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High impact weather

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

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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.

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

(FUG Associated with Cy49r1)