October: 20-year extreme shortage

Rainfall status Manilla Sep-Oct 2019

Most rainfall shortages are now extreme

The Rainfall Status graph for October 2019 at Manilla shows extreme rainfall shortages (below the 1st percentile) at most durations.
Of the 25 durations shown, 16 are now extreme shortages, and 7 are severe shortages (below the 5th percentile).
That leaves only 2 that are not far below normal: the October 1-month rainfall (21.4 mm) at the 16th percentile, and the 360-month (30-year) total (18696 mm) at the 14th percentile. Even this 30-year total is lower than any seen here since 1952.

A long-duration extreme shortage

An extreme shortage has now appeared at the very long duration of 240 months (20 years). In the 20 years since November 1999 the total rainfall was only 11893 mm – the 6th driest in history. Drier 20-year periods occurred only in the 1940’s and 1950’s.
Given that the median 20-year rainfall at Manilla is 13010 mm, this is a shortfall of 1117 mm, which is nearly two years of rainfall lost.

Only three new records this month

In this drought, rainfall totals in the 1- to 7-year range have broken records for low rainfall repeatedly. This month only three records have been broken: 24-months (615 mm), 30 months (850 mm), and 36-months (1111 mm).

How to read the graph

This graph shows all the present rainfall shortages at Manilla, short term and long term, as percentile values. The latest values, as at the end of October 2019, are shown by a thick black line with large circles. Those from one month earlier are shown by a thinner line with small diamonds. [The method is described in “Further Explanation” below.]


Further Explanation

The following notes explain aspects of this work under these listed headings:

Data analysis

Cumulative rainfall totals
Percentile values
Severity of rainfall shortages

Limitations of this analysis

Monthly rainfalls form a single population
Observations are not retrospective
The rain gauge failed

Data analysis

Continue reading