Summary
On 18 September 2007 several regions in Slovenia were affected by catastrophic flash floods, causing five casualties and a huge
material damage. A cold front was approaching the Alps and over Slovenia a
series of prefrontal convective developments took place.
The NWP models did not predict such extreme amounts of
precipitation. The reasons for the extreme precipitation in very short
time (> 300 mm) were:
- continuous flow of moist air from SW,
- strong instability,
- wind shear in the lower troposphere,
- orographic effects.
In the case-study it was shown how remote sensing
data and products can assist the forecaster in monitoring and nowcasting an event to
such extent that the extreme scenarios are very quickly identified.
Satellite data in combination with lightning detection proved as an
excellent tool for detecting and tracking such extreme flooding events.
The important information about the convective development in this
case was shown by the
colour enhanced IR images, since the "cold ring" shape was seen in the
hours form 6:30 to 7:30 UTC, indicating that cloud tops reached
the tropopause level. The lightning data are also showing that
very intensive processes were persisting and renewing over the same
small areas.
In this case Convective Storm RGB combination was not very indicative,
since the light yellow colour, a signal for severity of the storm, was
not present in NW Slovenia.
The reason was probably that the small ice particles were not present at
the very top of
the clouds.
In addition NWCSAF products showed that in the region where the
heaviest precipitation occured, clouds were classified as very high and
high opaque with some medium clouds at the southern edge of the cloud
system. Precipitating Clouds product approved to be very
useful in the regions where radar data are missing or insufficient.
Convective Rainfall Rate (NWCSAF V2.0) underestimated the real
precipitation amounts. However, the indication of large amounts of
precipitation is given in the right place over the whole examined
period. MOCON parameter calculated by mesoscale analyses and nowcasting
tool INCA with the combination with the INCA wind
field gave a good indication of convective development, showing
the possibilities of using INCA in nowcasting.
Local impact assessment however does require a good real-time
measurements coverage which, together with remote sensing data and
model output, can give
very good overview of the processes in the atmosphere and support
nowcasting process in the very best way. Such synthesis of all possible
data sources, like mesoscale analyses and nowcasting
tool INCA presented in this case-study, seems to be a very
promising tool for analysis and nowcast of the events which are not
predicted by the conventional NWP models.