Weekly Weather Overview and short-term outlook to Friday 07 February 2020
Visual: Spaghetti plume of 26 different predictive models to forecast the El Nino Southern Oscillation.
Source: International Research Institute for Climate and Society. Model consolidated by the Climate Prediction Centre in the National Oceanic and Atmospheric Administration of the US Government. www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf
For the first time in a very very long time, predictive models by which global weather developments are forecast, are showing a noticeable convergence. This week’s visual shows a similar trend among those highly specialised models which a large number of agencies spread across the world employ to predict the so-called El Nino Southern Oscillation(ENSO) in the equatorial Pacific.
These models are very technical but for those so inclined, the methodologies are generally available and can be read by any person with an interest in the nuts and bolts of understanding the bigger picture. In short, all the models incorporate various weightings of sea surface temperatures, deep water temperatures, downwellings, upwellings, wind speed and direction, cloud formation, convection and barometric pressure.
For Namibians, the only important aspect is whether these models agree or not. And this is the big change that has been observed during January, not in the actual weather but in the way the models handle the variables on a daily basis.
The consolidated spaghetti plume shows that all the incorporated models tend to show the same outcome. This is rather oversimplified but in December, the same model still showed wide variations from week to week and between the constituent models. In fact, the IRI/CPC model has converged to such an extent that both the dynamical and the statistical components show comparable outcomes.
Dynamical models refer to those that are based on current observation (day to day changes) while statistical models refer to those that are based on similar historical events.
The same broad analytical picture applies to Namibia. From own experience we can tell that weather forecasts have been all over the page for the past year. Often there was very little or zero coincidence between the forecasts and the outcomes, and at other times, different outlooks gave such contradictory views that it was hard to believe they all referred to southern Africa.
Over the past six weeks or so, this has gradually changed. For instance, while there were good expectations for rainfall across large areas in the North during last week, the outlooks agreed that this will be restricted to the areas north, north-west and north-east of the Etosha / Grootfontein line. The control is found in the relatively dry week in Babatwa and the eastern Caprivi part of the Zambezi region, also a forecast element for which there was general agreement.
This does not mean that it will suddenly start raining across Namibia but it does indicate that the predictive models now show a higher degree of confidence in their outcomes. In practical terms it means we can start trusting the forecasts for the rest of the season.
On the Radar
From Sunday to Monday, the 500 mB surface has come down from 5880 metres to 5850 metres. This may seem like a very small distance, given that it is relatively high up (around 18,000 feet) but it is a critical physical switch.
Rainfall conditions improve as the week progresses, starting on Tuesday reaching its highest potential on Wednesday with a strong bias in the western half, and then gradually shifting to the eastern half as the next approaching South Atlantic high pressure cell starts having an impact on the coastal plain by Thursday.
The outlook is favourable for the entire country above the escarpment on Tuesday and Wednesday, and for the northern half continuing on Thursday and Friday. For the next twelve days, cloud formation should be good which means that there is always a chance for thunder showers even if these may be isolated.