A subscriber wrote to ask us about strong winds that had developed after midnight at Port Hueneme in Ventura, California.
Since these winds occurred after midnight his thought was that these might be katabatic winds. Katabatic winds develop as air on a hilltop cools after sunset causing it to become more dense. Then gravity takes over and pulls the air downward, causing nighttime drainage winds. This is a plausible explanation since Heuneme sits on a plain. However, the surrounding hills are not very tall and are not in close proximity so that explanation didn’t quite fit.
And if these were katabatic we would expect the winds to occur most nights. So we pulled up the archive and found only 5 nights in the month of June when these winds developed. So we did a little more digging and found the likely culprit to be our friend, the Southern California Eddy. you will see us discussing the size, position and timing of an eddies development frequently in our forecasts because they have such a major effect on winds for So Cal.
Large eddies frequently develop along the So Cal. coast called the Bight. As you can see from the map (above) the coastline jogs inland starting just west of Santa Barbara. When we have a combination of strong NW coastal winds the winds inside this Bight will start to rotate, especially if we get some offshore flow and/or an upper-level Low sits over the area and provides some spin from above. These eddies can be small and short-lived or grow to fill the Bight. They usually grow overnight and will shrink during the morning. When they grow large the winds moving into Port Hueneme will be from the southeast.
So based on what we found in the data it appears the likely explanation is the formation of large eddies that develop near the coastline. Thanks for the observation and the question. If you have other questions please send them in the feedback.