If you have been reading major California newspapers over the past few weeks, you have seen a number of stories about a “monster” El Niño event headed our way this winter.  Most of those articles address whether such an event would end California’s four-year drought.  A small handful of them address what I consider a more interesting question: could record precipitation following a period of record drought result in major floods?

Note: I am not a climate scientist, hydrologist, or historian by trade; I’m a water and flood control attorney.  But I do know the Central Valley flood control system pretty well, and I am curious about whether there is really a connection between the El Niño phenomenon and the likelihood of flooding.

What is El Niño?

El Niño is a weather pattern characterized by unusually warm ocean temperatures in the Pacific, with weather consequences around the world including increased rainfall in the southern part of the United States.  You can get much more information on El Niño events here or here .

The Last Time We Had a Monster El Niño

Many articles discuss the 1997 El Niño event, because it was one of the strongest El Niño events in recorded history.  Indeed, that El Niño produced significant precipitation in California.  During the 1998 water year (which ran from October 1, 1997 to September 30, 1998), statewide precipitation was about 170% of the historic average.  (By comparison, during the 2014 water year the state received about 55% of historic average precipitation; and through July 1, 2015 the state has received just under 73% of historic average for the 2015 water year.) See the California Data Exchange Center for more detailed historic hydrologic data.

Did California Experience Major Flooding During the 1997 El Niño?

Also in 1997, California experienced one of the most significant flooding events in modern history.  At first glance there appears to be a connection between that flooding and the record El Niño conditions.  But as it turns out, they weren’t actually related.  The historic flooding in early January 1997 (which caused over $2 billion in damages, over 120,000 people evacuated from their homes, and more than 30 breaks on federal project levees) resulted from a three-day period during which 30 inches of rain fell over Northern California as part of a “pineapple express” storm event.  What’s more, reservoir levels were well above average coming into January 1997 after a wet December.  To add insult to injury, the early January event was followed by another significant storm in late January, when not all flood control facilities had recovered from the early January event.

The 1997 El Niño event didn’t actually develop until a few months later.

In fact, the other well-known significant flooding event in California’s recent history, in 1986, also was not associated with an El Niño year.  Flooding in 1986 also resulted from a “pineapple express” storm, which is typically warm with relatively high snow levels, and which is characterized by enormous amounts of precipitation over a short period (a few days), straining and in some cases overwhelming flood protection facilities.

El Niño events, by contrast, tend to be associated with colder storms with snow levels at average or even below average levels. What’s more, typical El Niño events usually bring wetter winters to Central and Southern California (but not always, which makes the actual effects of an El Niño event difficult to predict).

The Bottom Line

The bottom line is: whether the coming El Niño event leads to major flooding depends on a number of factors impossible to predict at this point, including:

• How early in the water year rain falls.  Our major reservoirs are very low, in some cases at historically low levels, after four years of record drought conditions.  If fall and early winter rains fill them up, it is more likely that continued rains or large storms in early 2016 will overwhelm flood control facilities.
• Whether there is significant rainfall in a short period of time.  These are the conditions that have led to major flooding in the Central Valley in the past, but these kinds of events are not typically associated with El Niño.

The best case scenario, of course, is that the El Niño event does materialize and fills up the State’s reservoirs (alleviating our drought situation) without overwhelming the flood control system (and causing a significant flood event).  Our fingers are crossed.

That Begs the Question(s)…

1. Did the 1997 floods lead to changes in the flood protection system to protect against future pineapple express storms?
2. What is the flood protection community doing to address future uncertainties related to water and flood control in the context of climate change?

Stay tuned for answers in future blog posts!

Sources/Citations

1. California Data Exchange Center
2. Flood Emergency Action Team Final Report, May 1997
3. “Flood Warnings: Responding to California’s Flood Crisis,” California Department of Water Resources, January 2005
4. “The California Winter of 1997-98: Status Report for Federal Emergency Management Agency,” Western Regional Climate Center, March 1998
5. “The El Nino Winter of ’97 – ’98: National Climatic Data Center Technical Report No. 98-02,” National Oceanic and Atmospheric Administration, April 1998