High Impact Weather

“Extreme” or “Severe” weather is coupled to both the small and large atmospheric scales and is mainly of three types:

  • Large-scale cold outbreaks or heat waves lasting for (say) three days or more.
  • Intense synoptic-scale dynamic precipitation and extreme synoptic-scale winds.
  • Strong and organised convection.

Ensembles are well equipped to forecast the first two types of anomalous events with the current resolution.  Extreme convective features are less well captured.  But a risk assessment can be added by experienced forecasters or via statistical interpretation schemes.

The Extreme Forecast Index (EFI) and Shift of Tails (SOT) highlight event probabilities in relation to climatology for each location and time of year.

EFI and SOT parameters for CAPE and CAPE-shear have skill in foretelling severe convective events that cannot be predicted directly by IFS itself.

EFI and SOT parameters aid impact forecasting as they indirectly relate to local return periods for different types of adverse weather.   Forecasters can also benefit from an understanding of the local resilience of man-made infrastructure in the face of natural hazards.

The forecaster’s role

Considerations of probability, jumpiness, and the forecaster’s role apply in particular to extreme weather forecasting.  Calibration or applying statistically based modifications to extreme weather events is very difficult because of their rarity.  However, forecasters can accumulate some experience of the capability of IFS models from extreme weather events in neighbouring areas.  Complementary information, particularly forecasts from other NWP models and/or IFS runs, might upgrade or downgrade probabilities.  Forecasters can also supply probability forecasts of events not explicitly covered by ECMWF forecast products (e.g. thunderstorms).

Perhaps the most important task is to help end-users (such as regional and national authorities) to make the optimal decision about protective action.

Probabilities or categorical guidelines? 

Severe or extreme weather is often characterized by low protective or insurance costs compared to high potential losses.  Thus relatively low probabilities can become highly decisive.  Using a cost-benefit approach can help the forecaster to enhance advice to the customer.  Protective action might be prompted at a level as low as 10% event probability, or even lower.

Advice does not have to be probabilistic.  Purely categorical guidelines may be developed if forecasters are very familiar with customer’s decision process and preferences.   In cases of extreme weather, the necessary actions may be obvious.  This might be evacuating the area or taking shelter.

But in the end the decision to advise on and/or enforce major actions such as these will generally be down to the customer.