The Hydrologic Ensemble Forecast
Service or HEFS produces an ensemble forecast
of river flows or levels for locations across the country.
Probabilistic forecasts offer a broader range of information
for users to decide the potential outcomes that may affect their community.
The National Weather Service River Forecast Centers distill the wealth
of probabilistic data into this short term river level guidance.
This combines a range of important information for our many diverse
partners into a single nationally consistent forecast graphic.
This product is a culmination of years of social science research on
how to appropriately convey a probabilistic river forecast
in a way that meets the needs of a widely varied user audience.
In this video, we'll explain how this short term probabilistic
guidance is created and how to interpret and use it
to support decision making.
Let's start with
a simple conceptual example of an ensemble forecast that contains
traces that span a period of 24 hours.
Let's zoom in on the forecast values on March 22nd at 12 p.m. local time.
The probability that the river level will exceed a given value is called
the exceedance probability, which we express as a percentage.
For example, 19 of the 20 or 95% of the ensemble members are equal
to or exceed the second smallest river level in this ensemble.
Following this logic,
there are a range of probabilities for exceeding various river levels.
The 50% chance of Exceedance is known as the median.
Alternatively, we can identify the probability
that the river level will be within a given range.
This gives us what is called a probability band.
Here we see the range of
river levels represented by 5% probability bands.
Here are the range of river levels within 15% bands.
Finally, we add the middle ten ensemble members,
which make up the 50% probability band.
We may consider the 5% bands to be less likely to occur and the 15% bands
to be more likely to occur, while the 50% band is the most likely to occur.
The width of these bands, as reflected in these percentages,
indicate the likelihood of each range of river levels.
Finally, the lowest and highest ensemble members each represent
a 5% chance of a river level outside the range of the probability bands.
Exceedance probabilities and probability bands are simply
two different ways of describing the ensemble.
Now let's bring back all of the other forecast times
in our example and connect them across the forecast period.
First, let's focus on the exceedance probabilities
by connecting the second largest river levels.
At each forecast time, we obtain a line that passes through the values
at each time step and have a 5% exceedance probability.
Notice that the river levels
connected by this line do not all originate from the same trace,
and this line does not represent a single forecast river level time series.
Repeating this process
for all exceedance probabilities results in multiple exceedance lines, each
representing the probability of exceeding a specific river level at a given time.
Similarly, we can add probability bands.
The blue shaded area, which lies between the 25 and 75%
exceedance probabilities, is referred to as the most likely band of river levels.
Repeating this for the 15 and 5% probability bands provides river
level ranges at all times, along with the probabilities that they may occur.
Now, going back to our toe ensemble traces,
there is one final piece of information we can extract the chance
of exceeding a flood category during the entire forecast period.
This is the percentage of traces forecasted
to pass the flood stage at any time during the period.
Here, 20% of the traces exceed the flood stage
at one or more times during our 24 hour forecast period.
So the probability of exceeding the flood stage
within 24 hours is 20%.
Probability bands, exceedance probabilities,
and the chance of exceeding a river level over the entire period,
are all different ways of communicating the ensemble forecast.
The HEFS short term probabilistic guidance communicates these
through a single, concise, graphic.
The HEFS short term probabilistic guidance was focus
group tested with hundreds of users ranging from water resource managers
to emergency managers to private citizens across six states.
Not surprisingly, we found that the needs of this broad set of users
varies widely, and the design of this product reflects this.
This product has a wide range of uses.
For example, an emergency manager, they might be focused
on whether a specific flood level is going to be reached.
While a forecaster might be interested in a range of possible hazards.
So they would be looking at probability bands.
Let's take a closer look at this guidance.
The red vertical line is where the forecast begins.
To the left is the observed river level and to the right is the forecast.
As in our conceptual example, the beige green and blue areas
are the five, 15 and 50% probability bands,
which are referred to as the less likely, more likely and most likely river levels.
Each probability band is bounded by exceedance lines.
For example, the exceedance lines on the edges of the 50%
probability band are in the 75 and 25% exceedance lines.
The middle blue line is the median or 50% exceedance line.
Remember, this is not a hydrograph.
We don't expect river levels to follow any of these probability lines exactly.
A typical river level time series can have values
at different exceedance levels at different times.
If you misinterpret these exceedance levels as forecast
traces, you will introduce errors, especially in the total water volume.
Recall from our conceptual example that we can also extract
the probability of exceeding a flood category over the entire period.
This is represented in the HEFS short term guidance
as the ten day chance of exceeding flood categories.
Keep in mind that this is in contrast to the probability
bands and exceedance probabilities which are for a specific time.
Providing more
detailed information about the range of potential river levels
allows users to understand the inherent uncertainty in the forecast.
HEFS probabilistic forecasts help our partners in planning
and decision-making, based on a range of possible outcomes, by providing
a better picture of the magnitude and timing of hydrologic events.
This more nuanced information allows decision-makers
to make better decisions to protect their community.
The HEFS product presents an exciting opportunity
for forecasters to share more detailed information with their users and
to have a more complicated conversation about what might be facing communities.
What's exciting about this guidance is that it helps the NWS and its partners
to start these risk based conversations earlier...
up to ten days in advance.
And this is really a game changer.
The HEFS short term probabilistic
guidance provides nationally-consistent, short-term,
probabilistic hydrological guidance for rivers across the U.S.
to support impact-based decision making.