Prior to the IFS cycle 47r1, the EFI (and SOT) for both CAPE and CAPE-shear has used were based on instantaneous CAPE (ShortName=cape; parameter ID=59) and CAPE-shear (ShortName=capes; parameter ID=228044) fields, in 6-hour steps, from which 24-hour maximum values have been were computed (more details can be found in ECMWF Newsletter 144). For example for the nominal T+24-48h EFI forecast, four CAPE values, at T+30h, T+36h, T+42h and T+48h have been , were used to compute the maximum for this forecast period. With the In IFS cycle 45r1 two new model output parameters, maximum CAPE in the last 6 hours (ShortName=mxcape6; parameter ID=228035) and maximum CAPE-shear in the last 6 hours (ShortName=mxcapes6; parameter ID=228036), have been were introduced (see details here). In the IFS cycle 47r1, for the respective EFI and SOT computations, mxcape6 and mxcapes6 are replacing will be used instead of cape and capes respectively. This change is aiming for a better sampling in the computation of the 24-hour maximum values necessary maxima needed for the EFI. In effect with the change we extract the maximum within 24 hourly values, instead of using 4 6-hourly values. The impacts of this change are described in detail below.
Firstly this changes has a neutral impact in terms of the EFI skill measured by the area under the Relative Operating Characteristic (ROC) curve.
The EFI fields using mxcape6 and mxcapes6 generally look smoother and in this the case of a tornadic outbreak in the USA earlier in March 2020 shown in Fig. 1 they even fit better to the severe weather reports. Please note that the EFI shows a wide area where the environment favours convective hazards whilst the actual severe thunderstorms develop along relatively narrow bands where sufficient lift is present to initiate deep, moist convection, e.g. along cold cold fronts or dry lines.
Fig. 1. T+24-48h CAPE-shear EFI/SOT forecast during the tornadic outbreak on 3rd March 2020 over Tennessee, USA.
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