EMForum Presentation — May 14, 2014

Storm Surge Forecasting
At the National Hurricane Center

Robbie Berg
Hurricane Specialist
National Hurricane Center

Amy Sebring
EMForum Moderator

[This transcript contains references to slides which can be downloaded from http://www.emforum.org/vforum/NHC/StormSurge.pdf
An audio recording of the live session is available at http://www.emforum.org/pub/eiip/lm140514.mp3]

[Welcome / Introduction]

Amy Sebring: Good morning/afternoon everyone and welcome to EMForum.org. I am Amy Sebring and will serve as your Host and Moderator today and we are very glad you could join us.

As you know, the official beginning of hurricane season is almost upon us and as we have done in the past, we have invited the National Hurricane Center to brief us on what’s new.  Please note, there is a link on today’s background page to the complete update for 2014, but today’s program will focus on storm surge forecasting.

[Slide 1]

Now it is my pleasure to introduce today’s special guest: Robbie Berg serves as a Hurricane Specialist at the NHC in Miami where his work involves the issuance of track, intensity, and wind radii forecasts as well as associated watches and warnings for tropical cyclones in the Atlantic and eastern North Pacific ocean basins.

Please see today’s Background Page for further biographical details and links to related resources.

Welcome Robbie, and thank you very much for taking the time to be with us today. I now turn the floor over to you to start us off please.


Robbie Berg:  Thanks, Amy.  I want to thank everyone for allowing me to give this talk today about storm surge forecasting at the National Hurricane Center.  The way the talk is going to go is I’m going to go a little first into storm surge forecasting in general—some of the older parts we’ve used.  Towards the end of the talk we’ll get into the new stuff coming along the way—some of which is experimental beginning this year.

[Slide 2]

To begin let’s just jump into how we model storm surge.  There are different ways to tackle the problem.  First off you can look at statistical methods.  The problem with statistical methods is that they do use only historical data and a lot of that time the data is nonexistent.  We have had hurricanes make landfall in the United States but not every location along the coast has been hit,  let’s say by a category four hurricane. 

We can’t use the historical data to extrapolate what kind of storm surge we might get at any location on the coast.  This method isn’t very helpful for us when we’re doing forecasting.

[Slide 3]

The second way we can do it is deterministic numerical models.  This is for forecasting storm surge solely based on physical equations of how things move such as how the wind from the hurricane pushes water along.  This is strongly dependent on accurate meteorological input—how accurate is the hurricane forecast coming from the hurricane center?

Unfortunately because of the uncertainty in the forecasts—we know our forecasts aren’t perfect—this particular method can be inaccurate.  I’ll have a great example coming up on why we can’t use this particular method. 

[Slide 4]

If we can’t use that, what do we use?  We like to use what we call the numerical model ensemble.  This is looking at many different runs of the same model but with different conditions, in other words like a family of storms.  This is the best approach for determining storm surge risk and vulnerability for an area since it does account for forecast uncertainty.  We’ll have illustrations coming up that will show this in a more vivid description.

[Slide 5]

What do we use at the Hurricane Center and the National Weather Service for forecasting storm surge?  The model we use is called SLOSH (Sea, Lake and Overland Surges from Hurricanes).  This was developed by the National Weather Service several decades ago—probably back in the seventies—and it is used to estimate storm surge height resulting from historical, hypothetical or predicted hurricanes.

[Slide 6]

Even though it is old, SLOSH is pretty good at what it does and it does a lot of things you would be concerned about. It knows about flows—water flows through barriers, gaps and passes along the coast.  It knows those passes can be deep between different bodies of water.  It knows about inland inundation or wetting and drying of cells.

There are some surge models that will raise the water level along the coast but they won’t bring that water inland in areas that are normally dry.  SLOSH model is able to do that so it can take normally dry areas along the coast and make them wet from the storm surge.

It can overtop barrier systems, levies and roads.  It can account for astronomical tides.  Unfortunately two important factors that SLOSH cannot include which are not storm surge related are breaking waves along the coast and normal river flow and rain.  While SLOSH cannot account for those two parameters by itself the Weather Service has other ways around that and in addition to that we are putting a lot of research into putting those parameters into the storm sured forecast and allowing SLOSH to be able to see those things within its simulations.

We are not there yet.  It is one of the challenges we are working on.  We hope within five to ten years we will be able to include breaking waves and river flow in the SLOSH model itself.

[Slide 7]

The way SLOSH is arranged is that you run the model in a grid.  What you see on your screen are the many different grids you can choose from.  In a real time storm we are only going to choose the grids that are affected by the storm. What that does is cut down on the run time that is required to run the storm surge simulation.

If we had to run the simulation for every grid it would slow the whole process down.  When we’re making a forecast we need the information very quickly.  In order to cut down the run time we only focus on the areas that are at risk from the surge in the hurricane itself. 

[Slide 8]

We know there has been talk about SLOSH being old and comparing it to higher resolution and higher performing storm surge models. One of those models is ADCIRC.  ADCIRC has its plusses and we don’t want to say that we don’t like ADCIRC but we don’t really think it is useful for our operations.  One of the reasons is because it is such a high resolution model it takes a long time to run. 

You remember before we were talking about we need to run a family of storms when we are looking at simulations and not just one run.  With SLOSH we can run one storm through the model in minutes on a desktop computer.  ADCIRC on the other hand takes a super computer, a much higher performing computer and it would take that one storm up to several hours to run.

If you’re thinking about running a family of storms we don’t have the luxury to waste that amount of time to get a storm run through the ADCIRC model.  That is one of the main reasons we have stuck with SLOSH—because it is a very fast model that gets us the information we need in a quick amount of time.

One thing I would note and you can see it on the screen—if we take the same storm and run it through both models, the flooding pattern isn’t much different between the SLOSH and ADCIRC models.  You could zoom in really close and see some differences but in an overall, community-wide sense, you are still seeing the same types of flooding in both models. 

In some sense the resolution of the model can only get so high for our purposes when it comes to operational real time forecasting.

[Slide 9]

To sum up how we forecast on surge, I think this is the point we really need to take away.   All storm surge models, no matter how high resolution they are, are strongly dependent on the accuracy of the meteorological input.  If the hurricane forecast is wrong by any little bit, whether it be the track, the intensity or size of the storm, it is going to have huge implications on how well the storm surge model runs and what type of flooding pattern you will see.

The meteorological uncertainty in that hurricane forecast will always dominate over storm surge model specifications and that includes the physics, resolution and other types of parameters.  Different vertical datums are not important here.  In reality storm surge is only one component in the real water rise in the storm.

We have touched on this already.  If you live on the coast you are worried about how high the water will rise from this storm and that includes the component of the storm surge, the tide, waves and the fresh water flow.  The SLOSH model does a good job with the surge and tides.  We have other models that can forecast the wave action along the coast and a model that does the river stages but right now the three models don’t talk to each other very well.

As I mentioned some research going on now is going into how to make those models talk to one another and in the end what we are really getting is the total water that will rise.   Fortunately in most scenarios it is the surge and tides that are the biggest contributors to the water that will rise.  While we are not explicitly including the other two factors in most cases those are not the two that are most important.
[Slide 10]

I talked about an example of why the family of storms is so important so let’s take a look here.  This is a case where we have a hurricane in the Gulf of Mexico to the southeast of Louisiana. You can see from the black line this would be the actual forecast track given by the Hurricane Center and in the white is the cone of uncertainty.

 In this case we have the storm to the southeast of Louisiana and the twelve hour forecast point is right along the northern gulf coast, just to the west of Mobile Bay.  Keep that in mind. We are only talking about twelve hours out from potential landfall.  That is a short amount of time.

[Slide 11]

If we were to take the forecast for that storm and run it through the SLOSH model this is the flooding footprint you would get.  Now you are seeing we are getting a lot of flooding in Mobile Bay and the barrier islands to the south of Mobile Bay.  We’re seeing values close to twelve to fourteen feet above the datum NGVD twenty-nine.

In this same figure look further to the east and you see in Pensacola you are not getting nearly as much flooding in that area as you are getting in the Mobile area.  You are still seeing some, about two feet or so but that is not as significant as what you are seeing in the west. 

The question is—here is our forecast for only twelve hours from potential landfall—is this the type of guidance you want to see to analyze the risk for this storm?

[Slide 12]

Here is what actually happened with this storm.  It continued moving north and although the black line was the twelve hour forecast the storm made a slight shift to the east.  This is not a huge shift.  This is a pretty good forecast.   You are only looking at about thirty miles to the east of the twelve hour forecast point.

Instead of making landfall to the west of Mobile Bay, now we are making landfall to the east of Mobile Bay.  If this is what the storm actually did, what do you think the storm surge flooding actually occurred in this storm?

[Slide 13]

Now we can take that track and put it in the SLOSH model and here is what you get.  Now you notice a big difference in the flooding footprint.  Now Mobile Bay, instead of having wind pushing water into the bay, you have winds blowing offshore and blowing out of the bay.  Instead of seeing twelve to fourteen feet of surge you are seeing four feet of surge.

In the meantime look to the east to Pensacola.  Remember from the previous diagram we showed two feet of surge.  Now in some parts of the bay near Pensacola you are looking at about ten to twelve feet of surge.  The track shifted a little bit of the east and the size of the storm grew more than was forecast.

A little secret when it comes to hurricane forecasting—we are pretty good at track, we are so-so at intensity and not very good at all when it comes to size forecasting.  That is one of the parameters you have to factor in uncertainty to a large extent.  What happened in this particular storm is that the storm grew in that twelve hour time frame and the storm extended far away from the actual track so we pushed more water into the Pensacola area.

With this storm’s evolution, this is the type of flooding we got.  It is a lot different from what was originally contained the twelve hour forecast.  We can’t have this kind of scenario occurring so we have to figure out a way to analyze the true risk and vulnerability to the surge of any particular storm.

[Slide 14]

There are perils for not accounting for uncertainty.  I like to use this diagram because it shows what happens if you don’t account for uncertainty in a hurricane forecast.  You are essentially putting all your eggs in one basket.  If something were to go wrong, the track were to shift a little bit, the intensity were to go up a little bit, the size to increase a little bit—you could end up looking like this.

You put all your eggs in one basket and you’ve made a really bad decision ahead of the storm.  We have to use the ensemble approaches in order to analyze our true risk.

[Slide 15]

How do we do that?  There are several ways.  We want to look at alternatives to single runs.  One way is to look at a map with pre-computed storm surge maps that are based on different direction and motions of the storm, different landfall locations, different intensities, different storm sizes and different forward speeds. 

[Slide 16]

The way we can do that is to look at these three different products.  I’ll go through each one by one.  The first is the MEOWS (Maximum Envelopes Of Water), the MOMS (Maximum of MEOWS) and the real time simulations which are the P-Surge (Probabilistic Storm Surge product). 

[Slide 17]

Let’s start with the MEOWS (Maximum Envelopes of Water).

[Slide 18]

If you have the SLOSH display program, the MEOWS are available to you if you open up the program and look for your area.  What happens with the MEOW is that as the user you select the SS category of the storm, the forward speed, the storm direction and the tide anomaly.  The Radius of Maximum Winds and landfall location are already built into this product.

[Slide 19]

The best way is to show you an example of the MEOW and how it is created.  Here is as simulation of the storm.  You can see at the bottom of this figure there is a black line moving into the Charleston, South Carolina area.  That is one storm.  When I run that one storm in the SLOSH model in this particular grid you get this type of flooding.

I want you to notice the grid cell here in somewhere in southern North Carolina that says 4.8 feet.  From that one storm it produced 4.8  feet at that location.  I selected a category three storm moving to the west northwest at fifteen miles per hour at high tide.

Let’s say we’re not so sure that the storm will make landfall at that exact location so what if it moved to the north of this and made landfall there?  Move it up the coast—and what if we had another storm and it moved further up the coast and we kept doing that?  Notice when we have storms the flooding changes based on that particular storm.

Also notice that at our inset that the values are changing as the storm gets closer.  As the storm gets closer you would expect a higher surge at that particular cell.  What if I have all this family of storms and I hit the coast everywhere with any storm with those parameters—category three moving to west northwest at fifteen miles per hour at high tide. 

Now what I do is retain the highest value at that grid cell from any one of those storms.  In this storm it was thirteen point eight feet.  The MEOW for that location with those parameters was thirteen point eight feet.  This is one particular MEOW.  For any phase in you can get tens to hundreds of MEOWS depending on the different parameters and how you choose them. 

One thing about the MEOW is that what you see on your screen shows flooding but this isn’t the actual flooding that would occur from that storm because we are looking at a family of storms.  This is more or less like a worst case scenario for a storm given those particular characteristics.

[Slide 20]

Thinking about what MEOW is we can go even further and say what the maximum of MEOWS are—the MOMs.

[Slide 21]

We’ll go back to the MEOW.  This is the one I just showed.  With the MOM you only select the category.  All the other parameters are baked into the cake for you.  I took the same MEOW I used and it showed me thirteen point eight feet at that location in southern North Carolina. 

What if the storm makes landfall at different directions into the coast using the same parameters of the category three storm?  Also what happens if the speed changes or if the tide level is different?  After I hit the coast at all different directions I still retain the highest value within that particular cell. Now you can see I have a category three MOM showing fourteen point seven feet at that location in southern North Carolina.

This is taking all the different uncertainty that exists with a particular storm or event and composites them to show you what the actual risk is along the coast from that category of storm. 

[Slide 22]

MOMs are important because that is the product that goes into determining evacuation zones along the coast of the United States.  A category three storm in this area of the country has the potential to do this sort of flooding along the coast.  Because of that simulation that output is used to determine what type of evacuation should be called for a category three storm threatening the coast of North Carolina and South Carolina in this particular case.

It is important to know about the MOMs in that they are the basis for the nation’s evacuation zones anywhere from Texas up to Maine.  This is an example of what I’m talking about.  If you look at this figure from New York City giving you the evacuation zones from the city—what I want you to notice the pause between the evacuation zones. Here I am showing the category four MOM.

Any category four storm could potentially produce this amount of flooding in New York City.  Watch again how the two phase in and how the evacuation zones line up very close to the category four MOMs.  That is not by coincidence because those MOMs are being used to create the evacuation zones along the coast.  I wanted to show you an illustration of how that occurs and how municipalities and states use that data to create their zones.

There are the two on top of one another.  The black line is the category four MOM and the colors would be the evacuation zones for the city.  All the products I showed you—the MOMs and MEOWs are available to you right now.  You can go into the SLOSH program and look at them. 

[Slide 23]

When you are in an actual event we will start producing the probabilistic storm surge product.

[Slide 24]

It is based on the NHC official advisory.  We produce a forecast that says an advisory goes out at five PM eastern time.  Based on that forecast we will run these probabilities and create a storm surge forecast.  It is available roughly forty-eight hours prior to the arrival of tropical storm force winds along the coast and another way of looking is when we have a watch or warning in effect for the United States coastline.

The P-Surge product is beneficial because it accounts for meteorological uncertainty in the hurricane forecast.  It accounts for track and landfall location uncertainty.  It accounts for uncertainty in the size and forward speed of the storm and in the intensity of the storm.  These uncertainties are based on historical errors that we’ve had in our hurricane forecasts.

We keep a very good running verification of how good or bad our forecasts are.  We use the past five years of our forecasts to determine our historical errors.  Those errors are input into the P-Surge product.  The latest version (2.0) which will be available this coming hurricane season also accounts for tide and can be available above ground level.

That is important because in the past, the previous version did not account for tide.   If you were looking at the P-Surge guidance you had to find a way to add tide into the guidance.  If you weren’t looking at the same vertical datum you could screw up the whole process.  This is a big improvement to the model because now the tide is accounted for and you don’t have to manually add the tide to what you are seeing in the output.

The product is going to be above ground level as well in addition to the normal datum which is typically NAVD 88.

[Slide 25]

Here is an illustration of how P-Surge works.  Here we have a storm.  It happens to be Hurricane Irene from a couple of years ago to the east of Florida just north of the Bahamas.  Here is the actual forecast from the Hurricane System in the black lines and we had the forecast landfall somewhere in eastern North Carolina and riding up the Atlantic Coast into New England. 

[Slide 26]

We know the forecast isn’t perfect.  We have to assume the track could shift to the east or west.  We have to assume the size could vary.  We know hurricanes come in all different sizes and even though we forecast in a particular size, that size could change.  We have to assume the speed is a little faster or slower than we are forecasting and we have to assume the storm could be stronger or weaker than what we are forecasting.

With P-Surge all these differences are centered around the official forecast.  It’s not like we’re ruling out the forecast but it is centered and weighted heavily on the official forecast.

[Slide 27]

Once you have run P-Surge this is the type of output you receive.  This is an example shot from Hurricane Sandy in 2012.  You’ll notice that P-Surge did a good job of illustrating the areas along the coast that had the highest risk from the storm surge from Sandy—in fact New Jersey, Long Island, New York and Long Island Sound including the Connecticut coast was ground zero for the highest storm surge. It realistically painted the highest risk from the worst surge.

[Slide 28]

Do you remember the example I showed earlier in the presentation?  Let’s go back to that one.  Here is our storm and there is the original forecast showing it making landfall to the west of Mobile Bay where we had a lot of flooding possible in Mobile Bay and not quite as much in Pensacola.

The deterministic SLOSH run sent a limited surge threat to Pensacola.  At twelve hours away from landfall we have a likely hurricane warning in effect which means P-Surge would be available.

[Slide 29]

Let’s look at what the P-Surge output would have shown for this storm.  What I’m showing his the probabilities that the surge would be greater than eight feet.  Now you can see that the P-Surge was showing a fifty percent chance of eight feet of surge in Mobile Bay, which makes sense based on the forecast, but it is also showing at least a fifty percent chance of eight feet of surge in parts of Pensacola.

The P-Surge product is realistically taking into account the uncertainties in the forecast and it is assuming that some of the tracks could shift to the east bringing the storm closer to Pensacola or it is assuming the storm could grow in size more than what is in the forecast.  When it takes those uncertainties into account it shows there is a significant storm surge risk in the Pensacola area as well as the Mobile Bay area.

During an event realistically somebody making a decision on things such as evacuations should be looking at a product like this and deciding that whether in Mobile or Pensacola people should be evacuated from the coastline because there is that amount of risk from that particular storm.

[Slide 30]

Here is reality of what happened.  The storm shifted to the east and created the storm surge in the Pensacola area.  This is Hurricane Ivan in 2004.  We didn’t make it up, this is an actual storm that occurred and where critical decisions had to be made based on that forecast.  This is showing how using P-Surge can help immensely when having to make those critical decisions.

[Slide 31]

I’ve thrown three products at you—the MOMs, the MEOWs and the P-Surge.  We like to show what we call the “storm surge decision support wedge” so you get a good understanding of what you should be looking at during different points of the forecast.  If you start at the bottom of the wedge in tier three this is your planning and mitigation stage.

This is more than five days from landfall.  You are looking now when there isn’t a storm out there.  You’re planning hurricane season.  At this point you should be looking at MOM.  MOMs are used to create your evacuation zones.  When you are well away from a landfall and more than five days out—more than five days—look at your MOM to assess your risk along the coast.  When you get within two to five days of a potential landfall, now you are at the readiness stage. 

I would still suggest looking at MOMs but now that you have a forecast from the Hurricane Center showing the storm approaching your area you can start to refine some of the parameters associated with that storm and looking at specific MEOWs because you may be able to pick a particular category and you may be able to eliminate some of the different directions that are available to you.

Maybe you know that the storm will move to the north, northwest or northeast, but not to the south so you can start to eliminate some of the possibilities and look at some of the MEOWs to further refine the assessment of risk.  When you get within forty-eight hours of a landfall within that watch warning time frame you are in the response phase.

We recommend looking at your MEOWs and doing further refinement to figure out which one is better to look at but P-Surge is available to you at this stage in a real time sense so you get a better handle on the risk from the storm.

In addition to those products you’ll also start receiving explicit numbers from the Hurricane Center within our advisory and also within the local statement provided by the National Weather Service forecast offices along the coast.  As we get closer to the landfall everything starts converging to more refined information.

When you are more than two days out there is too much uncertainty within the forecast to provide specific and extremely accurate information.

[Slide 32]

Those products have been available to people for several years but let’s look into the future and where we are going—especially coming up in this hurricane season and we are starting with the experimental potential storm surge flooding map, or you have heard it called the inundation map.

[Slide 33]

To add some background to this graphic, the first question would be—which product will drive the inundation graphic?  It is going to be P-Surge—in fact, P-Surge 2.0 which is coming online this year.  One thing I didn’t mention about P-Surge is you will select the level you concerned with.  For this inundation graphic we are going to elect the ten percent exceedance.

What that means is it’s a good way of looking at what you should plan for in this storm.  If you were to look at a zero percent exceedance that would be a worst case scenario—nothing could potentially top what you are seeing in a zero percent exceedance.  The problem with using a zero percent exceedance is you will end up over-evacuating people in a potential event.

We don’t want to over-evacuate.  If I was to look at a fifty percent exceedance product essentially what I’m saying is that there is a coin flip, that is a fifty-fifty chance that the level that is being shown on the map is higher or lower.  When it comes to critical decisions on things like storm surges which might be a life-threatening hazard we don’t have the luxury to do a fifty-fifty coin flip.  We have to exclude and not account for the fifty percent exceedance.

For that reason we use the ten percent exceedance.  What it means is that there is only a ten percent chance that the values shown on the graphic will be exceeded by the real storm.  It is not the worst case scenario but it is a plausible upper bound of what could happen.  It is what we suggest you should plan for in a storm given the uncertainties in the forecast.

The grids that are used for this product are from the latest SLOSH basin updates which have been updated in NAVD88 which is the newest datum.  Topography-wise we are using a Coastal Surge Center sea-level rise DEM.   They have been producing this topography data set to use sea level rise studies but it is being done in a timely fashion that we can use it as well for our inundation graphic.

It is at such a higher resolution that we have to do some re-sampling to make it a little smoother resolution because in some of the polling and focus groups we’ve done when it was too high resolution some people thought it was not accurate.  By doing a little bit of smoothing we were able to make the figure look at little more attractive to people when they are looking at it and they put more faith into what they are looking at.

In some areas when they haven’t finished this DEM we have been augmenting it with USGS NED data but that is being phased out as we go along to the higher resolution data.  For processing we are locally using the ArcGIS for server and desktop but we are working toward leveraging National Weather Service’s program, the IDP, which in the coming years when they get that set up it we won’t have to do that locally—it will be done within the larger National Weather Service framework.

[Slide 34]

Here’s an example.  One of the things we have to do is merge all of those grids I showed you later from the top.  This is an example of the merged grid.  On top of that you can also see the topography data that we are using to create this inundation graphic.  You can only calculate inundation if you know how high the land is.

The storm surge model produces a storm surge height.  You know how high the land is at any of these locations so you use subtraction to get how much water is sitting on dry land.  That is why we use the high resolution data.  You can see from this map that we include that data for every area along the coastline from Texas to Maine.

[Slide 35]

In a simplistic fashion this is what it looks like.  This would be raw output from the P-Surge guidance.  We get a grid of data showing storm surge height at different locations.  The post processing we do transforms that data into the inundation graphic or the potential storm surge flooding graphic.

[Slide 36]

Many of you on the webinar may have seen this example.  This is an example of a simulated storm that we’ve done in the Fort Myers, Florida area.  It shows you the potential storm surge flooding that might occur from a simulated storm.  The higher values in red are decreasing as you go further inland.  The lower values are showing in blue.

This is what the prototype looks like.  Again, this is the Fort Myers area.  The red would be extreme or nine feet or greater of inundation or storm surge flooding above ground level.  Orange is designated as high or six to nine feet.  Moderate would be three to six feet and low would be three feet or less.

[Slide 37]

How is this useful?  I haven’t mentioned it yet because this is in the experimental phase this year.  Because it is in the experimental phase we won’t actually be disseminating the raw data in any sort of format like the GIS shape file but in the coming years once we go operational with this product it will be useful when it comes to making important decisions.

Here’s an example of looking at southwest Florida in the Fort Myers area.  These are the evacuation zones starting with T for tropical storm, and then A, B and C.  I think they have a D and E but they are not showing on the diagram.  What is interesting and what we think the potential for this product is you notice as I flip back and forth, I can take this inundation graphic from this storm and overlay it on my evacuation zones to determine which zones are most vulnerable from the storm surge and I can make a more educated decision on which zones should be evacuated.

In this case we definitely evacuate zones T, A, B and zone C could be up for debate.  There are a lot of areas in zone C in a light pink color that aren’t quite touched by the water being moved inland by the storm in this scenario.  There could be arguments for zone C to not have to be evacuated or fully evacuated.

In a real time scenario we think this product will go a long way to help emergency managers make these critical decisions as far as evacuations.  It can be beyond that.  It can go into things like looking at shelters.  Are there shelters located in areas that could be under potential flooding?  Other critical facilities such as hospitals—you could look at transportation routes—which roads are at risk of being inundated by storm surge from this storm? 

We think this product has great potential going forward into helping make critical decisions before a storm and as a storm is beginning to make landfall so we can get ready for the response phase after the storm.

[Slide 38]

We have four examples that we’ve produced leading into the hurricane season.  This is the Fort Myers example I showed.

[Slide 39]

This is one from the Houston area and Galveston.  We can see the legends there with storm surge flooding with four categories, blue up to red.  This is a simulated storm.  It is not actually Hurricane Ike.  It is a simulated storm we created.  It shows there are some areas that are more susceptible to storm surge flooding because the flooding penetrates further inland than other areas.

To the north of Galveston there is a lot of potential for storm surge penetrating well inland even up to I-10 in this scenario and then showing down further south into the Galveston area.

[Slide 40]

This is an example from Charleston, South Carolina.  This is not Hurricane Hugo.  It is a storm we generated as a simulation.  It realistically shows the worst surge could occur along the barrier islands and the back bays of the islands especially closer to the ocean.  It also shows the surge can get into the creeks and waterways that extend outward of the Charleston area.

[Slide 41]

The last example we produced is the New York City area.  It is not Hurricane Sandy or Irene, just a simulation we ran.  It shows the areas that would be at risk from surge.  We got areas along the shore to Manhattan, Brooklyn, Queens and into the Jersey side like Hoboken and Jersey City and further inland into the Meadowlands.  You can notice from the diagrams that the inundation graphic knows where higher land is.

If you are familiar with the New York City and New Jersey area you know there is a bit of land extending through Hoboken and Jersey City that is elevated.  It is not going to inundate those areas because it knows the land is higher.  In the meantime it knows in the Meadowlands it is very low lying and very marshy so it allows the storm surge to get up in those areas and you can see the type of flooding that would occur in that kind of storm.

[Slide 42]

The takeaways—this product will be available in this hurricane season on an experimental basis via the National Hurricane Center website.  What the experimental phase means is that we can’t guarantee it will always be available.  The trial phase is for us to work out the kinks in this product before we go live in an operational sense.   There may be times when we can’t get it up during an event.

We want to make it available to people so they can look at it during an event when it’s available and to assess how useful it might be to them or if there are changes to be made before we go operational.  For that reason there will be no data dissemination during this phase.  We won’t be disseminating the data in a raw sense like a GIS shape file. 

It will only be available via the Hurricane Center website in a Google type format where you can pan and zoom in on your area.

[Slide 43]

The last thing I want to talk about in brief is storm surge watch and warning.  The National Weather Service has decided that based on some recent storms we’ve had that we need to move forward with initiating storm surge watches and warnings.  If you look at the hazards associated with a hurricane, there are four main hazards—the wind hazard, the storm surge hazard, the fresh water flooding and rainfall hazard, and the tornado hazard. 

If you look at which of those hazards we warn for, we warn for the wind in a hurricane warning.  By definition a hurricane warning means that hurricane force winds are expected in that area.  We warn for river flooding, flash flooding from rainfall and tornados with tornado warnings. The National Weather Service does not have an explicit warning for storm surge.

There’s a problem there because storm surge has been shown to take the most number of lives in tropical cyclones.  You only have to look back at storms like Katrina, Ike and Sandy to see that the storm surge in a lot of cases is the hazard that takes the most numbers of lives. Because of these more recent storms we notice there is a gap in our warning scheme so the National Weather Service has agreed and decided we are going to move forward with.

This is something we are looking to go experimental with in 2015 but not in this hurricane season.  One thing you might notice from this diagram on the screen is that the storm surge warning will be different from other warnings you’ve received in the past.  In the past you’ve received warning on the county scale. 

You might see a hurricane warning for Miami Dade County in Florida.  That would cover the entire county.  Some warnings are done in a fashion like tornado warnings where a polygon is issued by the NWS and you know if you’re in the warning whether or not you reside in that polygon.

The problem is that those methods are not really useful when it comes to storm surge.  We can’t use the county based method because in many cases storm surge only stays on the immediate coast line.  In cases where it penetrates further inland it is not going to cover an entire county.  It doesn’t make sense for us to issue a county based storm surge warning because we would be over-warning more area than we really should be.

The way we think NWS is going to go with this is to use the NDFD grid which is used to make the actual forecast.  You can see in this diagram why it looks boxy—it’s because it is using the grids in that dataset.  What you’ll notice is that this would be a Hurricane Irene example—the area that is painted onto the storm surge area is the area that is at risk of potentially life-threatening storm surge.

We are limited to areas along the coast and the barrier islands and the shores of the Pamlico and Albemarle sounds in eastern North Carolina.  It does a better job of painting the area that is at risk for that storm surge.  The warning itself—a lot has to be determined within the Weather Service in terms of criteria, the official definition of the warning.  I think we are pretty sure we only want to paint those areas that are life-threatening storm surge drifts.

We won’t be issuing a storm surge warning for every little flooding that might occur on the coast.  We are probably looking at thresholds of three to four feet of inundation which for most people would be up to their waist.  Many kids are only three to four feet high so it would be life-threatening for them.

A lot has to be decided on this internally in the Weather Service—how we are going to move forward.  It will be a collaborative process.  It won’t just be coming from the Hurricane Center.  We will be working with the local national weather forecast offices to create this storm surge warning.  It will be a collaborative process coming from both offices during a storm.

The experimental P-Surge flooding map will be available this coming hurricane season.  It will be available on our website when there is a watch or warning in effect for the United States.  Because it is experimental there will be no data dissemination.  For the storm surge watch and warning we are looking at 2015 experimental product.

In that sense it will be the same sort of ground rules—being in an experimental phase, it may not always be available.  We are going to work kinks out.  After a year or two of testing it should go operational in 2017 or 2018.  With that I am finished and I can take questions.

Amy Sebring: Thank you very much Robbie.  We will move to the Q&A portion.

[Audience Questions & Answers]

John Callahan: Is P-Surge based on SLOSH model runs?

Robbie Berg:  Yes, it is.  What happens in P-Surge is the SLOSH model is run for all the different families of storms centered on the official forecast.  So the SLOSH model is the base model for P-Surge.

Amy Sebring:  Is P-Surge available to the National Hurricane Center or something that will be available to emergency management people in future perhaps?

Robbie Berg:  It has been available on our website for people during a storm for several years now.  It is available in a Google type of framework where you can pan and zoom.  It is also available in GIS shape file so emergency managers and others can take the information, download it and use it as they see fit.

One thing I didn’t go into in depth is that you do select the levels you are interested in.  For the height of the storm surge the levels go from two to twenty-one feet.  Based on what you are interested in, you would select the level and it would give you the percentage or chance that the surge would reach that level all along the coast.

It is available now and has been available for a couple of years.  Whenever there is a watch or warning for the United States—that is when you would see it on the website.

Amy Sebring:  You can run it or just get the data?

Robbie Berg: You’re not running anything.  It has already been run on the super computer by the National Weather Service but what you are looking at is the output from that model run.

Amy Sebring:  Will people in the future be able to actually have access to the model itself and not just the data with P-Surge?

Robbie Berg:  P-Surge isn’t a model.  It is running off of SLOSH which is the model.  People can download the SLOSH display program.  That is available to you now.  You can look at the MOMs and MEOWs which I went over.  You cannot look at P-Surge in the SLOSH display program because it is a real time product.   The SLOSH display program itself is available for emergency managers to download and use.

Amy Sebring:  Do you update the P-Surge and inundation map with each advisory?

Robbie Berg:  It is updated every six hours with the new forecast so you could see some changes from forecast to forecast.  With the inundation graphic since we are using a ten percent exceedance we are already looking at the upper bound of possibilities.  You shouldn’t see too many major changes from forecast to forecast.  That is an excellent question and it will be available every six hours with the new issuance of a hurricane forecast.

Peter Slovinsky: Hello, Peter Slovinsky from Maine Geological Survey.  Questions:  has there been any feedback regarding the proposed storm surge depth color schemes?  (Intuitively, I think of darker colors as deeper areas of inundation).  The existing color scheme seems to correlate well with greater risk of storm surge, not necessarily inundation depths.  

Robbie Berg: I guess I don’t quite understand the question because the storm surge is what is creating the inundation depths so the highest depths are in red which mean more danger.  I should provide some background into the development of the product.  There was a very large social science outreach campaign in the marketing of this product.

We didn’t just sit back in our office and create this product ourselves.  We contracted with several social scientists that went out and produced surveys for emergency managers, the media, other weather service employees, the public and they also conducted focus groups with these audience numbers.

We went through many iterations of developing this particular graphic based on the feedback we got from all these different groups.  As you can imagine as each group had its own particular wants and needs it was hard to create one product that will fit and satisfy everyone’s needs.  Based on the overwhelming evidence we got from these focus groups and surveys, this was the final result we got.

As far as the colors go, we tested a bunch of color schemes whether it was one color blue with different shadings, different colors going from blue to red or green to red, and what you see was what bubbled up to the top as what was most liked and most interpreted correctly by the greatest number of people.

Peter Slovinsky: Does the inundation depth outputs take into account inherent model error (and what is the error of P-SURGE...SLOSH was supposedly 20%). 

Robbie Berg:  That is a complex question because the number that had been quoted historically about SLOSH being accurate within twenty percent is a little broad and hard to extrapolate that to how things are now.  I don’t have time to show you this but we have some recent studies on how SLOSH performed in the New York area for Sandy and we got some really good results.

I would say that the errors, comparing it to tide gages and other data we had, I think it would be less than twenty percent.  I don’t know how accurate the twenty percent is now but whatever models of accuracy is in SLOSH itself, since that is the base for P-Surge and the inundation graphic—that is the type of inaccuracy you might get. 

I would suspect the twenty percent that has been quoted historically has gotten a lot better especially since we have made the model much higher resolution than it was ten or twenty years ago.

Kris Ludwig: Thanks for the great and informative presentation. I may have missed this, but how are changes in elevation implemented into MOM/MEOW calculations? (e.g., is the DEM revised to include new elevation data after major storms like Sandy?)

Robbie Berg: What happens at the Hurricane Center is every year we update a handful of basins.  You remember the basins I showed you.  There are different ways that those are chosen to be updated—whether they are the ones that haven’t been updated in a while or there has been a storm in a basin and there have been changes in the coastline.

Those basins are updated based on any new data we receive.  A lot of that is LiDAR data that is being taken from the coastline.  In a sense we are at the mercy of the state that has collected the LiDAR data.  If the state hasn’t collected the data then we don’t have the new data to update the model or the basins.

It all depends on the states that have invested in updating the data and whether or not their basins get updated in subsequent years.

John Callahan: Are there plans within NWS for similar products for extratropical storms like nor'easters?

Robbie Berg:  Very good question. The short answer is yes.  We recognize that extratropical storms also produce storm surge events and some of them are quite significant.  The way the Weather Service is tackling this now is—we are trying to finish the tropical stuff first.  There is a process in play right now where we are trying to think about how we are going to do the extratropical storms.

One of the problems is that at the Hurricane Center we focus on hurricanes.  That’s our specialty.  There is no agency within the Weather Service that just does extratropical storms.  We have to figure out a paradigm shift as to how to handle that.  There are slightly different models that are used to forecast extratropical storm surge because of the structure of the storms.

The short answer is yes.  The long answer is it will be a couple of years out before we get to that stage but our hope is to be able to produce the same sorts of inundation graphics and have a storm surge warnings for extratropical storm surge events.


Amy Sebring: On behalf of Avagene, myself, and all our participants, thank you very much Robbie for being with us today and sharing this information with us.  We wish you continued success with your efforts and a quiet hurricane season!

Our next program will be June 11th when we will be pleased to welcome back the ever popular Doc Lumpkins who will be providing an update on the activities of the National Integration Center where he is now serving as Director.

Until then, thanks to everyone for participating today and have a great afternoon. We are adjourned.