To improve severe weather forecasting, KNMI has started a special programme in 2005. A number of research projects are included, like KOUW (Probabilistic forecasts of thunderstorms for the purpose of issuing a weather alarm) and MESOMOD1).
Other, non-research projects of the programme are aimed at better tuning of severe weather information to the needs and wishes of the user, at a clear and consistent communication to the user and media, and at a more dedicated role of the forecaster.
In this highlight the most important results of the KOUW project are presented. Because the skill of severe thunderstorm warnings was unsatisfactory, the KOUW project was initiated. The goal of this project was to develop a probabilistic forecast system for (severe) thunderstorms to be used by forecasters as a tool to decide whether a weather alarm should be issued.
After a selection process4), the most important predictors in the thunderstorm forecast system turned out to be the percentage of the total number of advection ensemble members with , ≥ 4 discharges (0-6 h projections only), the ECMWF 6-h convective precipitation sum, the HIRLAM Jefferson index, the HIRLAM CAPE of the most unstable level and the HIRLAM Boyden index (see Table 1 for definitions of the thunderstorm indices). The most important predictors in the severe thunderstorm forecast system turned out to be the HIRLAM Bradbury index, a number of predictors from the ensemble of advected lightning data (0-6 h projections only), the ECMWF 3-h convective precipitation sum and the HIRLAM Jefferson index. In the logistic regression2) equations for the three different thresholds (50, 100 and 200 discharges/ 5 min.) the same predictors are used, but of course with different regression coefficients. The equations contain minimally 2 and maximally 5 predictors. The maximum number of predictors has been set to 5, because more than 5 predictors often appeared to result in overfitting.
We conclude from the verification results for 2006 (Figure 2) that the overall skill of the MOS thunderstorm forecast system (> 1 discharge) is good compared to the 2000-2004 climatology. The average Brier skill score (BSS)2) of the 6 land regions (EMN, MMS, EMS, WXS, MXS and EXS)3) shows a clear diurnal cycle with the highest skill in the afternoon (12 and 15 UTC) and evening (18 UTC). The average BSS2) of the 6 coastal regions (WXN, MXN, EXN, WMN, MMN and WMS)3) shows a diurnal cycle as well, but with a smaller amplitude and a phase shift of approximately 12 h. This can be partly explained by the smaller and different diurnal cycle in the occurrence of thunderstorms in the coastal regions compared to the land regions. The Brier skill scores for the 6-12 h forecasts (Figure 2b) are generally smaller than those for the 0-6 h forecasts (Figure 2a), as expected. Apart from the fact that the skill of a forecast system decreases with increasing forecast projections, the loss of the most important predictor for the 0-6 h projections (i.e. the percentage of the total number of advection ensemble members with ≥ 4 discharges) is expected to play a role as well. Finally, the Brier skill scores of this new thunderstorm forecast system are generally higher than those of the operational system3) (not shown).
Severe thunderstorms were relatively rare in the warm half-year of 2006. Because the period July to mid-October 2005, in which their frequency was about the same as in the 2000-2004 period, was more representative, we have verified the MOS system for severe thunderstorms over both periods. In Figure 3 reliability diagrams2) are shown for the 0-6 h forecasts of 5 runs of the MOS severe thunderstorm forecast system based on the lowest threshold of 50 discharges/ 5 min. Figure 4 shows reliability diagrams based on the higher threshold 0f 100 discharges/ 5 min. The severe thunderstorm forecast system shows the highest skill in the evening (15-21 UTC) with the highest skill for the lowest threshold, as expected. The reliability diagrams for the period July to mid-October 2005 show a better overall skill than the diagrams for the warm half-year of 20067). Factors that may have contributed to the lesser skill in the warm half-year of 2006 are sampling effects, the increased ECMWF model resolution starting February 2006 and/or the low frequencies of severe thunderstorms. Nevertheless, the evaluation of the system by forecasters during the period May 24 2006 to mid-October 2006 was positive7). Apparently, a forecast system can be much more useful in practice than the (harsh) objective verification results might suggest. We conclude from the verification results shown here (Figures 3 and 4) that the overall skill of the severe thunderstorm forecast system is reasonable compared to the 2000-2004 climatology, at least for the thresholds of 50 and 100 discharges/ 5 min. Conclusions for the higher threshold of 200 discharges/ 5 min. can only be reached when a dataset with more severe thunderstorm cases will become available. Finally, we would like to stress that the verification results for the independent dataset during the development were good for all thresholds (not shown).