Why All the Heavy Rain this Winter? February 20, 2017

12-Hour water vapor satellite loop

If you’re wondering why we’re getting so much heavy rain in Southern California, and the state for that matter, it has everything to do with what’s called an Atmospheric River. An atmospheric river is a long, narrow band of water vapor in the sky, usually 250-350 miles wide.  On their own they bring beneficial rain and increase the California snowpack.  The rain forms because as the river makes land, the glide from sea level to the mountains lifts the moisture, condenses it and turns it into rain or snow.  In a typical year these rivers account for 30%-50% of the west coast’s annual rainfall.

 

But this season has been anything but typical.

On Jan. 22, 2017, 2.67 inches of rain fell on downtown Los Angeles — that’s the most rain in one day since December 2010.  And an all-time one day record rain of 3.97 inches fell on Long Beach, causing portions of I-710 to be under water. And, of course, we’ve seen what has been going on at the Oroville Dam.

I-710 in Long Beach

Laurel Canyon Downpours, Courtesy: Toni Guinyard NBC4

We’ve seen historic flooding in the state because of a few unique factors.  First, the amount of moisture in this winter’s atmospheric river has been equivalent to the average flow of water at the mouth of the Mississippi River.  Last week, a satellite image showed the entire atmospheric river from Japan to Northern California.  If the water vapor has its origin near the Hawaiian Islands, it’s called a Pineapple Express.  Second, since January the atmospheric river has stalled and focused mainly on the state of California. Last, the intensity of our storms has been enhanced by deep areas of low pressure and cold fronts moving though at the same time.  It all makes for record-setting rain.  But we have seen these patterns in the past.

The Great Flood of 1862 devastated Sacramento and Southern California.  This atmospheric river stalled for 30 days and brought wave after wave of heavy rain.

K Street, looking east from 4th Street during the flood of 1862, Sacramento

And the devastating flood of 1938 killed dozens of people in Southern California and destroyed more than 7,000 homes.  This event prompted the transformation of the LA River into a series of channels.

March 2, 1938: Flooding at West 43rd Place near Leimert Boulevard.

What makes atmospheric rivers hard to forecast from a seasonal perspective is they can arrive in El Nino, La Nina or neutral patterns.  They are most common in significantly strong El Nino patterns.  In 2016, atmospheric rivers brought record-setting rain to the Pacific Northwest and started the work getting Northern California out of a five-year drought.

Friday’s atmospheric river aimed toward Southern California

Advertisement

California – The wettest start since 2010… And more on the way January 31, 2017

I’m getting asked a lot of questions about our weather.  Why are we getting so much rain?  Why is it a year delayed from last year’s strong El Nino?  Is the drought over? What’s going on?

Four months into the water year we have the wettest start ever recorded.

To answer these questions I first have to give some perspective on climate in California and probabilities.

In case you don’t know, California is a state of extremes when it comes to precipitation.  Throughout recorded history we’ve alternated between drought and excessive rain.  Los Angeles is a good example of these extremes.  While we average 15 inches of rain a year, we are rarely near this amount.  If you look at the past 30 years, we’ve gone from extremely dry years to well above average years.  In the end, it averages 15 inches.

Next snow survey is February 2nd and we’ll near 200%.

The one question that is a little more difficult to explain is the why we’re getting the heavy rain this year instead of last year.  Last winter we were prepared for the potential of heavy rain. The reason we thought this was because in all of the significantly strong El Nino climate patterns, Southern California got soaked.  The odds were in our favor.  But the rain didn’t come.

The 2015/16 water year only recorded 6.57″ of rain.

What is fascinating this year is we are in a weak La Nina or neutral weather pattern.  Both of these patterns slant dry for the state of California and especially for Southern California.  The probability our state would be dry this year is 70%, leaving a 30% chance we’d be above average.  And, if you really study this image, there isn’t a weak La Nina pattern that is well above average for Southern California.

Instead what we are seeing is the wettest winter since 2010.  The storms of the past two months have recouped 37% of the state’s five-year snow/water deficits. And for the first time since January 2014, no place in California is in an exceptional drought.  Precipitation is more than 200% of average and the Sierra Snow pack is looking great!

So what gives? Why the two extremes in outcomes?  I could give the answer, “That is how the weather works sometimes.” And this reply wouldn’t be completely wrong because even with an excellent forecast for a 90% chance of rain, there are days it’s dry.

But the answer is a little more complicated.

Last year the water was so warm in the Pacific that it affected the jet stream and moved all of the heavy rain to the Pacific Northwest and Northern California.

This winter we’ve been aided by numerous atmospheric rivers or a Pineapple Express.  This type of pattern can occur at any time regardless of an El Nino, La Nina or neutral pattern.  These rivers of atmospheric moisture are responsible for much of the devastating flooding in California’s history.  And it played a big part in the flooding on Jan. 22.

Long Beach received the most rain ever recorded in a 24 hour period, 3.97″.

Most people don’t know about the mega flood of 1862, which was caused by an intense atmospheric river. It has a return period of 100-200 years, meaning it will happen again.

And this leads me to climate change.  The latest research shows that in a warming planet, droughts will become more severe and heavy rain events will occur more often.  Before we started getting the rain in December, Southern California went through the driest five-year stretch ever recorded.  And, if you look at tree ring data, it may have been the driest stretch in 1,000 years.

But if you look at the history of flooding in California, intense flooding events occurred before the industrial age and atmospheric rivers are responsible for most of these events.  Attributing climate change to future flooding events may be hard to do.  Upcoming research will need to address how a warming world affects atmospheric rivers.

Going forward, the key for our drought, especially in Southern California, is we need this kind of winter pattern for two more years so we can get out of a drought cycle.  And, of course, I’ll keep you posted.

 

Southern California is Making a Comeback! February 24, 2016

It’s the fourth quarter and SoCal is down by three touchdowns. The clock is ticking and 17 million people are wondering if this will be a disappointing finish.  Don’t give up yet, this is the rain forecast for Sunday and Monday with more on the way.

With one month to go before one of the strongest El Ninos on record fades, the question remains: How much more rain and snow is on the way, and why has Southern California been so dry?

Let’s start by going back in time and look at the El Nino rain forecast for winter. A couple of things stick out. First, notice how Southern California, Texas and Florida are all forecast to receive an above average amount of rain. The Pacific Northwest is dry and Northern California is a toss up. All of the previously strong El Ninos showed no correlation to NoCal, it was 50/50.

This is what has happened so far this winter. The bonus is most of the water sheds and reservoirs, which are located in Northern California and provide water for the entire state, are filling up. But Southern California has been incredibly dry, prompting people to ask: Where is El Nino? I call this forecast the 17-million person “bust.”

What is fascinating to me is what is happening this winter in the Pacific Northwest. Seattle, Washington and Portland have recorded their wettest winters ever! Record keeping goes back to 1894.  So the question is why.

The El Nino-fueled moisture is in the Pacific, as predicted, but the jet stream has moved the rain into the Pacific Northwest and Northern California. The dry slot has been Southern California and Southern Arizona. But why is this blocking ridge of high pressure here?

You’ve heard us say, “Every El Nino is different.” And that is certainly the case this year. The warmest water in the Pacific Ocean isn’t near South America like it was in previously strong El Nino patterns. The warmest water is farther west, south of the Hawaiian Islands.

That warmth is creating a large area of low pressure and rising air.

In turn, the sinking air has created a large area of high pressure keeping the rain away from Southern California.

The rain is back in SoCal March 6th and the longer term climate models have a wet March but these same models also showed a furry of storms in February and obviously that hasn’t happened. So, what do you think will happen?  You can Tweet or Facebook me at: @anthonynbcla.

Time is running out, the clock is ticking and 17 million people are wondering if SoCal can make a come back.

Allison Ten Years Later June 6, 2011

About 73,000 homes were flooded with property damage at $5 billion.  Allison was the only tropical storm ever to have its name retired.  Sixty-five percent of the homes flooded during Allison were not in a mapped 100-year flood plain.  This is exactly why flood insurance is vitally important. 

Check out house A.  It’s in a 500-year flood plain and has .02 percent chance of flooding this year.  But if you own that home 30 years, it has a 6 percent chance of flooding. That’s not very high, but when you get a storm like Allison you find yourself with that “once in a 500-year flood event.”  If you are out of the 100-year flood plain, flood insurance is relatively inexpensive. 

To view past weather quiz answers click here:

Past Weather Quiz Answers

Turn Around Don’t Drown January 7, 2011

The Texas Floodplain Management Association came out with a “Turn Around Don’t Drown!” 2011 calendar.  The artwork is by elementary and middle school students from across Texas.  The pictures show the imagination of kids and the danger we face in SE Texas.   Each month features good driving and flooding tips.  If you want one The Texas Floodplain Management Office has them.  Go to: http://www.tfma.org/ for more information.

Will Your Home Flood? August 27, 2010

This is an excellent graph that shows the risk of flooding at your home.  Most of you live in a 50-year floodplain, but, as you can see, you still have a 6 percent chance of your home flooding in the span of 30 years.  We see it happen all the time on the news: a homeowner doesn’t think their home would ever flood, but they are standing in front of their flooded home.  I can’t stress enough how important it is for all of you to have flood insurance.  If you live in a 500-year floodplain, the insurance is fairly inexpensive.

To view other weather quiz answers click here:

Past Weather Quiz Answers