  The predictability of eight southern European tropical-like cyclones, seven
of which Medicanes, is studied evaluating ECMWF operational ensemble forecasts
against operational analysis data. Forecast cyclone trajectories are compared
to the cyclone trajectory in the analysis by means of a dynamic time warping
technique, which allows to find a match in terms of their overall
spatio-temporal similarity. Each storm is treated as an object and its
forecasts are analysed using metrics that describe intensity, symmetry,
compactness, and upper-level thermal structure. This object-based approach
allows to focus on specific storm features, while tolerating their shifts in
time and space to some extent. The compactness and symmetry of the storms are
generally underpredicted, especially at long lead times. However, forecast
accuracy tends to strongly improve at short lead times, indicating that the
ECMWF ensemble forecast model can adequately reproduce Medicanes, albeit only
few days in advance. In particular, late forecasts which have been initialised
when the cyclone has already developed are distinctly more accurate than
earlier forecasts in predicting its kinematic and thermal structure, confirming
previous findings of high sensitivity of Medicane simulations to initial
conditions. Findings reveal a markedly non-gradual evolution of ensemble
forecasts with lead time. Specifically, a rapid increase in the probability of
cyclone occurrence ("forecast jump") is seen in most cases, generally between 5
and 7 days lead time. Jumps are also found for ensemble median and/or spread
for storm thermal structure forecasts. This behaviour is compatible with the
existence of predictability barriers. On the other hand, storm position
forecasts often exhibit a consistent spatial distribution of storm position
uncertainty and bias between consecutive forecasts.
