Rainfall Predictions- September – January

Ken Stewart, 05/09/2012

This page will be reposted at the end of September, as soon as I have enough RAINFALL and MINIMUM TEMPERATURE data.

Last month I tipped “For southern and central Queensland (Mackay to Longreach and south to the border), the next enhancements will be early to mid-August (which may last a week or two), another late September to mid-October, and maybe late October to early November.  Indications are that there will be heavier rain events starting in the period early to mid-November and early to mid-December.   These events may affect NSW as well, but I am restricting my study at this stage to Queensland.

Specifically, I’m expecting there may be a small rain event around 5-8 August and another probably larger one in the period 15-25 August, (I’m tipping close to 18-20 August).”

This is how we went:

The 5-8 August event did not eventuate, but the later one did, in fact for Mackay, the “18-20 August” event arrived on the afternoon of the 19th!  The system finally petered out by the end of August, again about a week later than I thought.  The system did not produce heavy rain, in fact this August was one of the driest we’ve had.

To put this in context, here’s a plot of 3 day average rainfall expressed as a percentage of median rain, for Rockhampton for 2012 (0 is median rain, -10 is zero, +10 is double the median; horizontal axis shows months):

As you can see, Rocky only received half its normal August rain.

It appears that my method indicates the timing, but not intensity, of a weather enhancement.  By enhancement I mean an upper level disturbance or upper trough, which can lead to anything from extra clouds, isolated showers and storms, through to widespread heavy rain.  In developing El Nino conditions such as we appear to be in, the influence is weaker and produces less rain, and is likely to come later.  In La Nina conditions the influence produces heavier and more widespread rain.

It is still too early to be specific about next year’s wet season rain just yet, however for Central and Southern Queensland indications for the next 5 months are:

Commencement period Description Likely result
September  18 +/- 7 days Possibly an upper level system Possible storm activity; probably only light rain
October 6 +/- 7 days Upper system likely to start Possible storm activity
November 15 +/- 10 days “ “ Possible storms.  This may last   to early December.
December 5 +/- 7 days Possible weak system Possible storms
December 16 +/- 7 days Indications of a stronger system Possible heavier storms or rain starting in this period, possibly   lasting past Xmas.
Late January to mid-February*.  *timing not clear yet, but not before 26   January. Probable upper system enhancing wet season begins in this period Widespread rain, amount unknown.    El Nino conditions would make it weaker and later.

All the above could be a week or so later.

I will monitor conditions and refine my predictions as the season develops.

Surface fronts will still come through of course bringing storms.  I would be surprised however if conditions are not generally dry until early October, and then again until early November or mid-December.  I expect January to be hot and dry.

Here’s the Bureau’s map of regions of Queensland they expect to have a 75% chance of exceeding various rainfall totals for September to November.

So we’ll see how we go with the timing.  Early days yet, and with El Nino, systems are likely to be later and weaker.

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2 Responses to “Rainfall Predictions- September – January”

  1. cementafriend Says:

    Good start to the predictions. The long dry spell in August & early Sept was broken on 18th (6.7 mm in the gauge)and we had a small amount of rain in the gauge on 29th (3mm) and 30th (2.1)
    I would love to see your formula.

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