Weather of 6 April 2024

 The forecast for the day was 'interesting' to say the least, with the prospect of 15-60mm of rain the standout attribute.  The day started with a very small amount of rain in the wee hours and then got strange, as I described in a Facebook post.

"I drove to Eden and back, leaving about 1030. I started to hit rain about Coolwater Creek on the way out and it was basically torrential all the way from Genoa to Eden and back to about Becker's woodyard. Say 90 minutes of heavy rain.

Check the Weather Station when I get home and it had recorded 1.5mm over the time of my absence. Pretty consistent with the recordings over the period at the Airport and Gabo. Given what I had driven through I was expecting at least 10X that amount."

In the afternoon some rain began.  We were watching TV after tea and became aware of the sound of lashing rain which went on for nearly 2 hours. My first graphs cover the rainfall, which totalled 41.6mm - pretty close to the middle of the BoM probabilities, so well done that Bureau.

There is a very similar pattern in the rain rates.

The highest rate, at which the gutters raised the white flag, of 96.8 mm/hr meant the day was 25th on my list of daily maximum rainfall rates. As an aside there are 3 days - of 1876 in total - with multiple records of rates >90mm/hr, and 5 days with multiple readings over 82 mm/hr.

On checking the results from my Weather Station (WS) I was struck by the consistency of the high wind speeds recorded. That is particularly remarkable as the wind was coming from the East or East North East from which the WS is protected by a copse of large Angophoras.

From 0400hrs to 1800hrs the average run was 23.1 kms (if that had been sustained for the whole day the run would have been 550km - the longest recorded here). The relative calmness after 6pm meant the total daily run was 402kms, which is the 8th highest recorded by my WS.  The highest gust recorded was 58kph, the 12th fastest for the site.

It is notable that the decrease in wind coincided with the wind swinging from ENE to S.  It seems to this layman that the East Coast Low approaching had winds from ENE as it came down the coast and as the core arrived with the heaviest rain the breeze switched to the South.

I have put some summary data in a Google Sheet if anyone is interested.

Comparison with BoM data.

Their latest forecast for the day at the airport site was (75% chance of at least) 15mm to (25% chance of at least) 65mm.   So that is the initial data of interest,  

One interesting item is that (according to a meteorologist with the Australian Meteorological and Oceanographic Society) the BoM rainfall forecasts are based on calendar days rather than the 0900hrs reset.  Obviously consistency is not always the top priority for "the Bureau".   For the calendar day the Airport site recorded 35mm which fits very nicely within the range forecast.  

An inconvenience relating to the differing reset times is that one needs to use the 'day' in the Latest Weather Observations page (WOP) on the BoM site rather than the summary under Recent months which is based on the reset.  This is inconvenient as the basic reporting unit for WOP is 30 minutes rather than hourly (which I use to cut down file sizes) and additional readings get shown when there is some significant change.  This is really only inconvenient because I am hopeless at SQL so have to fiddle around to get a comparable set: I can understand why the BoM does what it does.  (It also requires fiddling to get the time to a useful format as the BoM column for date/time is alphanumeric not numeric long time.)   Whatever: I think I have got there.

A second interesting fact is that the BoM advises in other notes that where there is a large range in the forecast (as in this case) it suggests storms are possible with a wide range over relatively small distances.  Thus the difference between the airport and WS records is not a problem as it is almost certainly due to storm cells (we heard one thunderclap which was also recorded by a friend's weather station and shown on Wunderground).

Other comparisons of WS and Airport data

The initial comparison of rainfall is 35mm at the airport and 41.6mm at my WS.  This is discussed above.  The pattern of falls through the day is extremely similar with a correlation coefficient of 96%!!

I normally expect the airport to have higher wind speeds than at home, as the airport site is more exposed (especially to Southerly and Easterly winds).  Such was the case yesterday with the average strongest hourly gust at the airport being 6kph higher than at home.  The hourly patterns are quite similar with a correlation coefficient of 89%.

Wind direction is also very similar, although the airport shows a more Westerly tendency after the system had passed.

 Comparisons with Gabo Island

Gabo Island recorded more rain than either on shore site with a total of 52.8mm for the day.  The pattern of falls is similar although the correlation coefficient cf the Airport is somewhat lower at 82%.  The main difference looks to be the rain continuing on Gabo after it had ceased on the mainland.

As is usually the case Gabo is windier than both the Airport and my WS with an average hourly gust of 53.5kph.   The peak gust on Gabo as the system passed at 1800 - 1900 was 78kph which is well above the peak gusts of 59kph at the airport and a (very brisk for the site) gust of 58 kph for my WS. 

Again the wind pattern was very similar.