Focus on surface wind events in Ireland on the morning of 18 th January 1 Michelle Dillon, Aviation Forecaster November 2007

 A record gust of 80 knots(148 km/hr) occurred at Dublin airport at 1000hrs on 18 th January 2007  In order to determine reason for extremity of gusts, review synoptic situation and wind events associated with Kyrill from 0000hrs on 18 th to 1200hrs on 18 th Jan  Compare surface ECMWF forecast and analysis pressure and also compare the aerodrome TAFs and warnings with the observations  Forecast Considerations with respect to gusts in similar scenarios 2

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 At 0000hrs low centre situated well to the North West of Ireland with the warm front over the South west of the country  Between midnight and 0600hrs the original low centre declined and the new smaller low centre with a greater angular velocity propagates eastwards  Typical of rapid cyclogenesis - Rapid pressure falls recorded at all our stations example falls of up to 16hpa in 6hrs in western stations from 18hr on 17 th to midnight and similar drops in eastern stations between midnight and 6am 4

 UK Met Office Analysis Charts at 0600hrs and 1200hrs 5

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7  Warm Sector Winds - typically in the region kts gusting knots for airport stations, stronger in more exposed western areas.  Strongest Winds and gusts recorded in areas to the South of the low centre(Northern half of country), during the passage of a line of showers involving Cb activity.  Belmullet : 0800hrs – 28046g73kt  Knock Airport : 0900hrs G68kt  Dublin Airport: 1000hrs g80kt, 1100hrs g74kt  Casement Airport : 1000hrs g71kt, 1100hrs g69kt

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 Surface Analysis 1000hrs  Surface Analysis 1100hrs

 Strongest gusts in Dublin at 1000hrs and 1100hrs where gradient very tight to the South West of Low centre and associated with a line of showers and cb activity 10

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12  Warning for Dublin airport issued at 2120hrs on 17 th Jan - STRONG W/SW WINDS DEVELOPING AFTER 0300Z WITH MEAN WINDS UP TO 35KT GUSTING 50-55KT LATER=  The forecast timings were accurate even from 24 hours prior to the arrival of Kyrill e.g the strongest winds and gusts were predicted for Dublin between 1000hrs and 1200hrs on 18 th

 The level of the gust at Dublin airport at 1000hrs and 1100hrs not predicted in TAFS (strongest gust predicted for Dublin airport- 65knots)  Generally the ECMWF gradient 2000 foot wind forecast coincides with the ECMWF analysis for most of the country except where there is a tighter gradient to South West of Low  Surface hourly analysis charts : gradient wind at Dublin roughly 60knots from 0400 to 0600, increased to 90 knots by 0900hrs, increased to its max 100 knots by 10 and 11hrs and then started to decrease  Gradient wind forecast on ECMWF more like 90knots near Dublin at time of max gusts

6HR FORECASTANALYSIS 14

12HR FORECASTANALYSIS 15

 The superposition of the upper-level jetstreak and the increasing angular velocity causes a well-defined high-velocity wind field to the south of the low pressure center  Dry stratospheric air aloft and moist air in cloud head giving rise to instability and development of cbs near cloud head  Tightest gradient to the south west of the small low centre.  Rapid pressure rises to the west leading to a tightening of the gradient to the south west of the small low centre which is trailing back from main low 16

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18 Conceptual model theory - "Very strong gusts in the transition zone between dark and white in the WV channels near the cold front and the cloud head"

 Keep an eye on the pressure falls and rises. Pressure at Dublin airport didn’t start to rise immediately after cold front. Rose after passage of line of showers associated with occlusion  Keep an eye on transition zone between dark and white in WV imagery  Make allowances for stronger gusts where Cbs are expected to coincide with tight gradients close to Low centres 19

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