There’s an interesting feature on the weather maps/charts this morning which will affect SoCal before the week is through.
FRIDAY AND SATURDAY WILL BE THE FIRST REAL SANTA ANA DAYS OF THE SEASON. — Southwest California Area Forecast Discussion, National Weather Service Los Angeles/Oxnard, CA
Santa Ana winds blow from the east through the mountain passes. They’re ‘katabatic’ or downslope winds, warming as they descend. Just like putting your finger over the end of a garden hose, squeezing these winds through the passes makes them stronger.
Starting Friday we’ll begin to feel hot, dry, strong wind.
In this area, which hasn’t seen widespread rain for months, Santa Ana winds represent extreme fire danger!
The BUFKIT chart at the top of this entry (click to get a larger view) is from the Monday 9/30/13 12Z GFS model for John Wayne Airport here in Orange County.
The timeline is opposite of what you expect. Today is on the right. As you look farther left you look farther into the future. The orange line is temperature, green line dew point and shading humidity.
On Friday morning the dew point plunges. High pressure is nosing into the region which will turn the wind from west to east. A very shallow layer of high humidity in place Wednesday and Thursday, the marine layer, will disappear. Its low clouds and gloom will be gone.
I did some quick back-of-envelope calculations from the chart at the top of this entry. With a Friday high of 88° and dewpoint of 16°, the relative humidity will be around 7%. That’s extremely dry… crazy dry.
The danger of fires quickly spreading will be high. I expect to see Red Flag Warnings in place Friday.
Every area has its weather peril. This one’s ours.
It’s scary because fires can quickly grown beyond the ability to control them. It happens every year.