October ebbs and November comes marching in with shorter days and generally cooler temperatures. At least that is usually the way it works.

However, a weak area of high pressure is slated to overspread Kern County and most of the state this “Fall Back” weekend. That means temperatures will be rising into the mid 70s making for a very pleasant couple of days. Early November is looking particularly nice with highs near 80 through about Wednesday. But more storms are sure to invade our region in the months to come with the advent of El Nino. Above normal storminess is forecast in the west, especially January through March.

I have been developing a Storm Index for California that quantifies the damage potential of storms affecting large areas. This involves looking back at past storms and their impact as well as objective techniques to estimate how powerful a weather system is by virtue of only two parameters- wind and rainfall.

You may remember our first significant storm of the season that hit the state on October 13th and 14th. It was dubbed “Storm Poseidon”, with any subsequent storms that rate high enough to receive a Greek god’s name. But what is that magic threshold? How strong must the winds be and how much precipitation must a Pacific storm deliver to be thus classified? Borrowing from the Bay Area Storm Index (BASI) concept developed by Jan Null to quantify storm damage potential in the San Francisco region, I have modified the evaluation process to encompass a broader and more precise estimation of impact that can be both forecast and observed.

My technique, called the California Storm Index (CSI), is calculated using 5 variables. They are: Average wind velocity, Peak gust velocity, Average 3-hour rainfall, Maximum 3-hour rainfall, and finally the 3-hour Flash Flood Guidance (which is the amount of rainfall required to produce flooding, based on current ground saturation). Additionally, none of this is triggered until the total area affected by an impending storm covers at least 50,000 square miles and the acutely affected portions of the state exceed 5,000 square miles. So, not every Tom, Dick or Harry low pressure system is graded on its strength. Finally, the CSI comes in 2 parts: Wind Damage Potential and Rainfall Damage Potential.

Big storms are evaluated using empirical equations I’ve authored and the output should describe how damaging the wind, rainfall or combination of both will be. The damage potential calculation utilizes exponential factors, so when a storm produces particularly strong weather the CSI values will be in proper perspective. That magic number is 35. In the days to come I will be featuring wind and rain forecasts of big storms with reference to that number. Wind or rainfall values of 35 bring about “Class 1 Damage Potential”. Additional increments of 35 raise the stakes one class at a time, i.e. a CSI of 70 is Class 2, CSI of 105 is Class 3, etc. I will publish definitions for each progressive wind and rain damage potential class in a future blog. BTW, Storm Poseidon rated a Wind Index of 48 (Class 1 Wind Damage Potential) and a Rainfall Index of 91 (Class 2 Rainfall Damage Potential).

So let the storms come and be evaluated. It’s not so much about which storm takes home the gold, but rather how prepared Californians are for fierce winter storms. After 3 years of drought, El Nino rains will bring much anticipated relief but with a calculated collateral downside. The CSI is being developed to better measure Mother Nature’s nasty side.