WMOD00527

• Title: Quantitative Precipitation Forecasting for Improving Reservoir Operations
• Date: 4/1/1995
• Weather Modification - Doc Type: Report

prognostic gridded field input data out to 48 hours, needs to be verified. To examine how
well the NWS large scale models estimate OAK upper-air atmospheric sounding profiles,
NWS personnel at the CNRFC are comparing prognostic grid point data with observed OAK
rawinsonde data. How accurately the orographic precipitation model computes QPFs depends
substantially on the quality of the input data. If the ongoing NWS model comparisons are
encouraging, as anticipated, the CNRFC intends to use the orographic precipitation model
operationally with NWS model prognostic gridded field data for input.

As mentioned previously, the orographic precipitation model has already been set up to run
for other Central Valley Project watersheds in California. However, the model still has to
be "calibrated" for those individual watersheds.

Of particular interest to the Mid-Pacific Region's Central Valley Project Water and Power
Operations Office are the Shasta-Trinity watersheds locatedin the northern part ofthe State.
The area that produces inflow into the reservoir behind Spring Creek Debris Dam is of
particular interest. Once this reservoir is full, the Spring Creek Debris Dam has a high
probability of spilling. Unfortunately, the topographic effects which largely control the
precipitation regime in the Shasta area are quite complex. Among other things, the airflow
is greatly distorted. Sample runs of the orographic precipitation model indicate that it does
not produce enough precipitation in the lower elevations of the Shasta area, even when
constraining the flow to be directly up-valley. Thus, other objective QPF techniques are
necessary for this area. From observation, precipitation rate is still usually directly
proportional to wind speed. A dependency on 700-mb wind direction also exists. Better
quantification of these wind dependencies is needed. Causal mechanisms that occasionally
produce slow moving to quasi-stationary narrow convective bands in the area also need to be
identified.

. Recommendation - Reclamation should perform a climatological study of major storms
that have occurred in the Shasta-Trinity watersheds during the past 10 to 12 yr for the
purpose of calibrating existing objective aids and developing new objective aids. The data
should be partitioned in 10-degree intervals based upon the 700-mb wind direction, using
Oakland, California, and Medford, Oregon, rawinsondes.

2.8.2 Future NWS Hydrologic Forecasting Support

The NWS has the hydrologic forecasting responsibility for the Nation. The mission of the
NWS hydrologic service program is to save lives, reduce property damage, and contribute to
the maximum use of the Nation's water resources. At the present time, the NWS meets its
hydrologic responsibility through the efforts of 13 RFCs (River Forecast Centers) located
throughout the United States (fig. 8). The CNRFC located in Sacramento is the forecast
center responsible for the Central Valley Project area. The NWS takes great care to ensure
that hydrologic forecasts come from only one source so as to not confuse the public.

. Recommendation - Reclamation's research work to improve QPFs and/or runoff forecasts
for specific watersheds, such as the work done in this study for the ARB, should be closely
coordinated with the NWS's RFC that is responsible for the area.

A good data reporting network is the most important element required to support hydrologic
forecasting. Reclamation is a close cooperator with the NWS on developing and maintaining
a large part of the hydrologic data reporting network (e.g., the Early Warning Systems). Of

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