WEATHER GURU | Weather predictions weigh many factors, but don’t always get it right

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While everybody was waiting for the promised rain that the models forecast, I remained cautiously optimistic, as I had been disappointed so many times recently.

Weather apps were dishing out 100 to 200mm for the event on Monday through to Tuesday.

Naturally there were once again warnings issued by the South African Weather Service, which further excited the population.

On Sunday, when the Baviaans started experiencing some fantastic thundershowers, I was inundated with queries as to when the rain would hit us.

The models forecast the bulk of the rain in our metro for Monday through to Tuesday.

As Monday arrived, I noticed that the models were up to their old tricks with rainfall amounts steadily decreasing.

I am sure that nobody was unhappy with the rain that we eventually received, especially the long-suffering dairy farmers in the Alexandria region who reported more than 150mm, however many have asked why the models got it wrong again.

We must always realise that a forecast is just what it says and implies the highest possibility of an event occurring under the given conditions.

Add to that with these upper-air systems, positioning is vital to create that “perfect-storm” scenario.

In this instance the bulk of the rain fell to the east of Gqeberha and mostly along the coastal belt west of Cannon Rocks.

On Sunday the upper Kouga Mountains received around 20mm, but with the main event, figures varied between 5 and 15mm in our main catchment, the Langkloof.

Although technology has improved drastically since the major 1968 and 1981 events, one cannot say the same about the number of surface and upper air observations which have not increased by the same proportion and in some cases decreased.

The large expanses of ocean where little to no observations are taken further affects models’ accuracy.

On the other hand, the number of private weather stations around the country has increased exponentially.

This is fantastic for after the fact verification of severe events, but the data is not fed into the system for use in modelling for a variety of reasons.

The main reasons are conformance to standards, data accuracy and correctness.

The local forecasters must therefore still rely heavily on their skills and years of experience, which often can and do outperform models, which are mere tools in the hands of a skilled forecaster.

With the advent of apps, everybody has become a professional weather forecaster and swears by his/her app on their smart, or stupid phone, as some call it.

Unfortunately, this leads to disappointment on many an occasion, as it did on this occasion.

Unfortunately, when numerous warnings are issued that do not materialise, it reduces public confidence in these warnings.

This is by no means critiquing the skills of our forecasters but is a mere fact of human nature. This is simply the “cry wolf scenario”, we were taught as children.

On the other hand, as highlighted in an earlier article in this publication “Weather alert exposes Nelson Mandela Bay’s vulnerability to flooding”, any heavy downpour warrants a warning in many parts of the metro.

The first that springs to mind is areas of Newton Park, with its uncontrolled and rapid development as a business hub.

As part of a meteorological risk assessment, I undertook a study to determine how much rain is needed to constitute flooding in our metro in 2007 and discovered that at the 50mm mark extensive flooding is reported in large parts of our metro.

Viewing recent events, I am sure that that amount has been drastically reduced.

This would imply that we should receive at least one flooding event per year, even under drought conditions.

As service delivery worsens, I shudder to think what will happen in a wet cycle, as happened in 2012, when it just did not stop raining.

In a nutshell, it is easier to organise stormwater drain cleaning than it is to precisely pinpoint the exact path of an upper air cut-off low and in so doing forecast a flooding event.

This week in history:

1921: Floods reported in various parts of the province with the Gamtoos and Sundays rivers bursting their banks.

Dam Levels

42.76% dropped from previous weeks 43.59%

Impofu slightly down to 40.23%

Weather Safety Tips:

After heavy rains dirt roads and especially those with a high content become very slippery and are difficult to navigate even in a 4X4. Wait for roads to dry, especially when traversing mountain passes

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