Decision support experiments and evaluations using seasonal to ...

The U.S. Climate Change Science Program Chapter 2
The existence in
digitized form of
the retrospective
archive of seasonal
forecasts is critical
for the verification
of forecast skill.
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The provision of a service that assists hydrologic
forecast users in either customizing a
selection of ESP possibilities to reflect, perhaps,
the users’ interest in data from past years that
they perceive as analogues to the current year,
or the current ENSO state, is a notable advance
from the use of “climatological” ESP (i.e., using
all traces from a historical period) in the prior
ESP-related seasonal forecast products. Some
western RFCs have also experimented with using
the CPC seasonal climate outlooks as a basis
for adjusting the precipitation and temperature
inputs used in climatological ESP, but it was
found that the CPC outlook anomalies were
generally too small to produce a distinct forecast
from the climatological ESP (Hartmann
et al., 2002). In some RFCs, NWS statistical
water supply forecasts have also provided perspective
(albeit more limited) on the effect of
future climate assumptions on future runoff by
including results from projecting 50, 75, 100,
125 and 150 percent of normal precipitation in
the remaining water year. At times, the official
NWS statistical forecasts have adopted such
assumptions, e.g., that the first month following
the forecast date would contain other than
100 percent of expected precipitation, based
on forecaster judgment and consideration of a
range of factors, including ENSO state and CPC
climate predictions.
Figure 2.6 shows the performance of summer
streamflow volume forecasts from both the
NWS and NRCS over a recent ten-year period;
this example is also part of the suite of forecast
products that the western RFC designed
to improve the communication of forecast
performance and provide verification information.
Despite recent literature (Welles et al.,
2007) that has underscored a general scarcity
of such information from hydrologic forecast
providers, the NWS has recently codified
verification approaches and developed verification
tools, and is in the process of disbursing
them throughout the RFC organization (NWS,
2006). The existence in digitized form of the
retrospective archive of seasonal forecasts is
critical for the verification of forecast skill. The
ten-year record shown in Figure 2.6, which is
longer than the record available (internally or
to the public) for many public agency forecast
variables, is of inadequate length for some types
of statistical assessment, but is an undeniable
advance in forecast communication relative
to the services that were previously available.
Future development priorities include a climate
change scenario application, which would
leverage climate change scenarios from IPCC
or similar to produce inputs for future water
supply planning exercises. In addition, forecast
calibration procedures (e.g., Seo et al., 2006;
Wood and Schaake, 2008) are being developed
for the ensemble forecasts to remove forecast
biases. The current NOAA/NWS web service
Internet web address is:
A contrast to these probabilistic forecasts is
the deterministic five-week forecast of lake
water level in Lake Lanier, GA, produced by
the U.S. Army Corps of Engineers (USACE)
based on probabilistic inflow forecasts from
the NWS southeastern RFC. Given that the
lake is a managed system and the forecast has
Figure 2.6 Comparing ESP and statistical forecasts from the NRCS and NWS for a recent 10-year period. The forecasts are for
summer (April through July) period streamflow on the Gunnison River, Colorado.