201603 - Windstorm - Katie

Status:Finalised Material from: Fernando, Linus

Discussed in the following Daily reports:

1. Impact

The wind storm Katie caused some damage across the south England on 28 March. The Metoffice issued a amber warning for the SE England (London region). Trees were uprooted, a crane was bent and incidents with scaffolding due to the force of the winds associated with Katie. Some flights were diverted from Gatwick in the early hours of 28 March. Wind gusts in excess of 50 kts were recorded mainly in the south coast and Channel during the passage of the wind storm Katie. Few places in the SE (inland) recorded gusts around 50 kts. Those maxima were observed between 04 and 06 UTC this morning

2. Description of the event

The sequence of surface analysis from MetOffice shows a surface low moving rapidly across the N Atlantic towards the UK, reaching Cornwall by midnight. From the analyses charts one can see that the storm reached the category of explosive cyclogenesis (28 hPa decrease in 24h). By the time the storm reached England the core pressure was 971 hPa.

The sequence of satellite images from midnight to early morning shows the a cloud band curling around the low centre forming a hook-shaped cloud. It is not clear from the satellite image if a sting cloud (jet) is formed during this period. Observations suggest that the max wind guts were recorded along the south coast and Channel which is to south of the sting cloud detected from the satellite image.

The weather station located in the University of Reading recorded strong gusts during last night. A peak was observed at 6 UTC reaching 23 m/s (83 km/h or 45 kts). The forecast based last Saturday shows values falling within the range of observations.

The plot below shows observations of 24-hour maximum wind gusts (27 March 12z to 28 March 11z).

3. Predictability

3.1 Data assimilation

3.2 HRES

The plots below show 24-hour maximum wind gusts from HRES between 27 March 12z and 28 March 12z and MSLP valid 28 March 00z.

24-hour wind gusts and MSLP HRES

1. 27 March 12z	2. 27 March 00z	3. 26 March 12z	4. 26 March 00z

5. 25 March 12z	6. 25 March 00z	7. 24 March 12z	8. 24 March 00z

9. 23 March 00z

Significant quantities of 24h precipitation was associated with the passage of Katie. Most of these values refer to 12- or less hours when Katie was crossing England during the night of Monday. Many places recorded 20-30 mm and at least half-dozen had more than 30 mm. Looking at the sequence of HRES forecasts (below) starting 8 days before the event one can see a gradual shift of the pattern to east as we get closer to the event. The amounts in the range 20-30 (30-50) were systematically underestimated in particular over land.