Rainfall shortages now
On this graph the black line with black squares shows Manilla rainfall shortages at the end of May 2018. Shortages are shown for short terms down to one month, and for long terms up to 360 months (30 years).
Extreme shortages
There were no extreme rainfall shortages at this date.
Severe shortages
There were severe shortages in rainfall totals as follows:
Total for one month (May): 1.2 mm, at the 2nd percentile;
Total for two months (April and May): 19 mm, at the 3rd percentile;
Total for three months (March, April and May): 45 mm, at the 4th percentile.
Serious shortages
Some other rainfall shortages were not severe, but serious:
Total for five months: 136 mm, at the 9th percentile;
Total for twelve months: 408 mm, at the 6th percentile;
Total for sixty months: 2765 mm, at the 8th percentile;
Total for seventy-two months: 3358 mm, at the 6th percentile.
General shortage
The first comment and reply below notes the fact that no rainfall total for any period reaches the 50th percentile. This has not happened for seventy years (1947).
[Later data
The following graph in this series is in the post: “Rainfall Shortages up to June 2018”. For the much worse situation in June 2019, see “June breaks more drought records”. ]
Comparing May 2018 with September 2017
The graph also has a grey line showing similar rainfall shortages at September 2017 (See the earlier post “A drought has begun”.). In the following month, October, there were no rainfall shortages, because the rainfall, 84 mm, was far above average. November, December and February also had rainfalls above average.
Since March 2018, shortages have appeared again. By comparing the black line (May 2018) with the grey line (September 2017), you can see that the rainfall totals are now lower for nearly all periods of time. Only four totals are now higher, including the 4-month total.
What are the classes of rainfall shortage?
We need to compare rainfall shortages. The best way is not by how far below normal the rainfall is, but by how rare it is. That is, not by the percentage of normal rainfall, but by the percentile value. As an example, when the rainfall is at the fifth percentile, that means that only five percent of all such rainfall measurements were lower than that.
Once the percentile values have been worked out for the rainfall record, each new reading can be given its percentile value. Percentile values of low rainfall are classed as extreme, severe, or serious.
For a rainfall shortage to be classed as extreme, its value must be at or below the 1st percentile.
A severe rainfall shortage is one that is below the 5th percentile.
A serious rainfall shortage is one that is below the 10th percentile.
A rainfall shortage that is above the 10th percentile is not counted as serious.
Long-lasting rainfall shortages
Rainfall shortages sometimes last a long time. The same classes of shortage are used for long periods, such as a year, as for short periods, such as a month. They depend on how rare such a shortage is on the average, and they all use the same percentile values to separate extreme, severe, and serious rainfall shortages.
climate by surly 
What is interesting to me is that now, there is no measured period from 1 month to 360 months where the rainfall has exceeded the long-term average, i.e. the 50th percentile. Clearly this is not the same thing as saying that every MONTH in the last 360 has been below average, but every one of the measured periods has been. I would expect this to be extremely rare in itself.
Rare, Allan, but not extremely rare.
Using the Excel COUNTIF function, I find that there have been 38 months out of 1600 months when every one of my 25 selected numbers of months (from 1 to 360) has returned a percentile value below the 50th percentile. That is to say, it has happened in 2.4% of the months.
This is not the sort of rare event that would be noticed. It could happen even if all of the rainfall shortages were barely below the 50th percentile.
What is truly remarkable is that this situation, which prevailed in January, March, April, and May of 2018, had not happened AT ALL in the seventy years since 1947.
The other 34 months with all their rainfall totals below the 50th percentile were in the two decades from 1927 to 1947. The only years with four or more such months were 1927 (4 months), 1940 (7 months), 1946 (10 months), and 2018 (4 months so far).
the last line I presume should read ‘extreme, severe and serious’?
DD
Thanks DD. Just testing.
There’s an interesting 2014 time series of graphs showing Australian rainfall in pentiles (is that the right word for 20%-iles?) and you can see from this that the severe year of 1946 was the lowest rainfall on record – but only locally for the Tamworth area. Most of the country was not in drought.
http://www.abc.net.au/news/2014-02-26/100-years-of-drought/5282030
Thanks for linking to that excellent set of rainfall decile maps, Allan. I get more value from the View showing all the maps at once, not the “interactive view”.
My main impression is that no two years have had similar rainfall in Australia. The most nearly similar times were not the driest, but the wettest: 1973, 1974, 2000, and 2011.
I have done a plot of Australian average rainfall (and temperature) 1950 to 2008 in the post “Australian climate Quasi-Biennial Oscillations”.
On that plot, the summer of 1973-74 was the wettest time and the summer of 1999-2000 the next wettest.
The rainfall decile maps show only data for whole calendar years. There could be more extreme six-month droughts that do not show at all. For droughts longer than a year, you can try to add two or more maps together in your mind, but it is very hard to do.
Even famous droughts, such as the “Federation drought”, and the “Millennium drought” are not well supported by what the maps show. Droughts that affect few people are not mentioned. One of Australia’s driest years was 1952, when the tropical monsoon failed, but coastal Victoria was wet.
The same series of maps, now updated to include 2017, is at:
http://www.bom.gov.au/climate/history/rainfall/