These forecasts are provided by the Bureau
of Meteorology Operational Consensus Forecasts system and should
be used in conjunction with official forecasts.
Operational Consensus Forecasts (OCF) provide the best-yet automated forecasting
guidance for specific sites around Australia. OCF combines the output
from up to 8 computer weather prediction models, gives weighting to those
that have recently performed best, and learns from its mistakes in real
time by making routine comparisons with actual weather. It was developed
by the Weather Forecasting Group at the Bureau
of Meteorology Research Centre (BMRC), and in verification it generally
performed better than the official (human) Bureau forecasts. It became
operational on 8 March 2005 and is now available unofficially for the
community.
OCF provides forecasts four times daily of up to 7 weather elements for over
600 locations around Australia for up to 8 days ahead. The elements forecast
are:
- Rainfall amount (mm) in the 24 hours FROM 9am daily (note FROM)
- The percentage of models forecasting more than 0.2mm of rain compared to the total number of models used in preparing the forecast. So 75% tells you that 3 out of 4 or 6 out of 8 models used to prepare the forecast for this day were forecasting more than 0.2mm rain.
- Maximum air temperature FROM 9am (note FROM)
- Minimum air temperature to 9am
- Minimum grass (ground, terrestrial) temperature to 9am
- Hours of sunshine during the calendar day
- Evaporation amount (mm) in the 24 hours FROM 9am daily (note FROM).
The locations for which forecasts are available are the Bureau's observing
network sites. Because OCF uses direct feedback from the measurements
at these sites, forecasts are only given for elements where at least 50% of observations over the past 30 days are available at the
site. Most sites report rainfall and the maximum and minimum air temperature,
but grass minimum, sunshine and evaporation are measured by relatively
fewer stations and in most districts there will only be one or two sets
of forecasts for these elements. Sunshine, in particular, is now measured at relatively few locations.
The models from which OCF is derived are the 00, 06, 12 and 18 UTC (4 and 10am and
4 and 10pm EST) runs of the Australian ACCESS-G, ACCESS-R and high-resolution
ACCESS-C models, as well as
the overseas ECMWF, GFS, UK and JMA models. However, OCF is not simply
an averaging of all available computer guidance for each location.
Models suffer biases, both over time and in different situations. Further
bias is introduced when output from a model is used to produce a forecast
for a specific site and time. OCF overcomes these bias problems by using
a bias-correction scheme using a 30-day running window of observations
for each site for which it forecasts. A bias estimate is calculated for
each component model over this period, and is then removed from the current
forecasts. In short, the forecasts remove, as far as possible, known
real-time bias for each model, then produce forecasts that favour the
models that have performed best over the past 30 days.
Critical factors in the preparation of OCF twice daily are the arrival
times of the models. The limited area models (ACCESS-R and ACCESS-C) become
available 2 to 4 hours after observation time but only
have a 36 to 72 hour outlook. The main global models take 6
to 10 hours to become available, but extend to 8 days or more. In the
operational real world, model runs are also sometimes delayed. There
is therefore no "perfect" time to begin the compositing process that
produces OCF, and the compromise that has been struck also has an eye
to presenting the latest information at the times it is most needed in
the preparation of forecasts (early morning and late afternoon).
Verification of OCF was carried out during the 12 months to 31 May 2004, and further work has continued as models have been upgraded and refined. In summary, OCF outperformed official Regional Forecasting Centre maximum temperature
forecasts for days 1 and 2, but was worse at days 3 and 4. However, it
outperformed official minimum temperature forecasts at all projection
hours.
AWN presents the OCF data district by district in a similar format to
the AWN Daily Climatic
Data for each month. The districts are the official rainfall districts
for Australia, and are similar to the Bureau's forecast districts.
5/6/2018
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