What makes a Sundowner wind event different than a Santa Ana wind event? I contacted Ryan Kittell of the National Weather Service earlier this year, and this is what he had to say:
Sundowners are a different beast caused by a different weather pattern than Santa Anas, but they do act in a similar way.
Santa Ana winds are generally a northeast to east flow formed by strong surface high pressure over the great basin. This high pressure, when strong enough, reverses the normal pressure gradient where pressure is usually higher over the Southern California coast. Air tries to flow from high to low pressure and as a result, air is forced from the eastern deserts toward the coast...and through the passes and canyons of the Mountains to the east from Ventura County down into Mexico.
The passes that typically get it the worst are the Cajon Pass, Banning Pass, Santa Ana Canyon, and the Santa Clara River Valley. Because of the orientation of the passes and canyons, the Santa Barbara South Coast is not a favored location to get these winds...though the Northeastern Santa Barbara County Mountains do get some of these winds.
Sundowners are also formed by building high pressure, however, this high pressure is much weaker but much closer to the Santa Barbara South Coast.
Usually when the winds are fairly strong along the Central Coast of Santa Barbara and San Luis Obispo Counties to the north, high pressure builds near Santa Maria behind the Santa Ynez range. As this high pressure builds, the same rule comes in play...air flows from high to low pressure. Air pushes through the passes and canyons of the Santa Ynez Range, especially through the Gaviota Pass, San Marcos Pass, Montecito foothills, and some smaller canyons. The wind that results is more of a northwest to northeast wind and is usually quite strong.
Although these two wind phenomena are formed from high pressure they occur in different places for different reasons.
The Sundowner winds affect a much smaller area than the Santa Anas. The timing of these winds differ as well, where the Santa Anas are usually strongest in the morning hours, Sundowners are usually strongest from sunset to sunrise.
The other thing to note, because these two wind phenomena require high pressure in two completely different areas, it would be extremely rare to have both occur at the same time.
Original post: 07/05/2008