Monday, 9 May 2016

Anatomy of a weather event

There seems to be a general consensus among the weather forecasters that we are going to get some rain next Sunday (8 May) and Monday (9 May).  Here is the official BoM forecast for Canberra on those days as at 0600 on 5 May.

 Here is the BoM Meteye forecast for Carwoola: consistent with the above.
I also look at the forecasts by Time and Date, which are usually quite accurate.  It is thus quite unusual to see that they are forecasting about 4 times the rain that the BoM is expecting.
On an hour by hour basis Time and Date (hereafter T&D) are expecting steady falls of about 5-6mm per hour from Sunday afternoon until early Monday morning.  Scoring 130mm over 2 days could be quite alarming!  Lets see what happens.  (As the event progressed T&D seemed to lose the plot: at one point the graphics and text suggested heavy rain from about 9pm Sunday until 2am Monday, but the numbers showed the heavy stuff from about 3am to 10am Monday.)

By the 1630 forecast from BoM the amount has already increased a tad.
I shall try to set up a way of tracking these forecasts, so expect some graphs in the final edition. It definitely looks like cleaning out a few drains will be a good idea. 

On checking Meteye at 1600 on 6 May the likely amount has increased a fair bit for Sunday.
Before we get to reviewing what has happened, here are some snaps explaining why a few mm of rain are not a bad thing.  The first is of our home dam, which is as low as I have seen it in ~9 years.
This sword is in the dam just to the right of the image above.  It is about 1 metre long, and in a wet year disappears completely under water.  The hilt is about 'full' level.
 This dam is well down Widgiewa Rd.
The graph to follow shows the average of the min and max falls offered by BoM and the estimate (from the 14 day forecast) offered by T&D.  I started recording on 5 May.
In essence the BoM forecasts were much more stable than T&D.  We ended up getting 30mm between 0900 Sunday and 0900 Monday which was on the upper end of the BoM forecasts but a fair bit below the lowest T&D forecasts.

As BoM forecast to 9am and T&D go on a calendar day some differences could be expected, but I don't think they are in this case significant.  Also note that BoM offer 2 forecasts a day while T&D appear to do ad-hoc updates.

Here is the chart for Monday. 
Not only is T&D much higher, but it is less stable.  Although the amount predicted seems extreme, it is pretty good for what was recorded at Perisher and Thredbo (and a Flood Advisory issued by BoM did suggest up to 150mm in that area) so given that I am sure the T&D are totally automated the errors are understandable with a slight, in global terms, misplacement of Canberra to the South-West.

WRT to BoM we actually got 15.4mm which is the maximum of the early forecast and the average of the final ones.  So, well done that agency.

Of course the final issue is what has happened to the water?  In January we had much more rain than this but it all soaked into the ground and there was no run off.  (Which is not all bad, since that keeps the crops and other vegetation happy).  This event has led t some run-off, putting a bit of life in Whiskers Creek
and a little water in the dam (compare with sword image above)  


Sharpen said...

A sword in the dam as a water level indicator, thats different.

Flabmeister said...

We found the sword in the garage when we bought the place and thought an Arthurian "Lady in the Lake" touch would be nice. Then realised that it actually served a practical purpose!