Weather Report December 2023

 We are now supposedly in an El Nino period which according to the BoM 

"In Australia (particularly eastern Australia), El Niño events are associated with an increased probability of drier conditions." 

That usually also means warmer temperatures, as explained in this BoM sheet,  in which case we have 1 out of 2!   Another very wet month with very warm daily minima and mild-warm maxima.  (I haven't looked at the other attributes of El Nino listed by BoM, although in a light-hearted way will note that snow depth in Mallacoota was, as usual, 0.0mm.)

I will do an annual report in the next few days

Rain

We ended the month with 187.4 mm of rain (275% of the median) with non-trivial amounts falling on 12 days. 

In terms of comparison over time we are well over the median and the fall in December 2022.   

The total fall was the second highest recorded for a December since 1976, beaten only by the 211.8mm recorded in 2021.  The variability between years is obvious and there is no trend in these data.

Temperatures

The overall assessment of temperatures can be represented by the difference between the average temperature for the month and the long term average for the same month, referred to as the anomaly.  The next chart shows how the value of the anomaly developed through the month: in effect warming early in the month and then cooling towards the end.

I am compiling a time series of values of the anomaly since I started to calculate this statistic in 2020. I have also compiled a 12 month moving average (to remove seasonal factors) and applied a 4th order polynomial trend to remove 'noise' (or random fluctuations).  This appears to reflect the passage of the last 2 years of La Nina rather well.

Another way of looking at the overall temperature is by comparing the daily extreme temperatures with the averages for that day of the year.  

The chart has a few interesting features:

• The current maximum series fluctuates considerably reflecting the passage of weather events, while the current minimum series is more consistently above the long term average;
• There is (as would be expected) much less fluctuation in the long term series.
• Both the long term series show an upwards trend though the month, probably achieving statistical significance in the case of the maximum series.

Minimum temperatures

The average minimum temperature for the month was 16.10C, the second highest value for December since my series commenced (the highest value of 16.22C was recorded in 2018.  Not surprisingly the 2023 value was well above those for 2022 and the mean.

Noting that all months since July have been above the long term mean offers an indication that at least some aspects of El Nino have been evident for a while.  12 of the daily minima were more than 1 Standard Deviation (SD) above the long term mean for the month and a further 12 were above the average but less than 1 SD above the monthly mean.  Only 7 days were below the mean minimum for the day.

A time series of average December minima shows no significant trend.

Maximum temperatures

The average maximum temperature for the month was 23.89C above the mean but well below the highest average recorded (25.3C in 2019).  It was also above the 2022 value.

It is a trifle problematic to make large statements about the older data (based on BoM data for the airport site, adjusted to make them more comparable with my Weather Station - WS - data).  Instead I offer  the chart below showing 5 years of data from my WS.  It is interesting that the only year with a hotter end was 2019.

Humidity

One word: muggy!  Another word: soggy.  Humidity levels were high through the month with only 2 days recording humidity below 50%.  The chart of readings at the 2 standard times is rather unusual with the 2 lines being much closer together than would be expected.

For both standard times the average relative humidity across the month was above average and the value for 2022.  THat applies particularly to the 1500hrs reading.

I wondered if there was a link between amount of rain and humidity.  The correlation coefficient was not great, at 42.8%, but the chart does show a vague coincidence of higher humidity (as measured by the daily minimum rH ) and rainfall.

Wind

Most days were reasonably calm although the passage of systems did give periodic breezy days (coinciding in most cases with spikes in maximum temperature).  The windiest day, with a run of 310km, was the 4th longest run of 2023 and the 32nd longest (of 1784) recorded at my WS.

Overall the month was quite calm with average run below the values for 2022 and the 5 year average.