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Tim Hewson. ECMWF. October 2022.

Summary

• The focus of this item is the northwest Europe flooding events that hit the headlines in July 2021; specifically the wettest day during that period: 24h to 06UTC 15th.
• Extreme rainfall on this particular day, following previous wet weather, is believed to have triggered the most deadly flash flood events to have affected Germany and Belgium.
• The synoptic drivers of the rainfall were a quasi-stationary cyclone, slow moving fronts, and related to those a plume of unusually warm, moist lower tropospheric flow curving west then south from the southern Baltic and areas beyond.
• 24h rainfall totals widely exceeded 75mm, with >150mm recorded in Cologne, and in the High Fens - Eifel area that spans the Belgian-German border, with the latter peak due in part to orographic enhancement over relatively steep north-facing slopes
• Rapid draining away, to the west and east, of the High Fens rainfall was probably the main cause of the flash floods and numerous fatalities in riverside towns such as Pepinster and Ahrweiler
• Operational IFS forecasts (cycle 47r2) gave reasonable broadscale guidance regarding the said 24h rainfall event, for lead times up to about 4 days in advance
• Whilst extreme rainfall details were generally not captured by the IFS, ENS and HRES did show signs of higher totals and probabilities in the High Fens area from 2-3 days in advance
• Whether IFS forecasts on their own (and a related hydrological model response) could have been used to anticipate and mitigate against extreme flood impacts is debatable, unless use of a low probability threshold such as 10% is deemed acceptable
• Reforecasts of the event, using cycle 47r3 with its new moist physics, had been expected to give a different flavour to the rainfall patterns: some features were as expected whilst others (e.g. lesser peaks) were not.
• Overall HRES forecasts from 47r3 were worse than those of 47r2, suffering in particular from a misplacement of rainfall to the east, although ENS forecasts from the two systems were of comparable quality.
• The shortest range 47r2 forecasts (for T+6-30h) were degraded relative to previous data times, showing less rain overall - the reason is not clear.
• Areally-integrated totals in short range forecasts from both IFS cycles were only slightly below observed values, so the major under-prediction errors one sometimes sees for big flood events were reassuringly absent.
• Analysis of 14,000 reforecasts per month, across an annual cycle, clearly highlights that in the affected areas a 24h rainfall event of the observed extreme magnitude was, climatogically speaking, far more likely to happen in summer than in winter.
• CAVEATS: this item has not directly examined LAM output, or hydrological model output, or flash flood products, or ecPoint forecasts, or sub-daily rainfall behaviour / totals

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Table 1: Domain average rainfall totals for the joined boxes as on Figure 3, in mm, for 06UTC 14th to 06UTC 15th July 2021, for forecast lead times of 6 to 30h (forecasts are on Figure 14, observations on Figure 3).

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