1. Hydrometeorological ensemble forecasts for the 28 September 2012 (IOP8) extreme flash- flood in Murcia, Spain A. Amengual and V. Homar Grup de Meteorologia, Departament de Física, Universitat de les Illes Balears, Palma, Mallorca, Spain e-mail: arnau.amengual@uib.esarnau.amengual@uib.es Universitat de les Illes Balears Reunión PREDIMED 2014A 5-6 Junio

2. Hydrometeorological ensemble forecasts for the 28 September 2012 (IOP8) extreme flash-flood in Murcia, Spain 1.The Guadalentín flash-flood event 2.Hydrological and meteorological tools 3.Probabilistic versus deterministic QPFs 4.Probabilistic versus deterministic QDFs 5.Conclusions and further remarks

3. 1. The Guadalentín flash-flood event: synoptic situation Entrance of a deep upper- level closed trough Generation of a surface mesoscale cyclone Advection of warm and moist air toward Almería and Murcia from the Mediterranean Convergence zone between easterly advection and westerly low-level flow + orographic enhancement quasi-stationary mesoscale convective system H500+T500+PV250 T850+SLP 27 September 2012 12 UTC28 September 2012 12 UTC

4. Torrential precipitation took place on 27, 28 and 29 September 2012 Daily precipitation amounts: 214 mm in Andalucía, 240 mm in Murcia and 230 mm in Valencia The Guadalentín catchment is a medium size basin with an area of 3343 km 2 and a length close to 121 km Accumulated rainfall in 8 h up to 214 mm inside the basin Peak discharges: - 616.3 m 3 s -1 in Lorca -1081.2 m 3 s -1 in Paretón de Totana 1. The Guadalentín flash-flood event: observations

5. 1. The Guadalentín flash-flood event 10 casualties. Material losses estimated at about 120 M€

6. 2. Hydrological and meteorological tools WRF model set-up Initial and boundary conditions: ECMWF forecasts (update 6h, 0.3º; 62 vertical levels) One domain: 4 km and 28 vertical eta-levels Schemes: Microphysics ─ WSM6; Long-wave radiation ─ RRTM ; Short wave radiation ─ Dudhia; surface model ─ NOAH; time-step ─ 30 s The experiments consider a 48 h period simulation (27/09/2012 - 29/09/2012 00 UTC) HEC-HMS model set-up Loss rate: Soil Conservation Service Curve Number (SCS-CN) model Transform: SCS Unit Hydrograph model Flow routing: Muskingum method Reservoirs: elevation-storage-outflow relationship + initial elevation of the water level The experiments consider a 72 h period simulation (27/09/2012-01/10/2012 00 UTC)

7. 2. Hydrological and meteorological tools

8. 3. Probabilistic versus deterministic QPFs Difficulties to correctly forecast precise location and timing of convectively-driven rainfall system affecting a medium size basin -88.4 -5.7 EV (%) -89.90.12control -2.60.91rain-gauges EP (%)NSEGuadalentín Flow observations only available for this study case: perfect-model assumption. Optimal estimation of the initial conditions and dynamical formulation after calibration.

9. 3. Probabilistic versus control QPFs Mesoscale EPS (WRF) Diversity source only from IC/BC (dynamical downscaling) Obtained from ECMWF-EPS forecast (Global Singular Vectors) 50 equally-likely members Study of the spatial and temporal uncertainties of QPFs into a medium-sized catchment

10. (d) Probability-matched ensemble mean WRF ensemble comprises 51 elements (control + 50 perturbed) Important spread on rainfall values Essential role of atmospheric dynamical forcing (c) Ensemble mean ( in mm, shaded) and standard deviation (in mm, continuous line starting at 10 mm interval) 3. Probabilistic versus control QPFs

11. 4. Probabilistic versus deterministic QDFs Elements of the HEPS are considered equally-like Cumulative distribution functions (CDFs) of driven runoff peak flows

12. 4. Probabilistic versus deterministic QDFs Elements of the HEPS are considered equally-like Cumulative distribution functions (CDFs) of driven runoff peak flows

13. 5. Conclusions and further remarks WRF control simulation is deficient for the Guadalentín event: maximum precipitation amounts are obtained quite far away from the basin EPS reduce biases obtained for the control forecast For civil protection purposes, a hypothetical first warning for a peak flow exceeding Qp (T = 25 yrs) would have produced a probability of exceedence of 0.4 and 0.3 at Lorca and Paretón. This fact points out the benefits of a HEPS versus a deterministic prediction system The performance of the hydrometeorological simulations strongly depends on the initial conditions of the databases and on the case under study References: Amengual et al. (2014): Hydrometeorological ensemble forecasts for the 28 September 2012 (IOP8) extreme flash-flood in Murcia,Spain. Quart. J. R. Meteorol. Soc [submitted]