Tuesday, 4 August 2015

Analysis of a rain event

I have in the past posted about the rainfall etc associated with spectacular Summer thunderstorms, but there were a couple of aspects of the passage of a front on 1-2 August that were interesting.

The first aspect relates to the weather forecasts.  I have used the plural since my memory is that the BoM forecast and that from Time and Date got it somewhat wrong.  (I have chucked an adjective there since it is very hard to get old forecasts: once gone, as far as the public is able to see, they are consigned to the garbage compactor of history.)  In this case the forecast was for a little rain on Saturday 1st (that is the 24 hours following 9am on the 1st) and a bit more on Sunday 2nd.  What actually happened was quite steady rain from about 11pm on the 1st to about 10am on the 2nd, then another burst in the afternoon.
When I use the word "steady" I mean the rain rate didn't vary greatly.  This graph shows the rain rate for the same period.
The similarity of shape of the two graphs is quite apparent, suggesting that it  rained pretty much all the time but at different intensity.  Perhaps I should have said "continuously" rather than "steadily" above?  However with the rate varying about 2mm/hr to 12mm/hr there is relatively little variation: by way of contrast in the storm analysed last year the rate varied during a 3 hour period from 11mm/hr to 151mm/hr.

The correlation between rate and amount is shown in this chart (taking note of the different scales).
The second aspect is what was the impact of this rain,which totaled 40.3mm here (but 57mm at a friends place on the Plain)?

A first point as that we put the small dog's raincoat on when we went for our usual walk at 7:30.  She made it about 300m and then suggested home was the go, so we obliged.

Secondly, I went out in the Pajero about 9:00 and to my surprise found that the Molonglo was well below the Brairs-Sharrow Rd crossing.  I couldn't see any point in photographing this!  The paddocks around were all very waterlogged.

This is the flow past the Carwoola woolshed (normally a dry paddock) ....
 .. and Chimney Creek was all over the paddock upstream from Captains Flat Rd.
The sheep at Foxlow were not looking that happy about having been shorn (but while they probably didn't put on much meat or wool there were no obvious former sheep in the paddock when I drove past on the Monday).
The Molonglo had a pretty good flow on under the Foxlow bridge but it didn't seem worth photographing - just a muddy stream.

On the Monday I decided to go over to the Plain to check the Banded Lapwings (present thank you) and found the Molonglo was well over the road.
So I had to go around via Foxlow.  As I went over the Foxlow Bridge, the Molonglo was very much (perhaps a metre) lower than it had been on the Sunday (I am judging by the height of the weeds in the river bed - all of which were underwater on the Sunday).  This implies to me that there was a flush of water that got to Briars-Sharrow Rd sometime after 9am on Sunday and that the road was likely to clear later on Monday.

I was hoping that Yandiguinula Creek, where the road splits towards Rossi, was crossable.  It was:  Of course, to continue my expedition I had to come back via Bungendore.

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