Black shows the driest or second-driest rainfall totals. In a record of more than 1600 months, they fall below the 0.1th percentile.
This contour plot shows the progress of the drought at Manilla up to May 2019. Colours show rainfall shortages as percentiles. Dates plot along the top, and durations down the side.
One month rainfall totals (on the top row)
This month (May 2019) like March, had rainfall above normal, but April had no rain. Months of serious rainfall shortage (light brown) had come much earlier, in May, June and July 2018, and also in September 2017.
Shortages lasting less than one year (rows 2 to 9)
As the effects of low monthly rainfall added up, extreme shortages appeared (dark brown). That is, rainfall totals in the lowest 1% of the historical record.
By June 2018, the 2-month and 3-month totals were already extreme shortages. Similarly, by July, the 3-month, 4-month, and 5-month totals were all extreme shortages. By September 2018, extreme shortages extended as far as 9-month totals. That total, adding up the nine months from January to September 2018, included only one month (February) that had rainfall above normal.
In these shorter durations, extreme shortages were rare after September 2018. March 2019 included no shortages (not even “serious” ones) for durations from 2 months up to 6 months. April saw some serious shortages return, but they were gone by May.
Shortages of 1-year to 3-year duration (rows 12 to 36)
By August 2018, an extreme 15-month shortage appeared. The 15-month total then included not only the dry months of winter 2018, but also the dry month of September 2017. By September 2018, the 15-month total became the driest on record at that time (400 mm). In January 2019, extreme shortages were current at 15-, 18-, and 24-months. By that time, some dry months in 2017 were excluded from the totals, but dry months in the current summer were included.
In February 2019, as the extreme shortages of the previous month persisted, the 12-month total became the driest ever (271 mm). March also had four extreme shortages in this group, but now they were at 12, 15, 24 and 30 months.
April saw lowest-ever or second-lowest values for all durations from 12 months to 30 months. In May, they persisted for 15-, 24-, and 30-month durations.
New lowest-ever values increased the percentile ranks of several earlier values that had been lowest-ever. For example, the 15-month total in September 2018 began as a lowest-ever value with percentile rank below 0.04% but, being replaced by new lowest-ever values in April and May, became third-lowest, with percentile rank 0.12%.
A line graph for May 2019
A profile of the rainfall status for May 2019 reveals that extreme shortages also exist at durations much longer than shown in this contour graph: durations of six years and seven years.
Data and method
This kind of graph simply displays the time sequence, month by month, of rainfall shortages that I have displayed on line graphs prepared for each month. In the post for a recent line graph (April 2019) I have described my method of analysis and its limitations.