Since the twelve-month drought of 2002, Manilla has been free from extreme rainfall shortage until now. Such a long gap between extreme droughts has not been seen here before. [See Note below: Dry May 2016.]
Rainfall shortages now
On this graph the black line with black squares shows Manilla rainfall shortages at the end of June 2018. Shortages are shown for short terms down to one month, and for long terms up to 360 months (30 years). [Shortages at the end of May are shown in a previous post.]
Three extreme rainfall shortages have now developed, all below the 1st percentile rank:
Total for two months (May and June): 6 mm;
Total for three months (April, May and June): 24 mm;
Total for four months (March, April, May and June): 50 mm.
There were five severe shortages in rainfall totals as follows:
Total for six months: 141 mm, at the 4th percentile;
Total for twelve months: 350 mm, at the 2nd percentile;
Total for fifteen months: 492 mm, at the 3rd percentile;
Total for sixty months: 2672 mm, at the 4th percentile;
Total for seventy-two months: 3317 mm, at the 4th percentile.
Some other rainfall shortages were not severe, but serious:
Total for one month: 5.2 mm, at the 7th percentile;
Total for five months: 120 mm, at the 6th percentile;
Total for nine months: 464 mm, at the 10th percentile;
Total for eighteen months: 658 mm, at the 6th percentile.
Comparing June 2018 with the month before
The graph also has a grey line showing the rainfall shortages in the previous month, May 2018. Apart from the single month totals (May total: 1.2 mm; June total: 4.8 mm), every rainfall total for June was lower than the same total for May.
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.
Note: Dry May 2016
The month of May 2006 had no rain at all, but that extreme shortage did not last two months.
An improved graph
This improved graph (added 6 July 2018) includes April 2018 as well as May and June. You can see that April had no short-term serious shortages, and that nearly all the shortages got worse with each passing month.
Actual rainfall totals are shown for the worst shortages as at the end of June. Normally, Manilla’s monthly rainfall is about 55 mm, and annual rainfall 652 mm. Reading the figures shown, the four-month total is now about one quarter of normal, the twelve-month total just over half normal, and the five-year total about 80% of normal.