September 2019 Weather report

Summary

Overall a quite dry, but windy month.  Temperatures were a mixed bag, setting a record for high minimum (on 20th) but the temperature anomaly was minus.

Details

Rainfall

We managed to get 41.2 mm of rain at Angophora Drive.  This compares with  60.4 mm recorded by BoM at the  Airport and  29.6 mm at BoM on. Gabo Island.  That pattern suggests that the rain was quite squally, with falls dependent on just where the cells passed.  While we haven't had any days of really steady rain Angophora did get 18.8 mm on the 9th: this was the first fall of more than 10mm since the 5th of May!

Overall we did better than last year (comparing, a little dodgily, Angophora this year with BoM last year) but were way down on median fall for the month.

Doing a pro-rata estimate of the annual rainfall, gives an estimate of 570 mm for the year.  The previous lowest fall recorded, since records began in 1975, was 657 mm in 1980.  The values given by prorating year to date totals show a fairly stable situation in recent months.

The BoM produces a wide range of rainfall maps both summarising history and looking forward.  The "history" set includes a map showing this area to be amongst the worst in Victoria in terms of rainfall deficiency.

Looking forward doesn't give much cause for optimism either.  We have a 25-30% chance of rainfall greater than median over the rest of the year.

I think Hanrahan is clearing his throat!

Temperatures

The best measure to summarise overall temperatures is the difference between monthly average temperature and the long term average temperature for the month.  For September 2019, using BoM Mallacoota data for both values, that came in at -0.2^oC.  Very close to no difference.  This was a balance between a majority of days (20/30) having an average below the mean for that day and a few days in which the average was up to 6^oC above the mean.

To finish this overview, here are the daily extreme temperatures through the month. Comment on the two series follows below, but the minima on 20 -21st were remarkable!

Maximum temperatures

The average maximum for the month was 18.1^oC, ~0.5^oC above the average. So far this year 8 of the 9 months have had an above-average mean maximum temperature (if that set of comparative adjectives make any sense).  The same applies whether using data from my WS or the BoM series.

The mean maximum temperature was higher than last year and the long term average.

A time series of average September maxima since 1994 looks like a profile of the Great Dividing Range with no obvious trend.  

Minimum temperatures

The average minimum for the month was 8.4^oC: a little above average and last year.  

Referring to the trace of daily minima in the preamble to the temperature section, the precise comparison of my temperatures on a sensible 24 basis against the BoM on a hoss'n'buggy 9am approach is difficult for minima.  However sticking to the BoM data the minimum of 16.9^oC on the 21^st was the warmest September minimum since 1993 by 0.6^oC.  The 15.7^oC recorded on the 20th was the fifth warmest.  (If those two days had recorded the average minimum for the dates the anomaly would have been 0.1^oC higher than observed: surprisingly little impact.)

Again the majority (7/9) of months for the year show an above average minimum temperature with BoM and WS giving similar patterns.  (I have also checked the daily minima for the two sites against each other and the correlation is very solid at 92%.)

The time series of average minimum temperatures for September shows no obvious trend .

Humidity

Humidity is rather difficult to make large statements about as it seems that for some months - particularly September - there are significant differences between the BoM site and my WS.  Until I have more data for my WS, perhaps enabling some calibrations, I will restrict this section to showing the daily lines for 0900 hrs and 1500hrs

It is far from helpful that the only detailed BoM data available is for the very dry 2018, when both humidities were significantly below the long term BoM averages.

Wind

Nearly all of this information comes from my WS as BoM publishes very little information about wind.

Speed

The daily wind run on the 20^th was 519 km.  The longest I have recorded at Angophora Drive.

The month as a whole was the windiest I have recorded since February.  The uptick in average daily run fits with BoM historic data.

Direction

The predominant directions of the wind were South West to West, followed by North West to North.

Compared to overall wind direction for previous months September had many more observations from West and a balancing less from North North West.  However when the directions are ranked in order of prevelance of wind the ranks only shift by +/- 1.