1. Impact Based Forecasting - Hanoi
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2. Recap... Introduction to weather and forecasting
Hazard and impact matrices
Impact based weather forecasting
Impact tables

3. Impact tables in the UK
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4. Impact and likelihood
Not just the nature of severe weather up to 5 days ahead – no weather thresholds
Impact based system
Two factors
Likelihood of impacts occurring
Confidence in forecast
The level of impacts expected – dependent on:
Location
Timing
Recent weather
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5. Considerations relating to Impact
General
Is the bad weather expected during a rush hour?
Are there reasons for there being more travellers/outdoor activities in the area of the warning? Greater risk of Road Traffic Accidents?
In the area of concern are there any local “hot-spots” i.e. areas prone to the particular weather hazard (such as wind and flooding)
Are there any significant outdoor events at which large numbers of people could be adversely affected?
Are there “politically” sensitive areas, such as areas which have flooded badly recently?
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6. Considerations relating to Impact
Rain
Does the rain follow a prolonged dry spell?
Is the rain a continuation of a prolonged wet spell?
Is the ground already saturated?
Is the heaviest rain falling over urban areas?
Is it falling during a rush hour or a major public event?
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7. Considerations relating to Impact
Wind
Are the trees in full leaf?
Is the ground saturated?
Is the wind from an unusual quadrant?
Is the wind unseasonal?
Is the wind affecting a major public outdoor event
Have there been previous strong wind events possibly weakening structures?
Will there be a significant cross wind component on key roads, especially bridges?
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8. The Matrix Colour determined by the combination of the likelihood and the impacts

9. Storm Timeline - 28th October 2013

10. Wednesday 23rd October The Guidance –
Moderate risk that some sort of “low prob, high impact” warning for wind will be issued tomorrow for Monday – still significant uncertainty in track and depth of low, but could be a very significant feature/event
Still outside the five day warning time period so nothing can be issued.
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11. Thursday 24th October
Increasing likelihood that a major storm will move across the UK.
Still much uncertainty over the track of the storm but…
Time to issue alerts
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12. Despite uncertainty over track of storm still enough confidence to highlight an AMBER area
Very low likelihood of High impacts
Low likelihood of High impacts

13. Still uncertainties so impact and likelihood assessments similar but areas now larger
Very low likelihood of High impacts
Low likelihood of High impacts

14. Saturday 26th October
Differing details still giving a spread to the possible tracks
Increasing confidence in levels of damaging winds so increased likelihood of high impacts
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15. Increasing likelihood of high impacts in the AMBER area
Very low likelihood of High impacts
Medium likelihood of High impacts

16. Sunday 27th October – the day before
Much narrower spread to possible tracks
Still a medium likelihood of high impacts, particularly to the south of the storm’s track
Lesser impacts expected to the north of the track so Yellow warning modified accordingly

17. Sunday 27th October Low likelihood of Medium impacts
Medium likelihood of High impacts

18. And finally, it blows through quicker than expected so…
Warning is cancelled

19. The damage
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20. Any questions?
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