A potential deluge!

The Bureau of Meteorology (BoM) has pretty consistently forecast a good drop of rain for us on Monday 6 November.  The current prognostication is for 30 - 45 mm.
It is noteworthy that the last change in the forecast (on 4 November) was to increase the range  from 15-30mm.  In recent months the BoM has tended to reduce falls as the forecast period has got closer.

The 27mm fall on 26 October, while welcome, was basically a short sharp event (done and dusted in 30 minutes) at such a rate that most of the water ran straight off.  This will be good news for irrigators when it eventually flows to them, but not much help here. 

In contrast this event looks to be a day of steady rain.  One depiction of this is from Time and Date - note the columns under the sunshine etc icons.
The localised BoM service Meteye doesn't offer a precise forecast by hour but gives probabilities for 3-hourly periods. 
The timing of the rain is pretty consistent with Time and Date, with the 50% probability summing to close to the lower limit of the main BoM forecast and the 25% probability matching the upper limit.

The Weather Channel agrees with a day of rain but isn't game to suggest amounts per hour.   Weatherzone doesn't offer an hourly forecast (that I could find) but does examine the entrails of domestic livestock for the next 28 days: in summary, damp!
Enough with the forecast already!  What does history tell us?

My first effort is to look at a scatterplot of daily falls in November from 1993 to 2016.
Of the 720 dates, there has been measured rainfall on 206 days (28% of days). Of those wet days, 10 (4.8%) have recorded more than 30mm and 3 have recorded >40mm.  The highest daily rainfall recorded in November in Carwoola is 50.5mm in 1995.  So what is forecast is definitely an unusually high fall.

If rain falls every day in which the Weatherzone forecast shows medium or high probability of  wetness we would have 10 rainy days.  This is 33% of days, slightly above the average.

Even if we get 45mm, there is a way to go to achieve an average fall for the month.  The mean November rainfall for 1993-2016 is 75.3mm and the median fall is 72.6mm.

My final chart looks at whether there is a long term trend in November rainfall.
 Not only is it visually obvious that the points are spread pretty evenly, but the trendline is flat and the value of R2 is about as small as I have ever seen!  In other words there isn't a trend in this series.  (Which I hope does not give any support to the ranting of NolongeraSenator Malcolm Roberts.)


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