1 December 2011
The winds are already gusting over 50 at Bullhead City Arizona and it looks like a major Santa Ana event is brewing for Southern California. The winds may exceed 50 mph across much of the Southland on Thursday and the fire danger will be extreme. These winds are an interesting phenomenon, and there are a lot of misconceptions about their causes. Here is a basic primer on the causes, and there are more than one!
You may think that these events happen when the winds blow through the canyons of the northwest-southeast trending coastal mountains. This is indeed the case but, it takes more than just that. You do need a strong pressure gradient that is blowing parallel with these canyons, but this alone does not mean a damaging wind event will occur.
Researchers at UCLA have found that sometimes a strong pressure gradient will develop across the mountains, and even though it’s channelling winds through the canyons, the winds just don’t get that strong. The researchers ran some high-resolution numerical models and developed a “Santa Ana” index to look at just what factors were involved in the strongest Santa Ana events. What they found, is that desert temperatures AND a strong pressure gradient seem to be the key.
It seems that if the deserts are cold, then the pressure will be high, compared to the warmer (less dense) air at the same altitude along the beaches. This cold, heavy air can be pushed by the existing pressure gradient over the mountains, where it then runs down hill to the coast! This added factor tends to increase the low-level winds greatly and damaging winds occur over a wide area along the coast.
Cold air is heavy, and when it flows off a high plateau, strong “katabatic” winds can develop. I experienced one at Pond Inlet in the Canadian High Arctic, and Antarctica regularly sees katabatic winds over 100 mph. You can see one for yourself if you open the fridge in are feet on a warm muggy day. Notice that cloud that streams out of the fridge and falls to your feet? In a sense the Santa Ana’s are apparently a combination of pressure gradient, winds being channeled through mountains and a katabatic effect.
I found a fascinating paper on this by Mimi Hughes and Alex Hall at UCLA on all of this, and it’s well worth a read for weather geeks. Their research shows that these events are strongest in December and the current temperature at Edwards AFB as I write this is 51 degrees with winds gusting to 39 mph. LAX is 58 degrees, but the deserts will get a lot colder overnight and the winds will likely increase. The pressure gradient alone is very intense tonight and it is now December.
Things should improve by Friday, but I suspect you will be seeing images of Southern California on the national news Thursday..
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