While it cannot be expected from a global NWP model to forecast individual convective cells, it can be used to determine whether the environment is favourable for development of deep moist convection (DMC). To facilitate forecasting severe convection two new EFI parameters have been added: CAPE and CAPESHEAR (or CAPESHEAR Parameter CSP).

• CAPE combines two of the necessary ingredients of DMC: instability and moisture. The higher the CAPE the more unstable the air mass is.
• CAPESHEAR is computed according to:
  
  and is targeted at forecasting organised DMC such as supercell convection as the deep-layer wind shear helps organising convection into long-lived cells. The deep-layer wind shear  is computed between levels l₁=925 hPa and l₂=500 hPa. The second term in the formula is proportional to the maximum vertical velocity in convective updraughts . CAPESHEAR is thus expressed in units of specific energy (energy per unit mass), m²/s².

   

Practical remarks

• The EFI for both CAPE and CAPESHEAR shows where severe convection is likely if it initiates. This means that in general high EFI values cover areas where DMC does not develop for example either because the lift mechanism is missing or the capping inversion (CIN) is too strong. To help determine whether DMC will initiate or not one can use for example the probability forecast for precipitation in conjunction with the EFI for CAPE and CAPESHEAR (Figure 1).
• The EFI shows the extremity of a given parameter compared to the model climate. Hence high values of the EFI may appear in the areas where no severe convection develops just because the values of the convective parameters in the model climate are too low. For example this can happen over the continents in winter.

More information about CAPE and CAPESHEAR parameters can be found in the ECMWF Newsletter No.144 at http://www.ecmwf.int/sites/default/files/NL-144.pdf.

Further details and practical information about the EFI for CAPE and CAPESHEAR are available in the help pages below the EFI web charts in the section EFI for CAPE and CAPESHEAR at http://www.ecmwf.int/en/forecasts/charts/catalogue?f[0]=im_field_chart_type%3A481&f[1]=im_field_product_type%3A1042. 

Figure 1. EFI forecasts for A) CAPESHEAR; B) CAPE; C) CAPESHEAR where probability of precipitation above 1mm/24h is greater than 5%; D) CAPE where probability of precipitation above 1mm/24h is greater than 5%. The EFI signal disappears over great part of Germany except the northern areas after filtering out the EFI values where probability of precipitation above 1 mm/24h is less than 5%. Severe convection developed just over the northern parts of Germany while it stayed dry and sunny over the rest of the country.