Weather Watch

New atmospheric river could bring huge rain to SLO County this week

At the end of February, average rainfall amounts at the Paso Robles Municipal Airport checked in at 125 percent of normal, while the Santa Maria Public Airport has recorded more than 11 inches, or 111 percent of average.

Cal Poly (official home of climatology for San Luis Obispo) has measured nearly 19 inches of rain, or 112 percent of typical, while in western San Luis Obispo recorded more than 22 inches and the rain gauge at Diablo Canyon was at 108 percent.

These averages will probably dramatically increase during the first half of March. Here’s why.

An area of high-pressure (blocking high) in the Gulf of Alaska combined with increasing mid-latitude westerlies (jetstream) winds will continue to steer a series of storms into California on a classic 72-hour storm cycle, with a new low-pressure system developing every third day.

Of course, these are long-range models, and they could certainly change; however, confidence continues to build that they will verify.

In other words, the Ides of March could see a wet weather pattern, indeed.

The U.S. Navy Fleet Numerical Meteorology and Oceanography Center model runs indicate an “atmospheric river” (AR) will develop on Tuesday into Wednesday.

This system is forecast to produce between 3 and 5 inches of rain, with higher amounts in the coastal mountains due to orographic enhancement. When the air mass is lifted over our coastal mountains, it cools and eventually reaches its dew-point temperature. When this occurs, rain will develop. Like squeezing a wet sponge, moisture from this air mass is released in the form of precipitation.

John Lindsey David Middlecamp

What is an AR? It’s an atmospheric condition that can dramatically enhance precipitation.

An AR needs a storm beforehand that can tap into vast amounts of water vapor, which most of the time along the California coast come in the form of plumes of subtropical moisture from the South Pacific. Like cloud seeding, you just can’t go out and seed clear skies with microscopic silver iodide particles and expect rain; you need the right atmospheric conditions with plenty of clouds that can produce rain showers.

The term atmospheric river hasn’t been around very long. None of my oceanographic and meteorological textbooks show any reference to it. It turns out; the phrase was coined by researchers Reginald Newell and Yong Zhu of the Massachusetts Institute of Technology in the early 1990s.

These rivers in the sky can stretch for thousands of miles across the world’s oceans, but they are only a few hundred miles wide. They can draw vast amounts of water vapor into narrow bands ahead of cold fronts and transport ridiculous amounts of water across vast expanses.

In fact, they can carry more fresh water than the Amazon River.

Along the West Coast, they are informally called the “Pineapple Express.” The Pineapple Express is a subset of an atmospheric river event that originates in the tropical waters near Hawaii; hence the pineapples. In the past, meteorologists simply referred to these as “the hose.”

In the February edition of the Bulletin of the American Meteorological Society (BAMS), a distinguished group of forecasters published a paper that rates the strength of atmospheric rivers. It’s similar to ratings used to classify the intensity of hurricanes and tornadoes.

However, unlike tornadoes and hurricanes, AR Cat 1 and AR Cat 2 atmospheric rivers are mostly beneficial, especially during drought years. Another difference in the AR scale is for a geographic location or fix point, along with its strength (how much water vapor is transported over a given area) and its duration (up to 72 hours).

Here’s a look at the AR category scale’s assessment of beneficial vs. hazardous impacts:

AR Cat 1: Primarily beneficial AR

AR Cat 2: Mostly beneficial, but also hazardous

AR Cat 3: Balance of beneficial and hazardous

AR Cat 4: Mostly hazardous, but also beneficial AR

AR Cat 5: Primarily hazardous

In March 1995, a storm developed about 900 miles off the Central Coast and caused an intense cold front to stall over our area that tapped into a plume of subtropical moisture (AR) that stretched to Hawaii.

The rain began to fall early on March 9 and continued through the next day, producing amazing 24-hour rainfall totals with Diablo Canyon reporting 8.4 inches of rain.

San Simeon saw sustained wind speeds of 70 mph with gusts reaching 88 mph. These winds produced an enormous amount of orographic enhancement. Locations in the hills above Cambria reported rainfall amounts exceeding 12 inches over 24 hours.

Flooding occurred throughout the Central Coast, with Cambria being one of the hardest hit. Cambria’s Fire Department used a boat to rescue people along Main Street.

The March 1995 storm would be classified as AR Cat 4, while Tuesday and Wednesday’s AR is expected to be ranked at AR Cat 3.

AR Cat 5 may have occurred in 1862 when much of the Central Valley of California turned into a vast inland sea. Remember, as the atmosphere continues to warm, the more water vapor it can hold, and the more intense rain and snow events will become.

PG&E has been preparing for storms like those while urging its customers to be ready for natural disasters. That includes having a family emergency plan and keeping emergency kits for your home, your office and your vehicle. PG&E offers emergency-preparation tips on its website at

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at or follow him on Twitter: @PGE_John.
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