Seasonal rainfall probabilities
Rainfall information will appear in a new format this month on Climate Explorer.
As well as the day by day rainfall accumulation and monthly totals that have been shown previously on figures similar to the one below, the new figures will include site-specific outlooks of three month rainfall accumulations.
In the figure below, monthly rainfall totals are shown by the vertical bars – grey for historical average monthly total, blue for last season (in this case July 2005 to June 2006), and red for the current season, starting with July 2006. The monthly totals can be read from the left hand vertical scale.
The day by day rainfall accumulations are represented by the curves across the figure, starting from zero at the first of July. Again, blue shows last season, and red the current season. The black curve shows the 50 percentile accumulated rainfall – that is, half of the rainfall amounts accumulated from 1 July are this much or less. The grey curves at the upper and lower edges of the shaded area are the 90 percentile and 10 percentile accumulations respectively.
The new feature of these graphs is the presentation of seasonal rainfall outlooks, shown by green ‘box and whisker’ symbols. The figure above shows symbols at 1 October and 1 November, representing seasonal rainfall outlooks for July – September and August – October respectively. The symbols indicate the expected total rainfall from 1 July until the end of each forecast period; in the example given, the most recent forecast period ends on 31 October, indicated by the positioning of the symbol at 1 November.
The gradations on the symbols, i.e., the ends of the ‘box’ and ‘whisker’ segments, indicate the 5, 25, 75, and 95% probabilities of accumulated rainfall totals for the current three month forecast period. These ‘probable’ totals are added to the amount of rain already measured before the forecast period; the resulting sum of the measured and forecast rainfall determines the position of the forecast symbols on the figure.In the example figure given for Wellington, the symbol shown at 1 November represents a forecast of near normal rainfall for August to October (see the outlook maps on page 3 of this publication) added to the rainfall total at the end of July of about 240 mm.
As each new forecast is added, the figure will reveal how closely (or otherwise) rainfall accumulations follow the seasonal outlooks.
It is important always to remember that seasonal outlooks like this can not forecast individual rainfall events or the length of dry periods. These may in the end have strong influences on seasonal outcomes. Seasonal forecasts can at best provide the general ‘flavour’ of the coming three months.
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