Ensemble storm surge forecasting in the UK during the exceptional 2013/14 winter Helen Titley Senior Weather Impacts Scientist, Met Office, UK With thanks to colleagues from the Flood Forecasting Centre and Environment Agency including Andy Lane, Dave Cox, Crystal Moore, and John Curtin

Outline Storm surge ensemble forecasting The UK 2013/14 winter Overview of coastal flooding events Performance and use of storm surge ensemble in 5 th /6 th December event Objective verification of 2013/14 season Future developments

Storm surge forecasting A storm surge is defined as the rise of sea level generated by the wind and pressure changes in a storm, over and above the predicted astronomical tide The overtopping of flood defences by coastal storm surges presents a significant threat to life and property Storm surge forecasting aims to provide advance warning of dangerous events, so that protective action can be taken Storm surge forecasts are highly sensitive to errors in the meteorological forecast, and often the uncertainty in the forecast is particularly high in situations with the potential for a large surge event, making storm surges ideally suited to an ensemble forecasting approach Newlyn Harbour Wall, Cornwall 23 December 2013

Met Office Storm Surge Ensemble Uses CS3X 12km shelf model developed by UK National Oceanography Centre, coupled to the 24-member 33km resolution MOGREPS-G Ensemble Runs out to 7 days to provide forecasts of surge levels and overtopping risk around the UK coastline The 2013/14 UK winter brought an exceptional run of deep lows across the UK, leading to significant coastal flooding impacts from several events Will show how forecasters in the Flood Forecasting Centre and the Environment Agency used the Surge Ensemble in their decision-making process and how this helped to mitigate impacts © Crown copyright Met Office Met Office UM MOGREPS NAE MOGREPS-G Driving met data: MSLP and 10m wind Deterministic CS3X Ensemble CS3X Output: forecast products and data from T+1 to T+162 Guidance and warnings to government and responders (FFC) and the public (EA)

The UK 2013/14 winter: coastal flooding December 2013January 2014 February 2014 SuMTuWThFSaSuMTuWThFSa SuMTuWThFSa East & west coast surge New Year flooding Further coastal and fluvial flooding Flood Guidance Statement to responders e.g. emergency services

Overview of 2013/14 coastal flood events DECNOVJANFEBMAR 5-6 th December rd January th February th Dec 2013: Lowest land pressure since 1886 (936.8mb, Stornoway)

5 th /6 th December 2013 Rapidly deepening low on 5th December, with strong NW winds to the rear of the storm Generated a significant storm surge down both the West and East coasts of the UK The largest North Sea storm surge for 60 years, threatening the UK’s East coast in a similar manner to the catastrophic 1953 event Affected 3 successive tides Highest total water levels ever recorded in many areas

Impacts 5 th /6 th December 2013

Products available to FFC forecasters

ScotlandN Wales & NW England East Coast Port Risk charts Aim to summarise the risk of alert level exceedence at UK ports Stacked probability for successively severe events 12 hour time-binning ensures true probability of alert level exceedence for a tidal cycle is shown: This example illustrates how time of greatest risk varied from region to region in Dec 5/6 event Alert level > 20cm above> alert levelwithin 50cmwithin 20cm > 20cm above> alert levelwithin 20cmwithin 50cm > 20cm above > alert levelwithin 20cmwithin 50cm VT 00-12UTC 5 th DecVT 12-00UTC 5 th /6 th Dec VT 00-12UTC 6 th Dec

6.5 days ahead Several members show risk of alert level exceedance, but not by the magnitude shown by observations Clear increase in spread and uncertainty in ensemble from T+144 showing potential for extreme event Led FFC forecasters to first include in guidance to EA on Fri 29 th Nov Lowestoft Indicated a low risk of an event a full week in advance Port time series

5-6 days ahead Large variation in surge timing and magnitude from run to run Main surge activity forecast 12 hours later than subsequently observed One member is forecasting a 2.5m+ surge coincident with a high tide!! 5-6 days ahead shows significant risk of very unusual/dangerous period of surge activity

4-5 days ahead Majority of members forecasting alert level exceedance at some point Thu-Sat – a very strong signal at this lead time Large uncertainty in magnitude and timing, showing importance of considering overall risk for the period, as considering one cycle only would underestimate overall risk FFC forecasters initiate National Flood Advisory Service teleconference on 1st Dec By 4-5 days ahead analysis and observations begin to fall within ensemble spread

Increase in signal strength at 2½ day lead time This increased confidence by c.T+60 enabled FFC forecasters to increase severity of the warning levels in their Flood Guidance Statement and enabled the EA to issue Severe Flood Warnings

12-24 hours ahead At the peak of total water the observations are within the ensemble spread at many ports e.g. Llandudno, Lowestoft, Dover, but are 15cm higher in several ports in NE England (under investigation) The analysis trace is within the surge ensemble at every port At short range the surge models also gave useful guidance for many ports

First included in guidance to EA on Fri 29 th Nov First National Flood Advisory Service teleconference on Sun 1st Dec Included in 5-day Flood Guidance Statement from Mon 2 nd Dec Flood Forecasting Centre: summary of actions FGS on Mon 2 nd Dec FGS on Wed 4 th Dec FGS on Thursday 5 & Friday 6 December

Summary of actions from customers in 5/6 Dec event Environment Agency issued 64 severe flood warnings Over 160,000 warning messages sent directly to homes and businesses 2,600 properties flooded, but 800,000 properties protected by flood defences including Thames Barrier (highest tide since completion in 1984) Mass mobilisation of emergency responders and emergency plans 18,000 people evacuated, and no deaths due to the storm surge Contrast with 1953 North Sea Storm Surge when were no flood warnings, and 307 UK deaths

(Dressed with fixed error distribution) (Dressed with magnitude- related error distribution) Objective verification of 2013/14 winter: Brier Skill Score for total water level exceeding the alert level Summarises overall skill of probabilistic forecast 1.0 is perfect forecast 0.0 is no better than climatological event frequency Surge ensemble beats dressed deterministic from 2+ days Shows surge ensemble is correctly quantifying situational uncertainty Still significant skill at the full 7-day lead time

Summary and future plans Surge ensemble provided useful guidance in UK 2013/14 winter e.g. for 5/6 Dec event: indicated low risk of high-impact event a full week ahead observations fell within spread from day 5 confidence grew as the event neared allowed FFC/EA to issue useful guidance and warnings well in advance, and responders to initiate emergency plans, thus mitigating impacts from the event Future plans include: development of a new storm surge model, based on the NEMO (Nucleus for European Modelling of the Ocean) ‘community’ ocean model, initially at 12km, but then increasing to ~1.5km deterministic and ~7km ensemble coupling of surge and wave ensemble forecasts