September 2023 Weather Report

 In summary very hot and very dry.  Which will come as no news to anyone who has visited the area in the last month.

Rain

We managed 19.6 mm this month of which 9mm came on the 1st!

This is the 4th month in a row of miserable rainfall.  The ranking over the past 4 months has been June, 4th lowest; July, lowest; August, 2nd lowest; September, 4th lowest. The total over those 4 months - 67.4mm - is the lowest total for 4 consecutive months on record for the area.  The second lowest, 83.2mm, was November 2002 to February 2003.

My prorata estimate of the possible annual fall for 2023 has been in free fall since early May and is currently at 601mm.  (The lowest annual total is 545mm in 2019.)

Temperatures

In discussing temperatures the phrase "Global Warming" often raises itself, particularly with a month like this.  My view is that my data set, which only relates to a single site (albeit with some data 'borrowed' from a nearby site) and for a relatively short period (31 years for most months), cannot say anything about the global situation.  So I won't, other to note I am not likely to argue with expert climatologists.

The difference between the monthly average temperature and the long term mean temperature for September was +1.83C.  The development of this anomaly shows the impact of the very hot spell in the third week of the month.

The companion chart shows the daily minima and maxima in comparison with the long term means of those values.

Minimum temperatures

The average minimum temperature was 9.66C, 1.27C above the long term mean.  The minimum temperature was above the mean for the date on 20 of 30 days.  On 11 days the minimum was more than 1 SD above the mean for the month and on 2 days was more than 2 SD above the monthly mean.  The highest minimum for a calendar day was 14.3C on the 18th and 19th.  (The highest minimum recorded by BoM with the stupid 0900 reset was 15.2C on the 20th.)  In contrast, on the 23rd the minimum (4.3C) was >1SD below the mean!

Somewhat surprisingly the maximin of 14.3C is only =20th high minimum for September: the record is 16.7C on 3/9/2021.  Again surprisingly the average minimum of 9.66 is only the 4th warmest recorded for the area.

Maximum temperatures

The average maximum temperature was 20.70C, 2.33C above the long term mean.  The maximum temperature was above the mean for the date on 20 of 30 days.  On 10 days the minimum was more than 1 SD above the mean for the month and on 6 days was more than 2 SD above the monthly mean.  The highest maximum for a calendar day was 34.5C on the 19th.  (The highest maximum recorded by BoM - unaffected by the stupid 0900 reset - was 34.7C on the 20th: the closeness of these readings can be attributed to the Northerly winds on that day affecting both sites equally.)  In contrast, again on the 23rd the minimum (15.1C) was >1SD below the mean!

In compiling averages I noticed that the average maximum for September was the highest on record, as was the value for July (August was 4th highest)  Looking at the average across the three months as a single set 2023 was the warmest (av. max. 18.38C) by 0.29C above 2013.

Heat waves and related matters.

My working definition of a heat wave is "3 consecutive days with both maximum and minimum temperatures are more than 1 SD above the long term mean."  This is a bit more precise than the BoM definition "A period of abnormally hot weather lasting several days" and I have started a note about whether my definition needs to be loosen a bit: I have no idea when, let alone how, that will end up!

The need to contemplate change arose as a result of the period 14 - 20 September this year.  For each of those 7 days the maximum and minimum were both above the average maximum for the day.  However: on the 16th and the 20th the minimum was less than 1 SD above and on the 18th the maximum was less than 1 SD above.   So there was not a heat wave as I have defined it in the past.

My inclination is to keep "Heat Wave" as is and to define a "Hot Spell" as 3 or more consecutive days in which both maximum and minimum temperatures are above average for the days and at least 2 readings are more than 1SD above the monthly mean for the variable.  On that basis, for September 2023 we have a 7-day hot spell for the period listed above.  Two other periods come close to being Hot Spells:

• 4th - 8th: of 10 readings, 9 were above average (2 >1SD).  However the minimum of the 6th was below average for the date so not a Hot Spell.
• 25th to 28th: of the 8 readings 6 are above average and all 4 minima are >1SD above, with the maximum on the 25th >2SD above.  However the maxima on 26th and 27th were below average for the date so not a Hot Spell.

Once I have finalised my ideas it will be possible to apply similar thinking to below average readings 

for Cold Snaps (≡ Heat Wave) and Cold Spell (≡ Hot Spell).

Humidity

An interesting month.

The striking features were the two periods of very low rH.  Of 38 readings below 25% rH (out of a total of 40,361 readings) 16 are in this month: most others are in October - December.  I have expanded the readings for the last 2 days in this chart. 

Wind

The month was windier that other recent months, as would be expected for September.  The longest run (201km on the 5th) is 140th out of 1600 records.