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Greetings and salutations, this is JJ Brost with the National Weather Service in Tucson,
AZ. Today is Friday, February 28th 2014 and this briefing is valid through Saturday, March
1.
You may hear the term “Atmospheric River” often over the next few days so I thought
you might like to know what it means. As it turns out, right now an Atmospheric River
is heading toward southern California and Arizona, and that’s why we are concerned
about heavy rainfall, or snow in the mountains, through Sunday morning.
I hope this briefing helps to educate you on this weather phenomena, and describe the
impacts from Atmospheric Rivers. I’m going to start by showing you some satellite
images of the Atmospheric River. This is a recent water vapor satellite image and it
show moisture in the air. The white to purple areas show moist air, and the red or brownish
areas show dry air.
This type of satellite data is great for detecting low pressure systems and Atmospheric Rivers.
I’m going to show you a loop of the images and then pause the loop every few seconds
and explain a few key features. Then I will replay the loop so you can see how everything
evolves over time.
The first image was taken late Thursday evening and the loop stops on Friday morning. -
I want first want to draw your attention to the low pressure system spinning several hundred
miles off the coast of California. Notice how the atmosphere is spinning in a counter
clockwise fashion and in toward the center of the low pressure (denoted by the blue “L”).
The white lines and arrows are there to help you visualize this counter clockwise circulation
pattern. -
Now this is very cool. The low pressure system becomes so tightly wound that you can see
what looks like the eye of a hurricane. A hurricane is a massive low pressure system,
but I want to make sure you understand this particular low pressure system is NOT a hurricane.
It just kinda looks like one from satellite. The eye does mean something though. In this
case, the eye means this low pressure system is strong and likely going to intensify. -
In fact, the system is so strong that it is able to pull warm and moist air up from the
equator or the tropics. This moist air streaming up from the tropics is the Atmospheric River.
I’ve tried to outline the atmospheric river in green here. -
Once that low pressure system moves closer to the coast, that atmospheric river moisture
will stream into Southern California, and eventually Arizona. In fact, by the end of
this loop, some of that moisture is already evident over Arizona. We just need some of
the energy from the low pressure system to start turning that moisture into clouds and
rain. That energy should arrive late Friday night into Saturday morning.
Atmospheric rivers are important weather features because they are packed full of moisture.
How much moisture you ask? Well, a strong atmospheric river can pack an equivalent amount
of moisture as 7 to 15 times the amount of water that flows out of the mouth of the Mississippi
river. Just to be clear, we are not referring to all of the water in the Mississippi river,
just the water that flows out of the mouth. Still, you can imagine just how much water
is packed into this atmospheric river.
They primarily impact the west coast of the United States, but they can and do push further
west at times. When they do occur, on average they produce 30 to 50% of the annual precipitation
across the west coast states.
This often results in flooding, mud slides or land slides. This image is from a strong
atmospheric river event in January, 2005 that produced widespread heavy rain. The heavy
rain caused a land slide in La Conchita, California. Unfortunately, 10 people were killed in this
land slide.
What about impacts to Arizona. Well, we will get some rain and portions of the state are
likely to experience localized flooding.
Now, not all of the impacts from atmospheric rivers are bad. When we experience these widespread
heavy rain events in the late winter or early spring, they often lead to a beautiful desert
green up.
The only danger is if we then proceed to dry out for several months. Then all of the new
vegetation triggered by the heavy rain can become fuel for spring time wildfires.
This image shows our precipitation forecast from Friday, February 28th through Sunday,
March 2nd.
You can see we are expecting everybody to get at least some rain with this system. For
southeast Arizona, that rain may start late tonight or early Saturday morning, but should
be in full swing by mid day Saturday.
The central portion of Arizona should be the big winners in this case with many areas looking
at 2 to 3 inches of rainfall. Locations along or south of Interstate 10 should be in the
one half to one inch of rain range.
Thank you for watching this presentation. I hope it helped you understand atmospheric
rivers and why they are important. Please use our current forecast information
for the latest rain or snow amounts. You can also find us on Facebook, Twitter
and Youtube.