MAKING USE OF UNCERTAINTY IN WEATHER AND FLOOD
FORECASTING IN MIDLANDS REGION OF THE ENVIRONMENT
AGENCY
RICHARD CROSS, MCIWEM, MSc, BSc
Environment Agency – Midlands Flood Forecasting Team Leader
Key Words: Forecast uncertainty, time lagged ensembles, what-if scenario modelling.
Abstract: Midlands Region of the Environment Agency routinely run over 200 real-time flood
forecasting models to support the region’s flood warning service. The National Flood Forecasting
System (NFFS) has been adapted to make use of forecast uncertainty as a positive attribute to help
inform preparedness and warning for possible flooding. Up to thirty hours of forecast rainfall can be
transformed to give multiple flood forecast outputs and time lagged ensembles generated to help
understand the range of possible forecast outcomes. Ten day forecasts using longer term rainfall
forecasts are prepared to scale the impact of potential flooding and ensure the effect of the
Agency’s response is maximised.
BACKGROUND
After nearly twenty five years of model use and development in Midlands Region of the Environment
Agency, October 2005 saw over 200 real time flood forecasting models transferred to the new
National Flood Forecasting System (NFFS). The Midlands application of NFFS now includes 120
rainfall runoff models (MCRM - Midlands Catchment Runoff Model) and 110 hydrological routing
models (DODO - Dobson and Douglas). These models cover the majority of the Regions rivers and
80% of the Region’s Flood Warnings are based on their forecasts. Figure 1 shows all forecast
locations.
Figure 1: Flood Forecast Model locations in NFFS – Midlands Region
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Prior to the use of NFFS the legacy forecasting system was fed with six hours worth of forecast rainfall
data from the Met Office (from UKPP - United Kingdom Post Processing system). This forecast
rainfall data was run with the MCRM model to increase forecast lead times and aid better warning
decision making, particularly in faster responding catchments. In some catchments acceptable
warning lead times were often only achievable through the use of forecast rainfall data. The use of six
hour rainfall forecasts was not without its problems as even at six hours ahead rainfall forecasts could
be inaccurate. Warnings were sometimes issued based on forecast rain that did not fall and
conversely warning lead times could be compromised due to rain that was not forecast.
EXTENDED RAINFALL FORECAST INPUTS TO FLOOD FORECASTING MODELS
Soon after the launch of NFFS forecast rainfall inputs from the Met Office were extended out to 30
hours (using NWP – Numerical Weather Prediction outputs). In the run up to a potential flood event
these forecast rainfall profiles provide an early indication of the flooding impact in both fast responding
catchments and in the lower reaches of the Regions major rivers.
It was soon obvious that the accuracy and uncertainty of such length of rainfall forecasts would limit
the use of the data for issuing reliable Flood Warnings. However, based on discussions with the Met
Office Forecaster the rainfall forecasts can be quickly manipulated using NFFS and the differences in
flood forecast output studied by overlaying the various forecast scenarios (see Figure 2).
Using NFFS the 30 hours of forecast can be doubled, halved or transformed in any other way based
on the uncertainties around the rainfall forecast as discussed with the Met Office forecaster. These
what-if forecasts allow the Flood Forecaster to explore worst or best case flood scenarios and
determine the risk of flooding occurring at specific river locations given realistic ranges in forecast
rainfall totals.
Figure 2: NFFS Forecasts with transformed rainfall inputs:
This technique is used in developing flood events and during flood events when further rain is
forecast. Risk averse actions based on such forecasts may involve opening of incident rooms, shift
working, an increase in regional communication and teleconferences, and readiness for potential
deployment of flood barriers. Additionally, early communication with Professional Partners and the
media can be initiated, especially worst case rainfall predicts high impact or widespread flooding.
Operational Use of Time-lagged ensembles
Model runs on NFFS have traditionally been viewed independently of one another with the next
scheduled run superseding the output from the previous run. However, a further feature of NFFS
allows superimposition of successive flood forecasts to enable the Flood Forecaster to assess the size
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and impact of changing forecast outputs. Such forecasts are known as time-lagged ensembles (TLEs).
The use of TLEs enables the knowledge gained by each model run to be accumulated into a suite of
forecasts that provides more information to the forecaster than if each model run is viewed separately.
Forecast rainfall inputs to NFFS models change as Met Office predictions update and as forecast
rainfall is replaced with measured rainfall captured by Environment Agency raingauges. Outputs from
models run at a two-hourly frequency and resulting in six model runs over a 12 hour period will include
at least one update of the 30 hour rainfall forecasts from the Met Office UKPP system (as UKPP is run
four times a day). Between each two hourly model run, shorter term 6 hourly rainfall forecasts from
NWP will also be changing as they are updated hourly. Whilst rain is falling, or forecast to be falling
within a 30 hour rainfall period of the T0 of the forecast, model outputs will change if rainfall forecasts
change. Under these circumstances the primary reason for variation in the output of each member of
the time lagged ensemble will be due to changing rainfall forecasts.
Figure 3: NFFS Time-lagged ensemble level and flow forecast for Bewdley on the River Severn
Where lead times are short and forecasting decisions need to be taken before or soon after the rain
has fallen, TLEs can be used to display the differences in one forecast run to the next. The spread of
forecasts both in timing of threshold crossings and peak level can help quantify the uncertainty
associated with forecasting for each location and help guide the Forecaster in what action to take.
For example, if all forecasts show a threshold is likely to be crossed then the forecaster would be
inclined to advise the Flood Warning Duty Officer to issue the appropriate warning. However, if the
impact of a threshold crossing is high then a recommendation to issue the warning may be made with
fewer ensemble members forecasting the threshold. Ensembles using longer lead time rainfall
forecasts are more typically of use for scaling a possible event and preparing for escalation rather than
issuing warnings.
However, the use of TLEs may give more confidence and justification in issuing a warning based on
modelling solely forecast rainfall inputs, thus extending the forecasting service to catchments with a
very short time to peak.
With all rainfall inputs complete, the variation in model output across the TLEs is mainly due to
changes in model updating. At downstream locations these changes can be due to the accumulated
effect of changing data assimilation at upstream model forecast points and at the downstream forecast
location. Manual verification of each model node will be required to determine how and why each run
is changing. However, the spread of post rainfall TLEs can help inform forecast verification and thus
flood warning decisions.
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Post Event Use of Time-Lagged Ensembles
Much can be learnt from a review of TLEs post event in attempting to understand what is creating the
variation in model output from one run to the next. Such studies will help to inform improvement
priorities. The impact of the variation in rainfall forecasts can be understood and such information
banked for the next event. Each forecast could be viewed as a what-if output which shows how much
rain is required for threshold crossings to be forecast. Such information can also help the Met Office
understand the impact of their rainfall forecasts on the Agency’s flood forecasting service.
TLEs generated after rain has fallen can reveal large forecast ranges which are likely to be those most
reliant on poor model updating to correct their output. Poor updating is often due to large differences
between the model simulation and measured flows which the updating model struggles to reconcile.
Improvements can therefore be focussed on minimising the work of the updating model through a
prioritised recalibration programme informed by the TLEs.
LONG RANGE FLUVIAL FLOOD OUTLOOK STATEMENT
Twice a week the Regional Flood Forecasting Team prepare extended flood forecasts using NFFS
and five day rainfall forecasts provided by the Met Office. NFFS has been adapted to display the
resultant predicted river flows out to ten days ahead which enables the peaks in the lower reaches of
the Severn and Trent to be forecast and displayed. In addition to the thirty hours of rainfall forecast
automatically added to NFFS, rainfall profiles out to 5 days ahead are manually added for each MCRM
forecast location using data and information from the detailed ten day weather forecast provided to the
Region by the Met Office. Flood forecasts are then run and total numbers and timings of Flood
Warnings are collated, currently by Flood Warning Duty Area.
Figure 4: Example of the Management Version of the 10 Day Outlook Statement
This warning information is summarised for Managers and a more detailed statement is issued to
Flood Incident Management Teams and Flood Warning Duty Officers. Due to the uncertainties within
the longer term rainfall forecasts specific details of individual Flood Warnings are deliberately not are
included on the statement but this detail is available if requested. The resulting statement provides an
indication of the forecast scale of flooding in terms of possible numbers of Flood Warnings. One area
of detail included is the timing of any forecast thresholds for the deployment of Flood Barriers on the
River Severn.
The statement has proved popular with Flood Forecasting, Warning and Operations teams. On the
basis of the statement timely action has been taken to ensure staff are available for shift working up to
5 days ahead. An early indication of the need for deployment of barriers along the River Severn has
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meant that Flood Operations teams can not only check on staff whereabouts during the anticipated
critical periods but can also ready the barriers for transportation if the forecast situation materialises.
Where the potential flood is severe Areas may also chose to pre-warn partner organisations using this
information.
Verification of eleven “wet” outlook statements has revealed that nine have correctly predicted the
scale of flooding with one false alarm and one event missed (but forecast by conventional forecasting
techniques as described above). Operational barrier thresholds on the River Severn at Upton have
been forecast correctly 3 times with one false alarm and one missed instance (but again the threshold
was forecast in a timely manner by conventional forecasting techniques).
SUMMARY
Techniques to explore flood forecast uncertainty using NFFS are being used in Midlands Region of the
Environment Agency. These techniques are helping Forecasters understand the variation in forecast
output due to uncertainties in rainfall forecast and model performance, providing Flood Warning staff
with more information on which to base warning decisions and providing Base Controllers with
information on which to set up and maintain a response to forecast and manage ongoing flood events.
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