Severe weather event of December 2021

Cutting to the chase, this has turned into a very  intriguing story, looking at the various measures of rainfall I have available.  Looking at the period 0901 on 8 December to 0630 on 11 December the totals I have are:

The Nylex and the Davis are sited close together and I took 5 readings from the Nylex over the event.  On one earlier occasion the Davis reported 0.8mm, which matched the wetness of our drive and the road, while the Nylex had no water at the bottom of the collector, just some drops up the side.  It seems to me that the Nylex usually understates the fall by a small amount.  However I assess that we recorded at least 165 mm over the period of the event.

The positions of the 3 sites are shown in this snip from Google Earth.  The red arrow indicates the dominant wind direction recorded by my Davis and the two BoM sites .

Looking at falls for the 3 days - taking the readings to a consistent - albeit stupid - 0900 reset time shows that the vast majority of the difference between my Davis and BoM Mallacoota is due to the fall starting at 0900 on 9 December.

For the other 2 days the falls are so similar that it reassures me that the Davis is functioning correctly.

I have looked at the hour x hour results from my Davis and the BoM Mallacoota site.  Of the 44mm difference some what over half (24.6 mm) comes from 5 hours.  In addition for 2 hourly periods BoM recorded more than the Davis.

The couple of negatives increase my confidence that the devices are working correctly and the periods of large difference suggest to me that most of the difference is due to embedded cells passing over the Davis but missing Gabo and the Airport.  (In this regard I recall that on 7 December it poured at the golf club for at least 20 minutes - at least, by a guess, 2 mm - but my Davis recorded 0.2 mm and the Airport 0 mm.)

So I conclude that all devices were working correctly, that there were cells embedded in the system, and by any measure we gotta lotta rain!

Back to the future!

The potential for this event  first came to my attention on 8 December when I noticed that BoM were forecasting 70-90 mm of rain for each of 9th and 10th December 2021.  

The forecast on Meteye showed a pretty solid amount of rain right through the 9th.

At that stage Meteye only showed the probability of 'any rain' - very high - through the 10th.  By the 9th they had filled in the rest of the table.  Again a solid amount right through the day.

I have compared the 25% probability falls with the actual, from my Davis.

Meteye did a pretty good job.  The peaks that they missed are as close as I can tell, cells my Davis recorded that BoM missed at Mallacoota.  These are pretty much random events that would be impossible to forecast.  The event did subside somewhat quicker than Meteye suggested possibly due to the low pressure system staying a little further North than suggested in the 10pm Friday forecast below.

The villain in the piece - you should excuse the anthropomorphism - was a low pressure system forming off the East coast, apparently just North of Mallacoota.

I only took one image of the radar plot at 0500 on 10/12. 

The system seemed to rotate as indicated by the green arrow so that by late on the 10th the heaiest falls were occurring along the line of the red arrow, just to our North-East.  This didn't seem to dump a great deal as Green Cape only scored 108mm (similar to Gabo).  

At one point it looked as though we might be heading for Floodmageddon but the Genoa River has backed off.

The Snowy River has not been so gentle, with the gauge at Jarrahmond currently well above major flood level.  (This would not have been helped by a 9 hour release from Lake Jindabyne on 9/12).

I'll finish with chart of rain rates from my WS.  It shows the passage of cells quite well.

I had thought that there hadn't been any heavy falls but the peak fall (of 72.4 mm/hr at 11am on the 9th) was the 33rd heaviest I have recorded (from 1401 rain observations).