Impact Based Warnings at NWS Norman

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 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!

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