Throughout the year, it can be useful to compare the current total rainfall for the year, or season with historical totals of rainfall. It can also be useful, particularly at times such as autumn when seasonal forecast models show low predictability, to look at what history says about the typical range of rainfall values for the remainder of the year or season.

For example, it may be useful to know, in the context of
making an informed farming decision, that there is a high
chance that the total accumulation over an entire cropping
season will be above average (median), even if the rainfall
in the coming months is reasonably low (e.g., around the
10^{th} percentile).

Similarly, if rainfall for the coming season is in the
highest 10% of all past rainfall totals (i.e.,
90^{th} percentile, or very wet) is still not
enough to lift the accumulated rain to above average then,
again, management decisions can be made to allow for such
an outcome.

The Rainfall Ranges graphs show accumulated rainfall to the end of the previous month as a blue line, starting at a specific month selected from the drop down menus.

To place this accumulation into an historical context,
the previous year's rainfall accumulation is shown as a
grey dashed line along with the historical range of
accumulated values. This historical range is indicated by
the 10^{th} percentile (a one-in-ten-year low
rainfall outcome), the 50^{th} percentile or
median, and the 90^{th} percentile (a
one-in-ten-year high rainfall outcome). The range between
the 10^{th} and 50^{th} percentile is
shaded brown. The range between the 50^{th} and
90^{th} percentile is shaded green.

The 90^{th} percentile is wetter than nine
tenths of all years, while for the 50^{th}
percentile it is one half of the years and for the
10^{th} percentile it is only one tenth of the
years. Another way of saying this is that the
10^{th} percentile is drier than nine tenths of all
years. Please see the accompanying example rainfall range
graph.

The rainfall ranges that look ahead from the present
are simply based upon the spread of past rainfall totals.
These scenarios for future rainfall show what conditions
may be if rainfall over the coming period is in the range
from the 10^{th} percentile (very dry past
conditions: red dashed line) to the 90^{th}
percentile (very wet past conditions: green dashed line).
Median / 50^{th} percentile conditions are also
shown (normal conditions: blue dashed line). These
scenarios are added to the current accumulation to arrive
at a range of rainfall possibilities.

The example graph for Tatura shows that rainfall during the previous year was in record low territory, with values easily within the lowest 10% of all records. The current season's rainfall accumulation is just below the median (normal) - far better than for the previous year. If very wet conditions occur it will still take Tatura a month to return to an above average accumulation for the year, however, if Tatura only receives average rainfall, it will remain below average until at least the end of the year.

The following points explain the details of the graph further, with numbers corresponding to those data points on the detailed example.

- The median line at November is simply the accumulation (in the example, rainfall from all past September to November periods) that sits in the middle of the full range of past rainfall accumulations for that set of months.
- The 90
^{th}percentile line at December is the accumulation (in the example, rainfall from all past September to December periods) that is wetter than 90% of past all rainfall accumulations for that set of months. - The previous year's graph shows the accumulation throughout the same period for the previous year. In this example, the value at February is the accumulated total rainfall from September 2006 to February 2007.
- This year's rainfall line shows the accumulation of rainfall to the end of last month. In this case, it is the accumulated rainfall total for the period September 2007 to February 2008.
- The "very dry" scenario takes the current year's
accumulation (in this case September 2007 to February
2008) and adds it to the 10
^{th}percentile accumulation total (i.e., the historical accumulation that is only wetter than 10% of all past accumulations) for the subsequent period (in this case, calculated from all past March to June periods). - The "very wet" scenario takes the current year's
accumulation (in this case September 2007 to February
2008) and adds it to the 90
^{th}percentile accumulation total (i.e., the historical accumulation that is wetter than 90% of all past accumulations) for the subsequent period (in this case, calculated from all past March to July periods).

It is important to note that the scenarios are not a forecast or an outlook. They are simply a range of possible values based upon long term rainfall records, with no one scenario favoured over another. A seasonal outlook would aim to use current climatic conditions, such as whether or not there was an El Niño in progress, to narrow the range of possibilities by providing the likelihood of each scenario occurring.

Not all Bureau rainfall stations are included in the list of Rainfall Ranges locations. At present, only those stations with at least 20 years of continuous data (i.e., no periods of missing data) up to the previous month are considered for calculating the Rainfall Ranges, as any shorter length of data will poorly represent the range of long term rainfall values. As new data comes to hand, it will automatically be included in the Rainfall Ranges. Further work is planned to fill the gaps in the data so even more sites will have a 20 year continuous record.