Summary and Conclusions
On September 12th a widespread trough over Western Europe was moving westwards. On this day, a cold front and a distinct PV maximum on the rear side of this frontal zone moved over France in conjunction with this trough. The performance lag of the ECMWF model shifted this maximum westward. With the help of satellite images and products the forecaster could have improved the prognosis.
One "new" nowcasting tool presented in this case study is the MPEF upper-level Divergence product. It has proven to be very useful for locating regions with a potential for convective development. As it has the same resolution as the WV-images (5km) it can show divergence on cloud scale much better than, for example, ECMWF model data.
As with any other convective nowcasting tool, it cannot be used all alone. The forecaster should use it in combination with some other parameters. The DIV product just shows the areas with significant upper-level divergence, which can be quite huge, but it does not show the instability. Therefore this product is best used in combination with NWP-fields that forecast the instability, or GII-fields.
This case study has also shown that the forecaster must not depend on the model but always has to follow the actual synoptic development with the essential nowcasting tools. Satellite images and derived products not only have a better resolution than global models but are also available every 5 to 15 minutes with a delay of only 15 minutes.