2012 Hurricane Season: Little Hope for Rain in California

May 19, 2012; 5:37 AM
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Only brief episodes of high humidity with the chance of pop-up thundershowers are forecast this summer in California. Odds of a tropical system's remnants reaching the Southland are remote as usual. (flickr image by ejbSF)

Compared to recent years, more tropical systems are forecast in the eastern Pacific, with more rain forecast for the Four Corners and northern Mexico. However, there is little hope for much rain in California.

The number of named tropical systems (tropical storms and hurricanes) is forecast by the AccuWeather.com Hurricane Center to be up from the past couple of years. The numbers are projected to be close to normal with 15 tropical storm, eight hurricanes and four major hurricanes.

While the big Sierra Nevada snowfall during the winter of 2010-2011 helped, California and the Southwest U.S. in general continue to experience drought conditions. In some cases, the lack of rain extends back through several years.

According to AccuWeather.com's Long-Range Experts, headed by veteran meteorologist Paul Pastelok, "The forecast of increased tropical activity, combined with an expected northward shift in high pressure over the West, will bring opportunities for needed rainfall into portions of northern Mexico and the Four Corners, but not necessarily California."

The shift of winds allowing higher humidity and pop-up showers and thunderstorms during the summer is known to locals in the Southwest as "The Monsoon."

This monsoon has been stingy, short-lived and suppressed rather far south in recent years.

However, a beefier monsoon prospect is not a guarantee for getting frequent rain into California. The high may direct moisture farther north, not necessarily farther west.

Prevailing northerly winds along the West coast of North America cause a phenomenon known as upwelling along much of the California coast. This process causes cold water to flow up from the depths. Cold water tends to prevent tropical systems or lead to their rapid demise.

As a result, direct strikes by tropical storms are extremely rare in California. Usually, only the remnants of a system, such as spotty rain and local thunderstorms, survive. And, these are uncommon.

According to Western Weather Expert Ken Clark, "This season, southern Baja California will have more tropical activity and the area from northern mainland Mexico into part of the Four Corners will have more rainfall, compared to recent years based on our analysis."

During the prior two hurricane seasons, we have been generally experiencing La Nina conditions.

A La Nina represents cooler-than-average sea surface temperatures over the tropical Pacific. Cool water tends to suppress tropical storm development.

The opposite of La Nina is El Nino. The somewhat irregular cycle of La Nina and El Nino is known as the El Nino Southern Oscillation (ENSO).

Currently we are experiencing neutral ENSO conditions.

However, along with the northward shift of high pressure in the West, we believe El Nino will begin to develop later this summer and fall.

According to Tropical Weather Expert Dan Kottlowski, "How quickly and strongly this occurs, as well as exactly where the high sets up will be key players."

Another factor is an oscillation of water temperatures in the northwestern Pacific, known as the Pacific Decadal Oscillation (PDO).

This variant of surface water fluctuates very slowly in comparison to the ENSO, often involving decades. Currently the PDO is in a cold phase and this cool water often extends rather far south along the West Coast of the U.S.

"The PDO has a tendency to create troughs of low pressure just off the West coast," Kottlowski said.

Troughs create wind shear, which dismantle tropical systems.

We have noticed the troughs have prevented tropical systems from wandering very far north the past couple of years.

"The effects of El Nino are not likely to be immediate, and the cold phase of the PDO is not likely to suddenly diminish, "Kottlowski said.

"This is why we are not going with a bumper crop of named systems or predicting substantial rain in California at this point," Kottlowski, Clark and Pastelok's team concluded.


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