On March 25, 2014, WFO Norman will join the Impact Based Warnings Experiment.
This video will explain the experiment, and show you how our warnings will be changing.
The experiment began in the National Weather Service’s Central Region, but this year
is expanding to include other areas.
The counties shaded in purple will be issuing Impact Based Warnings this spring.
The Norman office will join our neighbors in Dodge City, Wichita, Tulsa, San Angelo
and Lubbock in the impact based warning experiment beginning on March 25th.
That’s when you’ll see the changes appear in all of our severe thunderstorm and tornado
warnings.
We look at impact based warnings as being an enhancement to our current warnings versus
a major change.
And we hope these enhancements will make our warnings more effective and easier to use.
The special tags that appear at the very bottom of the warning should make it much easier
for broadcasters and others to find the most important information, and to help prioritize
when you’re dealing with multiple warnings.
The enhanced wording in the impact based warnings will give forecasters a way to express confidence
in the potential impacts of a storm, and detail the level of risk.
We hope that impact based warnings will allow us to make our warnings as effective as possible
without making substantial changes to the existing structure, and that the words we
use can help motivate people to take the proper precautions when they’re threatened by a
tornado or severe storm.
As far as the Norman NWS office is concerned, we want you to know that our warning philosophy
will not be changing as the result of impact based warnings.
We will issue warnings for the same reasons we did before, and do not plan to make any
significant changes to how we operate during severe weather.
We are also not changing our thresholds for issuing tornado or severe thunderstorm warnings.
The Norman office is fairly conservative when it comes to tornado warnings, and we have
no plans to change that.
Just because we have different categories of tornado warnings does not mean we will
issue more of them.
In fact, you should not expect any change in the number of warnings we issue because
of our involvement in the impact based warnings experiment.
The changes you’ll see will be in the text of the warnings.
Let’s take a quick look at how our severe thunderstorm warnings will be changing.
As you can see, the new impact based warning looks very similar to the traditional warnings
you’re used to.
The two main differences are the addition of three bullets in the middle of the warning.
Those bullets describe the expected hazard – whether it’s large hail, damaging winds,
or both – the source of the warning, which could be either radar indication or storm
spotter reports, and the expected impacts.
This is the heart of the impact based warning, where we describe the impacts that people
in the warned area might see.
The National Weather Service worked with social scientists to develop impact statements that
were as short and descriptive as possible.
This example is for a low end severe storm, but storms with larger hail and stronger winds
would include stronger wording about possible damage that could occur and protective actions
people in the warning should take.
At the very bottom of the warning, you’ll notice what we call tags.
This is the place where you will always be able to find the basic facts about the expected
impacts, whether it’s hail or wind.
The changes to our tornado warnings are a bit more involved, so here’s a brief summary
of how they will look.
Impact Based Warnings gives us three levels of tornado warnings, with different wording
and tags depending on what the forecaster thinks might happen.
The lowest level of tornado warning, which we call the base warning, will make up the
vast majority of warnings we issue at the Norman office.
The two tiers of warnings that deal with especially dangerous and damaging tornadoes will be used
much less often.
Next up we’ll discuss the individual warning levels.
To give you some idea of the distribution of tornado warnings, we looked at every tornado
warning we issued from 2011 through 2013, and assigned them the appropriate level as
if we had been issuing impact based warnings at the time.
You can see by the numbers that 92 percent of the tornado warnings during that three
year period would have been in the base - or lowest - category of tornado warning.
The second tier of tornado warnings, which would be issued when a tornado is capable
of causing considerable damage, came in at just over six percent.
And the rarest of tornado warnings, reserved for those exceptional cases when a violent
tornado is about to inflict catastrophic damage, and potential large loss of life, on a population
center, made up only 1.4 percent of the total.
Keep in mind that the three years we studied were very active years for tornadoes and tornado
warnings, and we think the actual longer term averages would be closer to 95 percent for
the base warning, four percent for the considerable tag, and 1 percent for the catastrophic category
of tornado warning.
So let’s look at each category, starting with the base level.
This is most common type of tornado warning, and is most often issued when radar shows
a potential tornado developing.
It may also be used when spotters or other observers confirm a tornado, but that tornado
does not pose an immediate threat to people or structures.
Here’s what the base tornado warning would look like.
Just like with the severe thunderstorm warning, the main changes are the addition of the hazard,
source and impact bullets in the middle of the warning, and the associated tags at the
very bottom.
If forecasters have credible evidence that a damaging tornado is actually occurring,
they can use the considerable, or PDS, tornado warning.
PDS stands for particularly dangerous situation, and should not be confused with PDS tornado
watches issued by the Storm Prediction Center.
They have nothing to do with each other, so don’t get confused.
Here are a few recent tornadoes that would have fallen into the PDS tornado warning category,
including strong and violent tornadoes around the OKC metro area from 2010 and 2011, the
April 2012 tornado that moved through the middle of Norman, and the May 19th Carney
and Shawnee tornadoes.
The text of the PDS warning has a couple of new lines of text we didn’t see in the severe
thunderstorm warning and base tornado warning.
First is a headline at the top of the warning indicating that this is a particularly dangerous
situation.
There’s also a new tag at the bottom of the warning, the tornado damage threat tag.
Remember the tags will always be at the very bottom of the warning and will give you a
very concise summary of what’s expected in the warning.
The final tornado warning type is the catastrophic, or tornado emergency.
This level of warning is exceptionally rare, and is only used for the most intense and
dangerous tornadoes impacting population centers.
Since the phrase “tornado emergency” was first used by the Norman office on May 3rd,
1999, we have only used it a handful of times, most recently during the May 2013 tornadoes.
The list of tornadoes that would receive the catastrophic designation is very short, and
includes the Wichita Falls tornado from April 10, 1979, the violent Bridge Creek-Moore tornado
from May 3rd, 1999, the May 8, 2003 Moore tornado, the May 20, 2013 Newcastle-OKC-Moore
EF5 tornado, and the powerful May 31, 2013 El Reno tornado as it approached the west
side of the OKC metro area.
Here’s what the warning text would look like, and you’ll notice the main difference
between this warning and previous examples we’ve looked at is the addition of the tornado
emergency headline at the very top of the warning.
Note the word catastrophic in the tornado damage threat tag at the very bottom of the
text.
All the changes you’ve seen in the warning text will also apply to severe weather statements,
which are warning updates.
You should always pay very close attention to severe weather statements – they are
used to update the warning after it is issued and are just as important as the warning itself.
In some situations, the severe weather statement will be used to change a tornado warning from
base to considerable or catastrophic, so it’s important to keep an eye on these very important
text messages.
Now here’s where you come in.
We want to hear what you think about the new impact based warnings.
Go to weather.gov/impacts and click on Customer Survey to submit your comments and suggestions.
Your input can help make the impact based warnings more effective, so let us know what
you think!
Thanks for watching!