201512 - Rainfall - British Isles - Part 1

Status:Finalised Material from: Ivan, Linus

Discussed in the following Daily reports:

Picture

1. Impact

Storm Desmond, the fourth named storm of the season, caused severe flooding, travel disruption and a power outage across northern England and parts of Scotland on 5 December. Flooding and travel disruptions were reported from Ireland as well. Cumbria in NW England is one of the worst affected regions with more than 200 mm of rain in 24 hours recorded in that area. Storm Desmond broke the United Kingdom's 24-hour rainfall record, with 341.4 mm of rain falling in Honister Pass, Cumbria. On Saturday, 5 December, UK Met office issued a red warning of heavy rain for Cumbria. The cyclone also led to flooding in southern Norway.

https://en.wikipedia.org/wiki/Storm_Desmond

2. Description of the event

The plot below shows the accumulated rainfall between 1 to 10 December.

3. Predictability

3.1 Data assimilation

3.2 HRES

Both the O-and E-suite forecasts are similar with peak values of the rainfall of about 100 mm in 24 hours in Cumbria. The E-suite looks marginally better. The UKMO global model was a bit better in terms of precipitation amounts but generally all three forecasts were quite a lot below the observed peak values. It's worth mentioning that orographical enhancement of precipitation played a major role and the models picked well the highest amounts over the orographical barriers.

Much higher resolution MOGREPS picked much better the extreme values of precipitation compared to HRES. The maximum value of precipitation over Cumbria in MOGREPS is 202 mm/24h.

3.3 ENS

The EFI values were above 90% more than 3 days in advance. High values of the EFI and SOT show pretty well the potential of extreme rainfall in the affected areas. For the ENS the extreme rainfall values were lower compared to HRES.

The plot below show EFI and SOT for precipitation on 5 December in forecasts from 3- days before the event.

3.4 Monthly forecasts

Verification of the monthly forecast valid the week 30 November - 6 December. The forecast from two weeks before the event had a wet signal for the northern British Isles and southern Norway. The early signal was connected to the prediction of a positive NAO phase, as seen in the MSLP anomalies below.

(The plots have been lost...)

3.5 Comparison with other centres

The plots below shows 24-hour precipitation (5 Dec 06z - 6 Dec 06z) from observations, ECMWF o-suite, ECMWF e-suite, UKMO Global CF, UKMO LAM CF, COSMO-LEPS Mem 1, HIRLAM-DMI CF. All forecasts are the last issued before the accumulation period started.