Flood Inundation Mapping for 30% of the US Population, A How To Guide

Overview

The National Water Center has successfully rolled out Flood Inundation Mapping (FIM) for 30% of the US Population covering portions of eastern Texas, through the Southern Plains and Mid-Mississippi Valley, portions of the Tennessee Valley, Ohio River Valley, Mid-Atlantic, and southern portions of the Northeast United States. FIM services are also available for the Pacific Northwest, and Puerto Rico. Unlike previous iterations of flood maps, this new service estimates the flooded area for not just large rivers and gauged streams, but every little creek, bayou, and headwater stream within the domain.

Set-Up

To access these services, point your browser to the National Water Prediction Service (NWPS) homepage. NWPS replaced the venerable Advanced Hydrologic Prediction Service (AHPS) you may be familiar with. When you land on the NWPS page, you’ll be greeted by the default look at the nation’s stream gauges with both observed and forecast stream conditions turned on.

The larger icons symbolize the forecast stream condition (e.g. Action Stage, Minor Flooding, etc) and the small icon is the current stream condition.

In this example, we can see a Flood Warning polygon under a stream with forecast Minor Flood and its current condition is normal/no-flood.

To use the FIM Services, I find it easier to prepare the map ahead of time. First, in the upper-right hand side, change the Map from “Topographic” to “Satellite”. If we’re concerned about flooding, hopefully that means it has not started to flood yet, so we can turn off the current stream conditions. Simply click on the word “Observations” in the map legend. Alternatively, you can click each checkbox off. After turning off the observed stream conditions, I then go and turn off all the Forecast stream conditions except for Minor, Moderate, and Major Flood categories, using the checkboxes in the Forecast column.

Flood Mapping!

To activate the Flood Mapping, we will first click the “Enabled” box next to Flood Inundation. Which will bring up this warning box explaining that these services are experimental and when the rest of the nation will receive flood inundation services.

After you click the close button, you will be presented with this on your map:

There are two features you should pay attention to on this first map:

• Thin red-lined polygon – FIM is only available inside of those areas.
• Cross-hatched polygon – Coastal Modeling Zone where FIM can be less reliable because the tidal portion of the modeling is not operational yet.

NWPS defaults to the “RFC Max Forecast” flood mapping when you first turn it on. This FIM, takes River Forecast Center (RFC) forecast streamflow and routes it through the National Water Model framework to produce FIM. Only areas within the red polygon and RFC forecast stream conditions of Minor, Moderate, and Major Flood will have FIM drawn. Further, the FIM will only be drawn for that reach of the stream gauge which is forecast or observed to be in flood. Within the 30% population, only a few Minor flood forecasts are currently active. Let’s take a deep dive and zoom into the White River near Augusta forecast point in eastern Arkansas, which is forecast for minor flood. In the image below, you can see what the forecast flood stage looks like…mostly wetland flooding. Although the impacts of this flood are limited, as a hunter, it would be useful to know ahead of time where I don’t want to set-up my blind or stand.

The grey polygon represents a levee protected area, and we should not expect to find inundation behind that feature. You will also see that the FIM service itself was updated at 10:51 AM CST. That information is important, because you will want to click on the stream gauge and open the hydrograph and check when the forecast was issued. It generally takes between 1 and 2 hours for new forecasts to work through the framework. In this case, this flood is on its recession limb, and the forecasts are probably not changing very much.

In the text below the hydrograph you will see a declaration of how much forecast precipitation is used in the forecast (24, 48, 72 hours). At this gauge, only 24 hours of precipitation is used in the forecast, so if you are in Augusta, Arkansas and a big system is more than 24 hours out, that system isn’t being accounted for, yet. But what if there was significant rain expected beyond the forecast horizon?

You click on the “Guidance Options” pull-down menu and select NWM Max Forecast. NWM stands for National Water Model, and it uses the GFS (American) model as the precipitation forcing and goes out 10 Days. Instead of Minor, Moderate, and Major flood categories, the streams are highlighted by AEP or Annual Exceedance Probability. AEP has replaced the Annual Recurrence Interval terminology in most of the weather service. A 100-year flood is descriptive, but often, the layperson will think if they survived a 100-year flood, then they won’t have to worry about a flood of that magnitude for the rest of their life. That same flood has a ~ 1% chance of occurring every year. So, 1% AEP. The NWM only models flood extents for High Water Threshold (regionally adjusted value) to 2% AEP.

The above image is the NWM Max Forecast. You will probably notice there is now flood inundation mapping active on the Cache River to the east of the White River. That was not being drawn in the RFC forecast version because that reach of the Cache River is either a) not forecast to exceed Minor Flood or b) not forecast at all. Another interesting feature is that the NWM FIM doesn’t draw below the confluence in the southern portion of the extent where the RFC FIM did. This is probably because the NWM is doesn’t have enough streamflow in those lower reaches to hit the High Water Threshold and the area where it is drawing is does because it is assimilating the observed river gauge streamflow.

Another thing you will want to do is click on and enable the National Water Model from the main legend menu. This will highlight streams in the National Water Model network and those become clickable objects.

Here we can see the Medium and Long Range forecast show relatively unimpeded recession until next Monday when several members of the Medium Range Ensemble suggest a renewed rise on the river.

If you have the National Water Model turned on, you can find the FIM in areas where streams are highlighted and within the FIM boundary and either not forecast for flood or ungaged.

National Water Model is signaling 50% AEP flow on the Green River near McClean, KY. If we zoom in, we can see what that looks like in the model.

Final Thoughts

The user needs to take care when reviewing these modeled FIM maps and let this list of ideas guide you on how to make sound choices:

• If you are using any of the National Water Model FIM types, remember, this is a deterministic model and there will be run-to-run variability and it should only be used as guidance.
• RFC Forecasts may not have the entire rain event in the river forecast, be sure to look at the number of hours of forecast precipitation.
• This service is experimental and there will be errors. Most commonly, areas of high urbanization with headwater streams may tend to model inundation into structures that won’t be inundated. The model is unaware of almost all urban stormwater drainage and flood infrastructure.

This service is going to be a revolutionary capability for the National Weather Service, Emergency Managers, and individual citizens to protect themselves and their properties to flood impacts.