This page gives the elements to start using the MISVA site for convective system forecasting. It is advisable to read the approach and the concepts of objects at the heart of the MISVA site, accessible here. In particular, the poor performance of rainfall forecasting models leads to consider the meteorological objects that drive it, such as African Easterlies, Equatorial waves, etc. In general, we proceed to a descent of temporal scale by analysing first the seasonal forecast, then the sub-seasonal and then the synoptic forecast (details here).
To start using the site, one can rely on a set of 5-6 products to master (explained in detail here) and focus on the following 4 steps:
- Know the biases of the forecasting model being used
- What are biases in precipitation, precipitable water (PW), eddies etc? What is the bias on African Easterly Waves? Some answers are provided in the section Documentation/Paramètres et Produits à maitriser/Eau Précipitable/ Intérêt pour la prévision de la pluie.
- To summarise, the precipitation parameter is a poorly forecasted parameter – both in terms of correlation and bias – whereas PW is a parameter that has good forecasting properties (low bias, good correlation) up to 8-10 days ahead.
- Analyse the situation of the day according to the season and the month in progress
- Use the ANASYG synthesis product for the day’s situation (African Easterly Wave, ITD, convection). Documentation here.
- Use the hovmollers products of equatorial waves and the weekly maps of velocity potential of the past weeks to identify if the period corresponds to the passage of an active MJO, of several active equatorial waves. Documentation here.
- Use sea surface temperature maps to analyse if SST anomalies bring moisture to the area
- Predicting areas and periods favourable to the development of convective systems
- Use cartes prévues of precipitable water (parameter=PW) and meridian wind integrated over the 925-600 hPa layer (Parameter=Mean-Vwind) to detect AEWs and track the areas favouring convection:
- Areas of positive PW anomalies and PW > 45 mm are areas of preferential occurrence of convection. Explanation ici.
- Integrated meridional wind tipping zones from the south and north indicate thalwegs and ridges and allow tracking AEWs. Use the maps of low-level average parameters, including vorticity, to qualify the intensity of the waves.
- Use cartes prévues of precipitable water (parameter=PW) and meridian wind integrated over the 925-600 hPa layer (Parameter=Mean-Vwind) to detect AEWs and track the areas favouring convection:
- Anticipating the occurrence of an extreme rainfall event
- Use the hovmoller of velocity potential at 200 hPa and the decomposition of equatorial waves to identify whether the period is favourable for an extreme rainfall.
- Use of mean products at low levels dans les basses couches (Parameter=LowLevel-Diag), in ârticular the mean vorticity in the 850-600 hPa layer to determine whether an intense vortex (red colour) is forseen.
- The probability of precipitation occurence greater than 50-100 mm/ j is high if the following conditions are met:
- A strong divergence at upper levels as detected by the wind potential at 200hPa
- PW > 45 mm
- The occurrence of a intense AEW as revealed by very pronounced mean vortex both in PW anomaly and cyclonic vorticity.