11.1.3 Quantitative Precipitation Forecasts (QPF)
The RWFS is designed to optimize forecasts based on feedback it receives from
observations collected at or nearby the RWFS forecast points. One of the most difficult
weather prediction problems is forecasting precipitation amounts. Part of the problem is
in the forecasting process (model physics and knowing the state of the atmosphere) and
the other part is related to the poor observation network and the inability to verify actual
precipitation amounts, particularly for winter precipitation (e.g., snow, sleet, etc.).
The lack of quality winter precipitation observations across Iowa hindered the RWFS in
that it was unable to tune itself for the QPF parameter. When the RWFS was initialized
in the fall of 2003, each of the ten forecast modules had a 10% weight. A few fall rain
cases were enough to move the weights from the initial values, but they never had a
chance to adjust fully before the winter precipitation season began.
Without the tuning process, all the QPF modules would essentially be averaged (10-
member average) and the MDSS would not be able to take advantage of the skill of any
particular model or dataset. This could result in a worse precipitation prediction than
directly using the more skilled models. It could also result in the inability for the system
to declare a light precipitation event that may have been well predicted by one or more of
the forecast modules. This of course is opposite of what the RWFS was designed to do.
The project staff monitored the weights during the first 45 days of the demonstration and,
based on the lack of adjustment, decided to intercede.
Because of the poor quality of real time winter precipitation measurements, it was
necessary to modify the RWFS to fix the QPF weights across the ten modules based on
expert opinion. The weights for the QPF parameter were fixed on 10 February 2004 as
shown in Table 11.1.
Table 11.1. Weights given to each of the ten RWFS quantitative precipitation forecast
modules.
Des Moines & RWFS QPF
Ames Area
Weights
TOTAL
Forecast
MM5 MM5
MM5 Total
WRF WRF WRF Total
MAV-
MM5+WRF
GFS Eta 2hr
WRF RUC
MOS Total % Contribution
Period
3hr
4hr MM5
2 hr 3hr
4hr
0 hr Forecast
9
11
15
12
10
37
17
14
12
43
0
0
100.00
80
3 hr Forecast
9
11
15
12
10
37
17
14
12
43
0
0
100.00
80
6 hr Forecast
9
11
15
12
10
37
17
14
12
43
0
0
100.00
80
99
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