Promises of rain may not eventuate locally

Neil Barraclough, Meerlieu


THE Bureau of Meteorology is predicting above average rainfall for the coming months with the likelihood of flooding.

Giffard farmers interviewed were sceptical.

Let’s check it out.

The BOM is basing its predictions on la Nina conditions associated with cooler sea surface temperatures in the Eastern Tropical Pacific.

This is somewhat intriguing, as we get a lot of our flooding rains from east coast lows that form to the north-east of us and move south down the coast, then bring the low pressure system inland over us.

The warmer the sea surface temperature, the greater the rain-forming evaporation from the sea, so you would think we should expect less chance of flooding.

As best I can work it out, it could be a bit more complex.

BOM is basing its predictions on changes in atmospheric circulations that occur associated with the la Nina conditions, and these favour greater rainfall over much of south-east Australia, much of which isn’t as dependent on rain from east coast lows as east Gippsland is.

So could I suggest the positives of the changed atmospheric circulations are countered by the negatives of the east coast low situation in east Gippsland and la Ninas may well tend more towards neutral in east Gippsland?

Weather predicting is an interesting hobby. You don’t just stroll out and see what the ants are doing.

This address takes you to the best information I can find on sea surface temps associated with la Nina and el Nino:

This takes you to East Sale’s rainfall from 1944 onwards: cdio/weatherData/av?p_nccObsCode=139&p_display_type=dataFile&p_stn_num=085072

I need more time, but a quick glance suggested little relationship between la Nina conditions and increased rainfall in East Sale.

Next, check out the situation regards the 18.6 year lunar cycle, the greatest short term influence on our rainfall.

Start at 2021 and go back 18.6 years and check it against Sale’s rainfall.

Yes, we do get floods in that part of the lunar cycle, but they don’t appear significantly more frequent.

Next we’ll look at long term cyclic solar activity.

There appears a clear relationship between solar activity and rainfall in the full rainfall records for Sale going back to around 1870.

Solar activity has been declining since 1996, and so has Sale’s rainfall.

So taking into account the factors I can see and accepting there many factors I can’t see with random floods, East Sale’s average rainfall in five months October to February totals 259.7mm.

I would say there is less than a 50 per cent chance of getting that total.

This was written on the morning of October 5 as I listen to the forecast on ABC radio of likely getting our month’s average rain during the following few days.