May 2021 Weather Report

The month was rather wet with several rain events.  Temperatures were bipolar with a warmer than usual start to the month and a cooler second half

Rainfall 

The total for the month at my Weather Station was 151.0mm.  This is 220% of the median fall for May and 185% of the mean.  Generally falls at home are similar to those at the airport so it is reasonable to say that this was the 5th wettest May recorded since 1975.  (The heaviest fall recorded at the airport in May was 1978 with 329 mm!  That month included the heaviest day of rain reported for Mallacoota with 192mm falling on the 20th - after 43mm on the 19th!)  

The fall was well above May 2019 (in itself above the median for May).

Calculating a prorata estimate of the fall for 2021 gives a value of 1303 mm.  The next chart shows how that estimate has developed since 12 January (heavy falls earlier in January gave unhelpfully high estimates - a total of 5m would be high for tropical Queensland).

Temperatures

The month began with a string of days with temperatures (especially minima) well above the long term average for the date.  Some cool days dropped the anomaly (average temperature for month to date less long term average MTD) to a plateau around 0.7C before a sting in the tail ended the month at +0.48C.

The daily comparison of BoM extremes with the averages is shown below.

I have also plotted a comparison of daily extremes from BoM and my WS.  The two series show very good comparisons with correlation coefficients of 94% (minima) and 96% (maxima).  (The monthly average minimum is the same for both measures while the maximum is slightly lower for the airport: the airport was warmer on only 5 days and 1.9C or more warmer on 3 days.)

Particularly for the minima it is probable that differences can be explained in part by the BoM using a day beginning at 0900 while my weather station is more logical using a day beginning at 0000.  I can (sort of) understand why BoM adheres to the old system, and recognise that the Government is unlikely to provide the funds to do the work needed to change the system, but it is annoying.

Minimum temperatures

The average minimum temperature for the month was above the long term mean and the value for 2020.  This is to be expected with much rain indicating much cloud cover.  

The average monthly minimum at my WS was 10.1C.  While this is the third highest value for May there were several other Mays early in the series only just below 10C.

Maximum temperatures

The maximum was higher than 2020 and the long term mean.  As the long term value is largely airport data that is not surprising, as received wisdom is that a sea breeze reduces the maximum temperature at the airport.  (This is something I need to look at in more detail.)

Humidity

A fairly humid month (although with cooler temperatures the word 'muggy' doesn't apply).

For both standard recording times 2021 was higher than 2020 and the average of the 3 years for which I have records.

Wind.

Most of the daily runs were moderate or less.  The outstanding day was 25 May, with a run of 292  km - the 20th longest of 841 days for which I have records.

In comparison to the adjusted value of BoM historic records the month was a little below avergae but slightly windier than 2020.

The direction of the wind was interesting in that there was more wind than expected from the ESE and consequently balanced by lower proportions from the WSW and NNW.  Looking at the daily record this was almost entirely due to wind on the 5th/6th and 11th/12th of the month, corresponding to heavier rain days: in effect a rain system sat off the coast to the East and bucketed down.  (This also resulted in Gabo Island recording more rain than the Airport in those two events.)