MOGREPS-W First-guess Severe Weather Warnings for NSWWS Estimating Impact – a Risk tool © Crown copyright Met Office

The New National Severe Weather Warning Service (NSWWS) Alerts/warnings based on likelihood and impact Alerts - issued more than 24 hours ahead Warnings - issued up to 24 hours ahead Regionally varying impact thresholds Alerts/warnings presented by shape areas on a map rather that just by county area As part of the Public Weather Service (PWS), the Met Office is responsible for the National Severe Weather Warning Service (NSWWS), which aims to give advance warning of extreme weather to: the public, businesses, emergency services and Government. © Crown copyright Met Office 2

Regional Impact Thresholds Rainfall Regions Wind Gust Regions Highlands and Islands Highlands and Islands Rest of Scotland Northern Island Rest of UK England and Wales London and SE © Crown copyright Met Office ECMWF Operations Workshop, Nov 2011 3

Introduction to MOGREPS-W Severe Gales: 3rd February 2011 DT: 06Z WED 02/02/2011 VT: 18Z THU 03/02/2011 Post-processing setup Uses MOGREPS-R 24members, 18km, 70L, T+54h The first version of the system created area probability forecasts for both severe and extreme rainfall, snowfall, and wind gusts, using criteria from the National Severe Weather Warning Service. Aims of the system Provide guidance of upcoming severe weather, hopefully increasing the lead time of any forecaster issued warnings Provide a more objective basis for assessing risk and making probability statements. Case Study: This 36hr forecast from MOGREPS-R shows the grid point probabilities (left) and the MOGREPS-W area probabilities (right) The grid point probabilities show greater than 99% probabilities for wind gusts exceeding 70mph (which was the old severe warning threshold) in the Western Isles, Western Highlands and up towards Orkney and Shetland. The wind speeds drop off over land – which is probably a slight under-estimation. Peaks in the Cairngorms National Park recorded gusts in excess of 100mph. Using the grid point information for each member, MOGREPS-W went for a “Be Prepared” in Western Scotland and the lowlands of the Scottish Borders, with a “Be Aware” in Aberdeenshire and some southern counties. A few of the Scottish counties are still Green. MOGREPS-W has a tendency to under forecast some severe weather events and this is demonstrated in the verification… MOGREPS-W Probability Severe Wind Gusts (≥ 70mph) Forecaster Issued Warnings of Severe Wind Gusts (≥ 70mph) MOGREPS-R Grid-Point Probs (≥ 70mph) © Crown copyright Met Office ECMWF Operations Workshop, Nov 2011

Calibration of MOGREPS-W Verification of 40mph forecasts Reliability Diagram (T+33h to T+42h forecasts) Forecast Thresholds All forecast thresholds are verified as if they are forecasting 40mph events September 2010 to February 2011 (6 month period) Under-forecasting  ~36mph forecasts might be best at forecasting 40mph events © Crown copyright Met Office ECMWF Operations Workshop, Nov 2011

Impact-based MOGREPS-W Weather Impact Matrix High Medium Low Very Low ≥60% ≥40% Likelihood ≥20% ≥1% Example of MOGREPS-W Wind Gust thresholds for the “Highlands and Islands”  Same probability thresholds for each county  Different impact thresholds for each county/region Impact Example of MOGREPS-W probability thresholds ≥70mph ≥80mph ≥90mph © Crown copyright Met Office ECMWF Operations Workshop, Nov 2011

MOGREPS-W Example Ex-hurricane Katia, 12th Sep 2011 MOGREPS-W first guess wind gust warning. Note small area of Red (but not enough to justify widespread) Good agreement with warning issued by forecaster on Sunday morning (right) © Crown copyright Met Office ECMWF Operations Workshop, Nov 2011

MOGREPS-W Example Heavy Rain on 24th October 2011 Good agreement with warning issued by forecaster (below) MOGREPS-R p(24h-ppn>50mm) T+42 3h Rainfall MOGREPS-G p(24h-ppn>50mm) T+48 24h Rainfall © Crown copyright Met Office ECMWF Operations Workshop, Nov 2011