Climates and Microclimates – Part 5
Southern California
By Rose Marie Kern
California is an amazing state: ocean shores, deserts, mountains, lush valleys, bays, and earthquakes. The weather is more predictable than the central plains, but also more diverse within areas as close as 50 miles.
The northern third of this long state has weather more akin to that of Oregon and Washington – so we will leave that for a future article and concentrate on the central and southern portions.
Let’s look at the geography first. A strip of beaches, bays and low hills stripe the westernmost portion, then a line of mountains from north to south. Two long valleys lie east of the mountains, the San Joaquin and the Sacramento. East of those is desert.
Most of the weather systems move from west to east, or northwest to southeast, crossing each of these topological areas in turn.
One of the most famous systems of the west coast is the “marine” layer. A weak high pressure out over the ocean contributes to a morning layer of IFR conditions, low clouds and fog within 50 miles of the coast that usually burns off by about 10am. This is prevalent from late spring through early fall.
Irregularities in the coastline determine which locations go IFR first and keep it longest. Up by San Francisco for instance, the fog rolls into the bay and builds faster to the east and north so that airports south of the bay will not be fogged in as quickly or as densely as those to the north and northeast. The exception is San Francisco itself which sits on a low spit of land area between the bay and the ocean – frequently causing it to be enrapt early.
High resolution satellite imagery can help to determine the density of the marine layer. Frequently you can see the tops of Santiago Peak and the other mountains between Carlsbad and John Wayne Airports – how much of them you can see reveals the thickness of the cloud deck.
During the winter months a trough forms in northern California to Nevada which forces a southwesterly flow into central and southern California. This creates a slightly unstable air mass and low stratus at the Marginal VFR levels. Though not as low as the summer marine layer, it still provides significant mountain obscuration.
This system also flows into the San Joaquin and Sacramento Valleys, which are essentially bowls with no drainage points, causing fog that is trapped under a temperature inversion layer. The low cloud condition can stay intact long after the original trough has moved on, frequently creating IFR lasting for weeks. It will only dissipate when a new system comes along and scours out the bowl.
A microclimate that surprises new west coast pilots, called a Catalina or coastal eddy, occurs when winds flow from northwest to southeast paralleling the central coast. The coastal mountain chain stops the airmass from moving inland and the offshore islands squeezes and accelerates the airmass. Once it passes Oxnard there is a break and the winds swirl and eddy into the LA Basin. Depending on how saturated the airmass is, it can cause diverse surface winds or sudden low ceilings.
If you have not flown on the west coast, you may not realize how much the mountain range acts as a tangible barrier to weather. Frequently you can have AIRMETs for IFR and Mountain Obscuration on the coast, but stay east of the range and you will enjoy fine VFR.
One of the effects of wind through the mountain passes is an eddy effect that makes landings difficult. At Ontario the wind indicators at one end of runway 8/26 may show an easterly windflow, while the one at the other end shows winds from the west!
The best known feature of California weather is known as the Santa Ana Winds. This wind stream can be shallow, only a couple of hundred feet thick or extend upwards several thousands of feet.
Created when there is a trough of low pressure off the coast and a surface based high in the Great Basin of Nevada and Utah, the Santa Ana winds flow from the northeast to the southwest. It is a stream of air that careens toward the mountain chain, heated by compression as it flows downslope and accelerates as it funnels down the mountain passes.
Pockets of cooler denser air on the surface may falsely signal calm wind conditions, but what is really happening is this warmer and less dense river of air is overriding the local surface conditions. An unfamiliar pilot will innocently take off into the bright morning for a local training flight, then about 80 feet off the ground he will be slammed by a high northeasterly winds, windshear and turbulence
Many times you can see this effect through the METARs. Ontario and Riverside will placidly report surface winds calm, while pilots at nearby Chino suffer 45 knots gusts. Since these winds flow from deserts in the northeast, they dry out the airmass and the vegetation, creating conditions ripe for intense forest fires.
For the most part the eastern side of California is the easiest to fly in terms of terrain and weather. The few thunderstorms which occur are frequently due to a low pressure centered over the Baja that flings moisture northward. The moisture tends to drift at midlevels until afternoon thermals rise off the desert to create towering cumulus.
As you can see from the information above, it is not frontal activity that determines the weather of central and southern California nearly so much as the placement of high and low pressure systems. Knowing where they are and their effects on terrain makes planning your flight a lot easier.
Rose Marie Kern has worked in ATC for over 25 years. The depth of this article was greatly enhanced by the personal insights and experiences of former Riverside Flight Service Specialist Charles Andries. To ask Rose a question contact her at author@rosemariekern.com