Status:Ongoing analysis Material from: Linus, Fernando, Tim
On 16 January the windstorm Gerard hit western France.
2. Description of the event
The Met Office chart sequence below shows the development of cyclone Gerard, starting out (on these charts) as a warm front wave over Newfoundland early on 14th, crossing the Atlantic as an innocuous looking frontal wave feature, then deepening explosively by 37mb/24h up to 06UTC 16th, or 15mb/6h to 00UTC 15th. The former value exceeds by some margin the definition of explosive development (24mb/24h at 50N) whilst the latter ranks highly alongside other famous windstorms (see graph below). Peak intensity was reached over the western part of the English Channel. Extreme winds hit western France (ref 00UTC 16th chart below), and also western Normandy including the Cotentin Peninsular (ref 06UTC 16th chart below). There were suggestions of a sting jet (unconfirmed) which, based on the structure and evolution of fronts and mslp on these charts, would have most likely affected northern and western Brittany and western Normandy.
Met Office analysis charts for 14th - 16th January 2023.
Context of max 6h deepening of Gerard (vertical back line) versus other 'famous' windstorms (From Hewson and Neu, Tellus, 2015). We don't of course have, for Gerard, the ERA-Interim equivalent max deepening rate shown on the y-axis.
The plots below show HRES analyses of MSLP and 6 hour rainfall from 14 January 00UTC to 18 January 00UTC, every 12th hour.
3.1 Data assimilation
The plots below show observations from SYNOP (first plot), concatenated 6-hour forecasts (2nd plot) and HRES forecasts of 24-hour maximum wind gusts from 16 January 00UTC - 17 January 00UTC, from different initial dates.
The plots below show EFI for 1-day maximum wind gusts on 16 January, from different initial times.
The plot below shows the forecast evolution plot for 24-hour maximum valid 16 January 00UTC - 17 January 00UTC for 1 degree boxes covering southern Brittanty (left) and central Normandy (right). Mean of observations - green hourglass, concatenated 6-hour forecasts - green dot, HRES –red, ENS blue box-and-whisker, Model climate – red box-and-whisker. Ensemble median as black box and ensemble mean as black diamonds. Triangle marks the maximum in the model climate based on 1200 forecasts. Note that only one observation was available in the Normandy box and might not be representative for the maximum in the full box.
3.4 Monthly forecasts
3.5 Comparison with other centres
4. Experience from general performance/other cases
5. Good and bad aspects of the forecasts for the event
- While the prediction of a windy day for the Bay of Biscay was well predicted, we northern extent was only captured in the short-range