September 2025 Weather Report

 The dominant feature of the weather this month has been lack of rain.  The cause of this appears to be a Sudden Stratospheric Warning event which first came to my notice in a Facebook post by cyclonesOZ on 27 September.  This article from the ABC has some detail: I haven't been able to find a definitive statement from BoM on the subject, although a spokesmeteorologist is quoted in the linked article.

Rain

With the aid of 1mm on the 30th we managed to achieve 17mm for the month.  This is the only 3rd highest September fall since my records (at the BoM site at the Airport or my WS) begin in 1975.  Here are the daily falls.

The highest daily fall was 6.6 mm: there are only 34 months (out of 570) in which the maximum daily fall was lower.  Not surprisingly the monthly total was very much below median and last year.

The pro-rata average estimated annual fall for 2025 is still high, reflecting the heavy falls in earlier months.  I note that the trend line of this series takes a major dive in September.

Temperature

My analysis of the temperature anomaly began with finding that I had used a series of averages incorporating temperatures from the BoM site rather than, as has been my recent practice, restricting the analysis to my WS data.  It is of interest that correcting this cock-up reduces the value of the anomaly from +0.65C to +0.13C! After a cold start the anomaly hovers around 0.0C for the rest of the month.

The monthly values of the anomaly are highly variable but the 12 month moving average shows relatively stable high values for the last 2 years.

The chart of daily extremes shows considerable variation especially for the maximum series, but higher values are much greater than those below the mean.

Minimum temperatures

The mean minimum temperature for the month was 8.7C, 0.57C lower than the monthly mean minimum.  The lowest temperature recorded was 4.0C, very significantly below the mean low for the month.  After running on the high end of recent experience for the first half of the year, the last three months have been on the lower side. 

There were 3 days - early in the month - significantly below average monthly low and 2 days - late in the month - significantly, or very significantly, above the mean monthly minimum.

Maximum temperatures

The mean maximum was 19.5C, 0.85C above the average maximum for the month.  The highest  temperature recorded was 25.2C on the 8th, one of 5 days through the month significantly above the mean low for the month.  The lowest maximum was 14.2C. one of 3 days significantly below average monthly high.  After running on the high end of recent experience for the first half of the year, the last three months have been variable but September is on the higher side.

Humidity

The daily readings for the standard times are reasonably 'normal' apart from 8th - 10th and 16th when the morning humidity was higher that that in the afternoon.  It is possible that these values reflect a wind shift to the South, although the phenomenon is not so evident in the BoM data.

The values for both standard times are a little lower than average.

Wind

The month was relatively calm with the maximum daily run only 127km.

Although the run is a little longer than that of August it is still well below the Average and 2024.

Tempest data

I am able to access data for a WS at the Wharf which uses the Tempest network and records some further very interesting data.  

Lightning strikes

As well as data from the WS Tempest brings in information from other sources including a tally of lightning strikes within 40km radius of the WS site.  For the Mallacoota site the raw data is made almost useless by a record of 6900 strikes (I believe real) in February 2025 dominating the chart.  It does give a good indication of relativities if plotted as log10 values (thus 6900 becomes 3.84).

The number of strikes in September was 23 (log10 =1.36).

Solar Radiation etc

The system has a range of measures of Solar Radiation and Brightness which are very relevant in these days of solar heating and power generation.  The chart below shows the value of solar radiation received and a 12 month moving average which eliminates the very obvious seasonal variation.

Applying a linear trend to the average suggests that the series is trending down, but the correlation coefficient suggests this is not significant.