Saturday, 2 December 2017

A mesodiluvian post

People use the term antediluvian to refer to the period before (ante) the flood, usually in a Biblical context.  I have used the prefix "meso" meaning middle as this is starting off in the middle of a flood.

I use the term flood as Whiskers Creek has just started to go over the drive.

We have had warning that a dump was coming for several days.  It was mainly a question of "How much".  Yesterday morning we were outside the extreme weather warning area defined by the BoM.  However by mid-day we were just inside the area and there was talk that we might get up to 100 mm out of it.  Other forecasts suggested about half that amount (still significant - probably putting the fall in the top 20 days since 1983 - WOT A PREDICTION!!  see below).

On the evening of 1 December we had a storm come by, dropping 8.6mm.  It then didn't rain until about 5am on the 2nd.   By 10am on the 2nd we had scored another 31.8mm.

I'll begin the imagery with three radar shots from about that time.  The first is the Weatherzone image of 128km around Captains Flat.
 We are just SE of the big yellow blob.  Obviously more to come.

Switching to the BoM site and going out to 256km shows a pretty clear patch to the NW of us.  Perhaps it is going to shut down a little sooner than expected?
 Back to Weatherzone, at NSW level to get the lightning strike information.  This shows a line of really nasty looking storms from Lake Cargelligo to Bourke.
Depending on just how the system moves we might cop those storms (bad) or just the bunch of green around Wagga.  (30 minutes later and it looks as though the line of storms may drop by!)  We shall see: in the meantime here are a few photos from our block.  All were taken around 10am.

 Whiskers Creek is just starting to flood over the road..
It is running strongly below the drive.
We had a period - perhaps 45 minutes with no rain.  Then a reasonably squally period came through.  I believe this is represented by the yellow linear patch in this Weatherzone 128km radar plot.  (I've added a red X for our house.)
The yellow linear feature is the South end of the line of storms shown in the NSW level graphic above.  So with luck that has gone to our North.  At 12:30 the wind had risen also.

By 5pm the sun had come out and we had received 43.8mm for the day (52.4mm for the 24 hours).   Checking the WZ radar ...
.... shows some unpleasantness North of Wagga (Wagga Wagga if you wish to be formal) so we may get a little more yet!  I think everything went to the North as we only got another 0.2mm for the day.

Looking at the 2 days overall shows the two patches of rain.  On 2 December I recorded 44mm: while not "unprecedented" (the day-record for Carwoola since 1983 is 96mm on 10 December 1996) it is the 20th highest daily fall over that period which I think noteworthy.
 The different nature of the two patches is revealed by the rain rate.  The highest rate (for 1900 on 1 December) was 93.6mm per hour which is the 25th highest rain-rate since I started recording in November 2013. (Rain rates weren't available from the older records.)  Again noteworthy.
My final graph of data from my Weatherstation is the external temperature, showing a very steady decline during the day of 2 December.
The final element of this post is from a BoM River Height monitoring station on the Molonglo River, about 2.8km NE of our house (and upstream of the junction with Whiskers Creek).  I have been aware of the data from this station for a number of years but never really looked at it before.  Here is an extract of the data in chart form.
In one of the community Facebook threads about the weather the question was asked "Is the Briars -Sharrow crossing open?" The height of the Molonglo is currently at 1.2m and my understanding is that water is across the road at 1.4m.
What struck me about the graph was the relative suddenness of the rise. Between 6:30 PM and 8PM (after it had finished raining here) the Molonglo rose 0.3m . Looking at the data as a table the interval between readings varies a lot: when the River level is stable (or dropping) it may be up to 3 hours  between readings. However during that period of rising levels the interval was pretty constant at 5 minutes.
So the good news from this is that when it is needed the readings are very timely. A second point is that another surge like the one yesterday evening (0.3m) will get the water well over the road, so check before leaving home.

There has been a bit of discussion (on the community Facebook page and elsewhere) about the cause of the rise.  Opinion seems to come down on the side of it just taking a while for the runoff to filter through the catchment and down to Briars-Sharrow Rd.

This next image is basically here to let me get a URL of the image!

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