Description
These West Africa Synthetic Analysis (WASA) maps are produced twice daily at 06 UTC and 18 UTC by Météo-France forecasters at the CISMF under the name Anasyg. This product is available on the MISVA website under Anasyg – Misva (aeris-data.fr) Anasyg – Misva. They are also produced by ACMAD forecasters under the name WASA, but not on a regular basis ACMAD | ACMAD.
These maps summarise the day’s situation as seen by the forecasters. The following eleven parameters considered to be important appear on the WASA maps in the form of symbols (listed in the table opposite), in order to understand the main characteristics of the situation and to forecast the general weather over West Africa, in particular deep convection and sand and dust phenomena.
- The intertropical discontinuity (ITD).
- The heat low (HL).
- The subtropical jet (STJ).
- Features associated with mid‐latitudes.
- Mid‐level dry air.
- The Tropical Easterly Jet (TEJ).
- The African Easterly Jet (AEJ).
- African easterly waves (AEWs) and cyclonic vortices.
- The African monsoon layer and monsoon trough (MT).
- Dust monitoring and forecast is extensively treated in Section 5.2 of the Handbook.
- Convective activity with the distinction between three cases:
- suppressed convection areas;
- unorganised isolated convective cells;
- mesoscale convective systems (MCSs) and squall lines.
developed by Météo‐France and Meteo France International.
The drawing of each object on these maps is detailed in chapter 11 of the Handbook available on the MISVA website Manuel du prévisionniste en Afrique de l’Ouest (Handbook).
Methodology
A forecaster’s task is to analyse a large number of observations and Numerical Weather Prediction (NWP) products, before producing a weather forecast for a given location and time period. It’s a complex process involving both objective and subjective criteria, with the forecaster’s experience having an key impact on the quality of the final forecast.
The difficulty is even greater for tropical regions (and West Africa in particular) where rainfall forecasts are poor or even bad. As a consequence, the forecaster has to rely on other, better predicted parameters (wind, temperature) with the help of conceptual models to make the link with expected rainfall. Another challenge concerns the speed with which the forecast production and dissemination process must be carried out, for optimum use.
To help forecasters in West Africa with this difficult task, a conceptual framework was developed and tested in 2006, during the special observation period of the African Monsoon Multidisciplinary Analysis (AMMA) programme. This framework proposes an object-based approach to forecasting, based on conceptual models. These objects, of which there are 11 listed above and which appear on the WASA maps, are deduced from the analyses produced by the NWP models for the first 9, while the last two (dust and convection) are mainly determined by satellite observations. The forecaster can thus characterise the synoptic situation and compare it with the conceptual models. A similar synthesis can then be produced on the basis of the forecasts from the NWP models for the first 9 objects, to then forecast precipitation.
Illustrations
The figures below illustrate the WASA for three very different synoptic situations:
- the first for the case of an easterly wave in the mature phase during the monsoon period,
- the second for a more frequent case of interaction with temperate latitudes in spring,
- and a dust event in March.
An Archetype of African Easterly Wave
The WASA map shown opposite is a typical case of an African Easterly Wave (AEW) during its mature phase. It captures the structure of the wave with ridges and troughs, the modulation of the AEJ, the undulation of the ITD… and its coupling with convective activity, corresponding for this case to the conceptual diagram proposed in chapter 2 of the Handbook (Figure 2.10).
This case study is documented on the bilingual website handbook (umr-cnrm.fr)
Interaction with mid-latitudes
The figure opposite illustrates the different elements shown on the WASA maps, for a sandy situation associated with an interaction with mid-latitudes, for 19 October 2012 at 1200 UTC. The ITD is normally a continuous line, except at the coast, where there may be a transition zone (particularly at the beginning and end of the rainy season), with two lines joining and extending into a single line over land.
Sandstorm
Opposite WASA map and coloured composite image (CC-Dust) for the Sahara and Sahel sandstorm driven by the Libyan anticyclone on 15-18 March 2012.
This case study is documented on the bilingual website handbook (umr-cnrm.fr)
