The Catalina Eddy and the cutoff low are two types of low-pressure systems or cyclones that can make weather forecasting a real challenge along the Central Coast.
A cyclone is an area of relatively low pressure, characterized by air spiraling inward.
Like the vortex of water going down a kitchen drain, the winds flow in a counterclockwise direction in our hemisphere and produce lower air pressure toward the middle of the circulation.
Weather charts and models often have a difficult time forecasting the development of the Catalina Eddy because it usually covers a small area in the Southern California Bight (a portion of the coast from Point Conception to just south of San Diego).
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But when it does form, it can quickly change the temperature forecast along the beaches.
The formation of the Catalina Eddy is still not completely understood.
The coastal eddies seem to develop when strong to gale force northwesterly winds travel down the rugged California coast, move pass Point Conception and interact with the topography of the Southern California coast.
This interaction causes the winds to flow in a counterclockwise direction and creates an area of low pressure in the vicinity of Santa Catalina Island.
When the coastal eddies develop, the winds will shift out of the south and usually produce a rapid increase in the depth of the marine layer, resulting in a thick blanket of fog and much cooler temperatures.
It can occur all year but happens most often from May through July during the peak of our northwesterly winds.
Another weather forecast challenge is the cutoff low.
Longtime weather forecaster George Fischbeck of KABC-TV in Los Angeles used to say, “A cutoff low is a weatherman woe.”
Last week’s cutoff low certainly proved it. These cutoff low-pressure systems are notorious for being unpredictable and can humble the most confident meteorologist.
Last week an upper-level low-pressure system developed off the Northern California coast and broke away from the jet stream, becoming a cutoff low. In other words, the jet stream shifted to higher latitude and left a circulating low-pressure system behind.
At our latitude, the jet stream is typically a tubular ribbon of high-speed winds some 18,000 to 40,000 feet high, flowing in wavelike patterns from the west to the east for thousands of miles.
The jet stream is the main mechanism that drives storms from the Pacific Ocean east toward the Central Coast.
When a low-pressure system breaks away from the main flow, it becomes difficult to almost impossible to predict the speed and direction of these weather systems.
These lows have been known to stay nearly stationary for days, or even move westward back out to sea.
Often when a cutoff low develops, the many numerical weather prediction models that meteorologists consult for guidance can diverge wildly, much like the cutoff lows themselves.
Last week’s cutoff low parked itself south of Point Conception and produced scattered thunderstorms and rain showers over San Luis Obispo County for days.
It finally moved southeastward into Arizona, ending the unsettled weather. Although it was difficult to forecast, the weather did provide us one important benefit: much-needed rain.
This week’s forecast
The leading edge of moisture from a large 956-millibar storm moving into the Pacific Northwest will merge with a weak front currently crossing the Central Coast and will produce light scattered rain showers early this morning.
A few breaks of sunshine will develop later this morning.
However, another and slightly stronger cold front from this storm will follow later today into tonight with rain showers.
Total rainfall amounts from both of these systems should remain less than 0.33 inches. Snow levels are expected to gradually rise to 11,000 feet by this afternoon, and all major Sierra highway passes should be well below the snow level.
Today’s temperatures will range between the low to high 60s throughout the county.
The main effect of the system along the Central Coast will be a steep pressure gradient that will produce moderate gale to fresh gale (32 mph and 46 mph) force northwesterly winds along our coast Monday and Tuesday.
These winds will also help to mix out the marine layer, leaving behind mostly clear and sunny weather.
Fair weather will return Wednesday, with gradually warmer maximum temperatures but also cool minimum temperatures, especially Wednesday morning.
It continues to look like another weather system will produce increasing southerly winds and rain Friday and Saturday.
Surf and sea report
This morning’s 5- to 7-foot northwesterly (300-degree deep-water) swell (with an 11- to 16-second period) will continue at this height but with a gradually shorter period through Monday morning.
A northwesterly (320-degree deep-water) swell from the Gulf of Alaska combined with northwesterly seas generated by gale force northwesterly winds along the California coast will produce a 10- to 12-foot northwesterly sea and swell (with a 5- to 16-second period) Monday through Tuesday.
Note: The waves’ heights will be much higher at the offshore buoys.
This northwesterly sea and swell will decrease to 6 to 8 feet Wednesday, further lowering to 3 to 5 feet (with a 7- to 11-second period) Thursday through Friday.
Another series of storms in the Gulf of Alaska will produce increasing swell conditions along the Central Coast late next week, with a possible high wave event Nov. 1.
Sea temperatures will range between 55 and 57 degrees through today, decreasing to 52 and 54 degrees Monday. Sea temperatures will further lower to 51 and 53 degrees Tuesday through Wednesday.
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John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for more than 23 years. To subscribe to his daily weather forecast or ask him a question, e-mail firstname.lastname@example.org.