YoTuT
The Catalina Eddy
For a better understanding of how the eddy works, take a look at what happens to SCAL’s coastal winds when the Eddy forms. Strong springtime NNW winds barrel down the California coast until they reach Point Conception and the southeastward turning of Southern California. Some of the air curves toward the coast, swirling back around forming a loop, or mini low-pressure system. This is like the swirls that form on the backside of rocks in a stream.
There are two basic setups of the Catalina Eddy. The first is a complete loop (image above), when thermal low pressure is situated near Las Vegas. This usually means that southerly winds will prevail most of the day. The second, is when the thermal low is near Yuma, Arizona. In this case, we may have the southerly winds in the morning, but winds shift to W-SW during the afternoon. Either way, morning surf is usually shot for San Diego and Orange County during an eddy circulation. There is a good chance for an evening glass off though.
Since we can get parked in this circulation for weeks, causing early-morning “bummer’’ south winds and all day lasting depressing gray skies - forecasting the demise of the eddy is important.
To destroy the eddy completely, we’ve got to stir up the marine layer, either from a storm, Santa Ana, or some kind of tropical surge that helps relax the strong winds off the Central California coast. Unfortunately, a weather front or a Santa Ana will require waiting until fall, and a tropical surge, later in the summer. But in the early season we’ve got to watch for subtle changes in the marine layer — super deepening or lowering — either of which can return the coast back to a normal sea-breeze pattern of offshore winds in the morning, and onshore during the afternoon. A good rule of thumb says if wind speeds fall below 1 0 kts at San Nicolas Island, or the wind blows from a direction other than northwest there, the eddy is gone.
The following bullets summarize some of the more anal aspects of forecasting the eddy and its effects on Southern California’s weather:
* When onshore flow is forecast, a north-northwest wind of 20 knots or greater is usually sufficient to spark an eddy; provided the 500 mb forecast shows weak troughing over the West.
* Upper level charts should reveal troughing over the West. A cyclonic circulation aloft will help to promote an eddy even if the surface winds are borderline strength (below 20 knots) near Point Conception.
* A check of the visible satellite imagery during the day, and the fog imagery at night would show stratus off the coast, and an advancement of that stratus up the coast from the south. The cloud will usually form off of San Diego, and progress up into Orange County. Often, fingers of stratus are visible curving around the outer Channel Islands, evidence that winds have increased off Point Conception.
* Coastal wind reports should back into the SE quadrant overnight, and may even be ESE in direction during the morning.
* Occur all year long, but most eddies form during the spring and summer months. During winter, the upper troughs and associated surface fronts are often too strong for an eddy, causing northwest winds to pour out across Southern California, instead of an eddy forming.
* Onset can chill air temperatures, by as much as 15 degrees F along the coast!
* A rapid deepening of the marine layer, swift inland advance of stratus, and associated cool marine air, birthing the radical cooling.
* Sluggish coastal clearing, and the likelihood of an overcast day is enhanced.
* Warmer sea temperatures. If warm water exists offshore to the south, the potential for that water to be pushed shoreward on southerly winds is good, raising sea temperatures by two to six degrees along the south coast.
* Slight rise in coastal water levels. A minor increase in water levels can be expected as south winds tend to pile water towards the coast. This may add to coastal erosion if accompanied by large long period swell, and high astronomical tides.
* A disrupted sea/land breeze pattern. The typical offshore morning winds may be from the southeast, and the normal westerly afternoon winds would be southerly in direction. Winds usually veer to the SW through the day, sometimes backing-off all together for an evening glass-off; only to return after midnight, provided the necessary conditions continue.
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The Catalina Eddy
For a better understanding of how the eddy works, take a look at what happens to SCAL’s coastal winds when the Eddy forms. Strong springtime NNW winds barrel down the California coast until they reach Point Conception and the southeastward turning of Southern California. Some of the air curves toward the coast, swirling back around forming a loop, or mini low-pressure system. This is like the swirls that form on the backside of rocks in a stream.
There are two basic setups of the Catalina Eddy. The first is a complete loop (image above), when thermal low pressure is situated near Las Vegas. This usually means that southerly winds will prevail most of the day. The second, is when the thermal low is near Yuma, Arizona. In this case, we may have the southerly winds in the morning, but winds shift to W-SW during the afternoon. Either way, morning surf is usually shot for San Diego and Orange County during an eddy circulation. There is a good chance for an evening glass off though.
Since we can get parked in this circulation for weeks, causing early-morning “bummer’’ south winds and all day lasting depressing gray skies - forecasting the demise of the eddy is important.
To destroy the eddy completely, we’ve got to stir up the marine layer, either from a storm, Santa Ana, or some kind of tropical surge that helps relax the strong winds off the Central California coast. Unfortunately, a weather front or a Santa Ana will require waiting until fall, and a tropical surge, later in the summer. But in the early season we’ve got to watch for subtle changes in the marine layer — super deepening or lowering — either of which can return the coast back to a normal sea-breeze pattern of offshore winds in the morning, and onshore during the afternoon. A good rule of thumb says if wind speeds fall below 1 0 kts at San Nicolas Island, or the wind blows from a direction other than northwest there, the eddy is gone.
The following bullets summarize some of the more anal aspects of forecasting the eddy and its effects on Southern California’s weather:
* When onshore flow is forecast, a north-northwest wind of 20 knots or greater is usually sufficient to spark an eddy; provided the 500 mb forecast shows weak troughing over the West.
* Upper level charts should reveal troughing over the West. A cyclonic circulation aloft will help to promote an eddy even if the surface winds are borderline strength (below 20 knots) near Point Conception.
* A check of the visible satellite imagery during the day, and the fog imagery at night would show stratus off the coast, and an advancement of that stratus up the coast from the south. The cloud will usually form off of San Diego, and progress up into Orange County. Often, fingers of stratus are visible curving around the outer Channel Islands, evidence that winds have increased off Point Conception.
* Coastal wind reports should back into the SE quadrant overnight, and may even be ESE in direction during the morning.
* Occur all year long, but most eddies form during the spring and summer months. During winter, the upper troughs and associated surface fronts are often too strong for an eddy, causing northwest winds to pour out across Southern California, instead of an eddy forming.
* Onset can chill air temperatures, by as much as 15 degrees F along the coast!
* A rapid deepening of the marine layer, swift inland advance of stratus, and associated cool marine air, birthing the radical cooling.
* Sluggish coastal clearing, and the likelihood of an overcast day is enhanced.
* Warmer sea temperatures. If warm water exists offshore to the south, the potential for that water to be pushed shoreward on southerly winds is good, raising sea temperatures by two to six degrees along the south coast.
* Slight rise in coastal water levels. A minor increase in water levels can be expected as south winds tend to pile water towards the coast. This may add to coastal erosion if accompanied by large long period swell, and high astronomical tides.
* A disrupted sea/land breeze pattern. The typical offshore morning winds may be from the southeast, and the normal westerly afternoon winds would be southerly in direction. Winds usually veer to the SW through the day, sometimes backing-off all together for an evening glass-off; only to return after midnight, provided the necessary conditions continue.
Uploaded with the Flock Browser