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As we transition into the rainy season, there is a tremendous amount of speculation on the potential impact of the current El Nino weather event. Some are even calling this the “Godzilla” of El Ninos, predicting it to be one of the strongest on record. Those of us who deal directly with stormwater compliance are especially concerned, due to the wide-ranging effects that an extra wet rainy season can have on our workloads, water quality, and compliance with stormwater regulations. There’s no getting around it: when the skies open up, there is a ton of work to do before, during, and after the rain event.

Rainy Days

I suspect that everyone will be relieved when we do, indeed, get significant rainfall this year for the obvious reasons related to improving California’s dire drought situation. However, for some of us who work in the construction and industrial sectors, anxiety rises with the onset of winter, as rain days lead to lost production. Typically, we plan for rain days in our bids and in our operations plans, based on averages and norms. But it’s often too early to predict a spike in rainfall for the coming year. During a typical rainy season (Oct-April), the norm suggests that we plan for approximately 10 rain days per season and a rainfall total of approximately 15”. So, what type of variance can we expect from the norm this year? The historical record tells us that during the ’97-’98 El Nino event, we experienced 34 rain days between October and April and in ’04-’05, during a milder El Nino, we had 27 rain days. This is well over double the norm and don’t forget that we must also account for post rain day periods in which conditions are too wet to work. From this, we can see that a strong El Nino season has the potential to drastically increase our rain days on sites and facilities to upwards of 40 days.

What impacts will El Nino have on the drought?

I think one big question surrounding the rainfall topic is: Will this El Nino season help end the current California drought? First, let’s take a quick look at rainfall totals. The average annual rainfall in a given year (July 1st-June 30th) in Los Angeles is 14.81”. The past four years have averaged about half of this at 7.23” per year. During the ’97-’98 El Nino year we experienced 31.01” of rainfall, slightly double the 138-year average and approximately 4 times the annual rainfall for the past 4 years. During the ’04-’05 El Nino year, we experienced the second most rainfall in recorded history at 37.96”. So, the data suggests that during strong El Nino years, we do receive significant amounts of rainfall above and beyond the norm. It’s difficult to predict if the drought will begin to end this season but the NOAA predictor can help us understand this a bit more. If you plug in the precipitation required over the next 6 months to end the drought for the Los Angeles area, you get a figure of 15”-18”. Looking back to the previous El Nino seasons, specifically looking at the rainfall over the 6-month period of November to April, things look positive. During this 6 month period in ’97-’98, Los Angeles received 26.08” of rain. During the same period in ’04-’05’, 20.23” of rain fell. So history tells us that this El Nino has the potential to end the drought. Of course, you should take all these predictions and forecasts with a grain of salt; it is the weather after all and I am not a climatologist. But I’ve got a hunch this El Niño is going to meet expectations and we’re going get rain…lots of it!