moisture they carry and how fast they are moving will help forecasters warn local communities.

“At a given moment in time there are typically three or four of these somewhere in the northern hemisphere and they’re spread out. They’re very small relative to the global scale. And yet they produce about 90 percent or more of all the water vapor transport in the mid-latitudes is in these few filaments or rivers in the sky,” Ralph said.

New Tool Helps Forecasters Measure Systems Carrying Rain
San Diego researchers have developed a new weather forecasting tool that will give them a better idea of the impact of incoming storm systems. You can hear this story and other local news every morning by subscribing to San Diego Stories, KPBS’ daily news podcast. Subscribe via iTunes, Google Play or your favorite podcatcher.