April 2019: 7 lowest rainfall totals

In April 2019, rainfall totals were the lowest-ever values for 1 month and for 15, 18, 24, 30, 72, and 84 months.

Rainfall status Mar-Apr 2019

Graph of Rainfall Shortages

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 March, are shown by a thick black line with large circles. Those from one month earlier are shown by a thinner line with smaller circles. [The method is described in “Further Explanation” below.]

Record and near-record low rainfall totals

There never has been a month at Manilla when rainfall totals have included so many record or near-record low values.

  • The 1-month total for April is zero (as in April 1912, 1925, 1942, and 1971).
  • The 12-month total of 283 mm is second-lowest after February 2019 (271 mm).
  • The 15-month total of 397 mm is a new record, beating September 2018 (400 mm).
  • The 18-month total of 510 mm is a new record, beating April 1966 (514 mm).
  • The 24-month total of 745 mm is a new record, beating July 1966 (766 mm).
  • The 30-month total of 1005 mm is a new record, beating March 2019 (1078 mm).
  • The 72-month total of 3060 mm is equal-lowest with February 1903.
  • The 84-month total of 3653 mm is a new record, beating February 2019 (3672 mm).

Other changes from March to April

When rainfall in April was 55 mm less than in March, the short-duration totals for 3, 4, 5, and 6 months became serious shortages again.
New severe rainfall shortages appeared at the very long durations of 144 months (12 years) and 240 months (20 years). Such shortages have not been seen in half a century: not since 1969 in the case of 144 months duration, and not since 1950 in the case of 240 months duration.


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

March rain leaves drought extreme

Above-average rainfall in March reduced the shortage of rainfall in the last 3 months. It did not relieve extreme shortages at durations between 12 months and 7 years.

Rainfall status Feb-Mar 2019

Graph of Rainfall Shortages

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 March, are shown by a thick black line with large circles. Those from one month earlier are shown by a thinner line with smaller circles. [The method is described in “Further Explanation” below.]

Good rain in March

A rain front at the end of March 2019 that brought about 40 mm took the March total up to the average. This raised the 2-month and 3-month totals nearer to normal. It did nothing to increase longer-duration totals.

Extreme rainfall shortages

By February, six of the eleven rainfall shortages measured over durations from 12 months to 7 years were extreme. (That is, those totals were in the driest one percent in history.)
Despite the high rainfall of March, March figures also record six extreme shortages. The 18-month total is no longer extreme, but the 30-month total has now become extreme.
Two of the rainfall totals (plotted on the 0.1% line) are near-record low values. The 24-month total of 769 mm is the second lowest after July 1966 (766 mm). The 30-month total of 1078 mm is equal lowest with October 1966.
Data for February, plotted on the thinner line, show the record low values for 12 months (271 mm) and for 84 months (3672 mm).

The previous 24 months

The development of this drought through the previous 24 months is shown in the later post “Rainfall Shortage Sequence 03/2019”. A contour graph shows severity of shortage by contoured layer tints, with serial months on the x-axis and duration of shortage on the y-axis.


Further Explanation

[Update 5 April 2019.]

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

This graph is based on analysis of monthly rainfall totals from 1884. Using the spreadsheet application Excel, I calculate cumulative totals and their percentile values. Using these values, I identify rainfall shortages as serious, severe, or extreme .

Cumulative rainfall totals

I prepare two tables. The rows in each table are serial months, more than 1600 in total. The columns in each table are headed by the selected number of months, n, as specified below. In the first table I cumulate the rainfall totals. First, I add each month’s rainfall total to that of the previous month for a 2-month total. Using the previous two months, I get a 3-month total, and so on. In this way, I get n-month rainfall totals from n = 1 up to n = 360 (30 years). However, I calculate for only the following 25 values of n:

n = 1, 2, 3, 4, 5, 6, 9, 12, 15, 18, 24, 30, 36, 42, 48, 60, 72, 84, 96, 108, 120, 144, 180, 240, 360

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One- and two-year droughts recur in January 2019

Drought contour plot to Jan 2019

This contour plot shows the progress of the extreme drought at Manilla up to January 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)

By January 2019, there had been six months without serious monthly rainfall shortages. The months of serious rainfall shortage (light brown) were earlier, in May, June and July 2018. The only other month with such low rainfall was September 2017.

Droughts lasting less than one year (rows 2, 3, 4, 5, 6, 9)

As the effects of low monthly rainfall added up, extreme droughts appeared (dark brown). That is, rainfall totals below the 1st percentile. (They are the lowest rainfall totals, that have occurred in less than 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 extreme shortages. By September 2018, extreme shortages extended as far as the 9-month total. That total, adding up the nine months from January to September 2018, included only one month (February) that had rainfall above normal.
In these durations of less than one year, extreme droughts were rare after September 2018. Because the last six months had no serious monthly shortages, the final month (January) includes no serious shortages for durations from 2 months up to 6 months.

Droughts of 1 year to 2 year duration (rows 12, 15, 18, 24)

By August 2018, an extreme 15-month drought appeared. That 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 (400 mm). By October all four droughts in this group (12-, 15-, 18-, and 24-month droughts) were extreme. This became true again in January 2019. By that date, some of the dry months of 2017 were no longer included, but dry months in the current summer replaced them in the total.


Related graphs

A similar contour plot with data to October 2018 is here.

A line graph of the rainfall status for January 2019 reveals that extreme shortages also exist now at the much longer durations of six years and seven years that are not included in this contour plot.


Notes

The following notes include:

Classes of rainfall shortage

Rainfall rate versus percentile rank

Limitations of this analysis

Note: Classes of rainfall shortage

Continue reading

The 2002 drought contour chart

Contour chart 2002 drought at Manilla NSW

The 2002 drought at Manilla was a failure of winter rainfall. [See Note below: “Manilla’s rainfall seasonality”.]

The top line of this contour chart shows that monthly rainfall shortages occurred in all the six months of winter rainfall dominance (April to September) of 2002. Shortages in May and July were severe, below the 5th percentile. In the summer rainfall months (October to March) that preceded and followed, rainfall was near or above normal. [See Note below: “Classes of rainfall shortage”.]

Lower down the contour chart, rainfall shortages of longer duration are shown. For droughts of 3 months duration, the rainfall shortage was extreme (1st percentile) by July 2002, as it included the serious shortage of May as well as that of July. In the same way, one sees extreme 6-month shortages in September and October, as all the monthly rainfall shortages since March added up.

By November 2002, one sees extreme droughts of 9 months and 12 months duration. The 9-month drought incorporated the consecutive months of below-normal rainfall from March to November. The extreme 12-month drought (307 mm) that was evident in November 2002 began earlier, with below-normal rainfalls in December 2001 and January 2002. That was the fourth driest 12-month period on record, after October 1965 (288 mm), August 1946 (302 mm) and November 1965 (304 mm).

The 2002 drought had no extreme rainfall shortages longer than 12 months. There were, however, some severe shortages of 18 months duration and some serious shortages of 24 months duration, due to some low rainfalls in the previous winter (2001).

By April 2003, hardly any serious rainfall shortages due to the 2002 drought remained. [See Note below: “Limitations of this analysis”]

More about the 2002 drought

Graph of monthly percentile rainfall in a droughtAnother approach to describing this 2002 drought is in the post “The 2002 rainfall shortages at Manilla”. That post has a graph showing selected monthly profiles of percentile values. It also links to two earlier posts with graphs of smoothed values of climatic anomalies.

The 2018 Drought

Drought 2018 contour chartA similar contour plot for the drought of 2018 reveals similarities and differences.


Note: Classes of rainfall shortage

Continue reading

An Extreme 24-month Drought

The 830 mm of rain that fell in the last 24-months is the lowest in 50 years.

Rainfall shortages September and October 2018 at Manilla

Graph of Rainfall Shortages

This graph shows all the present rainfall shortages at Manilla, short term and long term, in terms of percentile values. The latest values, as at the end of October, are shown by a black line with black circles. Those from one month earlier, at the end of September, are shown by a thinner line with smaller white circles.

Changes this month

October rainfall that was normal (51.6 mm) also raised the totals for 2 months and 3 months, so they did not qualify as serious shortages. The 4-month total of 101 mm just qualified.
Extreme shortages formed a new pattern. In September, two values had been exceptionally low: the 6-month total had been the third lowest ever, and the 15-month total had been the lowest. By October, no rainfall was far below the 1st percentile value. They clustered at 12 months, 15 months, 18 months and 24 months.
As the 24-month rainfall shortage was extreme (below 840 mm), this became the worst 24-month drought in half a century. Extreme 24-month droughts had come in 1902, 1913, 1946, and 1966, but never since.


Further Explanation

Much more detail was given with last month’s graph of rainfall shortages, in the post: “Record 15-month Drought in 2018”. Notes include: “Long-term shortages”, “Classes of rainfall shortage”, and “Manilla rainfall records”.


Drought development plot

Drought 2018 contour chartThe development of the 2018 drought at Manilla is shown in the post “Contours of Manilla’s 2018 Drought”. The graph there shows contours of drought severity plotted against date and duration.

Contours of Manilla’s 2018 Drought

Drought 2018 contour chart

Displaying drought severity, date, and duration.

This image shows how an extreme drought developed at Manilla, NSW in 2018. The contours and colours indicate drought severity. They show whether the rainfall shortage was extreme, severe, or merely serious. At times without drought, they show when the rainfall was above normal.
[See Note below: “Classes of rainfall shortage”.]
Calendar months appear in order along the top, showing how the severity of drought has changed as time has passed.
Down the side, the duration of drought is shown, from one month duration at the top to thirty-six months duration at the bottom. Each month is shown on one vertical line, with a severity value plotted against each duration value.
[In other posts (like “Record 15-month Drought in 2018”) each individual month’s drought data is plotted as severity versus duration.]

The pattern of the 2018 drought

Development

In each of the months May, June, and July 2018, the monthly rainfall was a serious or severe shortage, below the 10th percentile. As rainfall had been below normal also in March and April, each month after April saw severe and even extreme shortages that extended to durations of 3 months and longer. By August, there were extreme droughts of five-month and six-month duration, despite reasonable rain (28 mm) having fallen in August.
By September, the nine months to date were in extreme drought. That is to say, the first nine months of 2018 had a total rainfall lower than any but 1% of all nine-month periods in history.
However, in that month (September 2018), the rainfall shortage for a 12-month duration was not extreme, but merely severe. Yet the 15-month shortage for that month was also extreme. In fact, it was the lowest 15-month rainfall total in history (400 mm).
The source of the extreme 15-month shortage at this date is obvious. One year earlier, the month of September 2017 had a serious rainfall shortage, and the two previous months had little rain. From these short-term shortages in spring 2017, shortages of longer duration descend as an arc across the graph. They become mild nine-month shortages during the summer, then worsen to extreme by merging with 2018 shortages.

Seasonality

Manilla’s rainfall is seasonal, with two distinct modes, each dominating half the year. There is a major summer (monsoonal) mode and a minor winter (westerly) mode. The summer rainfall mode dominates from October to March and the winter rainfall mode dominates from April to September. [See Note below: “Manilla’s rainfall seasonality”.]
This graph documents a failure of winter rainfall. The 2018 months that had rainfall shortages were the months of dominance of the winter rainfall mode (April to September). Rainfall had been near or above normal in the preceding summer rainfall mode (October 2017 to March 2018), and was so again in October 2018. [See Note below: “Limitations of this analysis”]
The rainfall shortages in the previous year (2017) were also restricted to the months of the winter rainfall mode, but began very late. Shortages did not develop until the final three months (July, August, September). Only when the extreme drought of 2018 developed did those 2017 shortages have a big effect.


The 2002 drought contour plot

Contour chart 2002 drought at Manilla NSWThe most recent extreme drought at Manilla before 2018 was the drought of 2002.
A contour plot in the same format is in this post.


Note: Classes of rainfall shortage

Continue reading

Record 15-month Drought in 2018

The 15 months to date is the driest 15 months in the Manilla rainfall record from 1883.

Rainfall shortages August and September 2018

Graph of Rainfall Shortages

This graph shows all the present rainfall shortages at Manilla, short term and long term, in terms of percentile values. The latest values, as at the end of September, are shown by a black line with black circles. Those from one month earlier, at the end of August, are shown by a thinner line with smaller white circles.

Changes this month

The September rainfall total of 12.5 mm was at the 18th percentile. That raised short-term totals (2-, 3-, and 4-month totals) compared with those of a month ago. However, totals fell very much lower for 5-, 6-, 9-, 15-, 18-, and 24-months.

Extreme shortages

Extreme shortages, seen less than 1% of the time since 1883, are now seen for the durations of 5, 6, 9, 15 and 18 months. This drought is now worse than the drought of 2002. In that drought the longest duration of extreme shortage was only 12 months: 307 mm (at the 0.2th percentile) from December 2001 to November 2002.

A record rainfall shortage

The 15-month total of 400 mm is the lowest in the 136-year record. It is a rate of 26.7 mm per month, 49% of normal, and at the 0.06th percentile. It beats the previous lowest 15-month total of 404 mm that was set in May 1912.
The last time that records for low rainfall were set at Manilla was nearly 50 years ago. Those records were: only 1 mm rainfall in the two months to April 1971, and only 14 mm in the four months to June 1971.

Long-term shortages

The 6-year rainfall total (3244 mm) is a severe shortage, unchanged for three months. These values are lower than any 6-year rainfall totals since 1962. When rainfall shortages of such long duration persist, rainfall does not maintain the groundwater levels or river flows required for irrigation or town supply.
A serious 20-year shortage (9.7th percentile) has developed in this month. Such a very long-term shortage has not been seen since 1949. Up to that date, the Namoi River had suffered decades of low flow, which was followed later by much higher flows. Manilla’s mean annual rainfall has been above normal from 1949 until recently.


Classes of rainfall shortage

I have adopted two classes of rainfall shortage from the classes of “Rainfall deficiency” defined by the Bureau of Meteorology in their Climate Glossary as follows:

“Serious rainfall deficiency: rainfall lies above the lowest five per cent of recorded rainfall but below the lowest ten per cent (decile range 1) for the period in question,
“Severe rainfall deficiency: rainfall is among the lowest five per cent for the period in question.
“Areas where the rainfall is lowest on record for the given time period are also shown.”

The Manilla rainfall record allows me to be more exact than the Bureau. Because the record extends back 134 years, it includes more than 1200 cumulative monthly rainfall values. I can identify percentile ranks even below the 0.1th percentile.
To the Bureau’s two classes of deficiency I add a third:

“Extreme deficiency (or extreme shortage): rainfall lies below the lowest one percent for the period in question.”


Manilla rainfall records

Manilla Post Office rain gauge, Station 055031, was read daily from 1883 to 26 March 2015. Then, for 15 months there was no official Manilla rain gauge. A Bureau of Meteorology automatic rain gauge was re-located to the museum yard and operated as Station 055031 from 23 May 2016 to 7 October 2016 (4 months). It failed, and did not operate for 5 months. After repair, the gauge was read automatically at 9 am daily as Station 055312: Manilla (Museum). It failed again after 6 months, on 24 September 2017. The web-page for Station 055312 shows that, since its repair on 15 March 2018, the gauge has been unreliable, with readings frequently missed. No readings were recorded in September 2018.
Since April 2015, I have read my rain gauge in Monash Street Manilla daily. I have used these readings when official readings are lacking. The gauge is not precise, the site does not meet specifications, and it is 1 km from the Post Office. However, the daily readings are seldom more than 3 mm higher or lower than available official Manilla readings.