CHICO, Calif. - In the fall, the mountains get much cooler while the valley can still be pretty hot. The jet stream also drops south which means we see more weather systems passing near our region. A strong change in temperature and pressure can cause strong winds and the fact that these winds are offshore winds and downslope winds can emphasize the strength of them via the Venturi Effect and gravity.
When you think of fall in California you probably imagine, rain, cooler weather, and the beautiful colors of mother nature.
But Fall often brings devastation to California. The Camp Fire, the Tubbs Fire, the Thomas Fire are just a few reminders.
Why is fall so dangerous?
In the fall, there are several factors that change compared to summer. The mountains get a lot colder at night while the valley can still see some pretty strong heat in October and sometimes even in November.
This cold air rushes down the mountains at night and the stronger the temperature difference between the valley and the mountaintops, the stronger the wind could be. This temperature difference is not as strong in the summer. In early spring, it's not a concern since vegetation is not dry.
Pressure difference also plays a factor in strong winds in the fall. In the summer, the jet stream is very far north. So all the strong low-pressure systems are usually too far north for us to worry about. But in the fall, the jet stream begins to drop south. This allows for much stronger low-pressure systems to get closer to California. When they enter our region, in opposition to a strong high-pressure system, the stronger the pressure gradient will be. A pressure gradient means how much pressure changes over a distance. The stronger a pressure gradient is, the stronger the wind will be. When low-pressure systems pass to our East they cause strong downslope and offshore winds across California. It is often referred to as offshore wind season. A low-pressure system nearing our area Wednesday through Friday is a big reason for stronger winds this week.
There are other factors that play into strong winds in our area not just in fall but at any time.
First, is the fact that downslope winds can be dry. When air travels downslope it can become dry. Rising air cools and condenses (meaning it becomes more moist) but falling air warms and drys (losing its moisture).
Also, air traveling downslope into the valley can often funnel into canyons and become a bottleneck. This causes much stronger winds locally and is known as the Venturi Effect. The Jabo Gap winds are a popular example of this locally and happen every night.
Climatologists believe the evidence shows wildfires will continue to get worse in the fall as significant rainfall has been arriving later, in recent years.