Analysis and forecast of a weather situation Severe precipitations and thunderstorms on the 7 th of October 2009 J. Neméghaire RMIB – Belgium November 2009

Table of contents 1.Introduction 2.Observations : short description of the weather event 3.Analysis : a global model analysis to diagnose the weather situation (identification of the perturbations, scales,....) +summary of comments 4.Forecasts : global and regional (LAM) deterministic models are exploited by forecasters to elaborate a 4D short range weather scenario +summary of comments 5.Conclusions and recommendations for forecasters NB: regional Satreps analysis are documented for discussion

1. Introduction Motivations heavy precipitations including a large thundery activity have been observed over a large part of Belgium but also in surrounding areas (North of France – Netherlands – Germany)  synotic and meso (even convective) scales are involved -precipitations more severe than expected by forecasters altough significant precipitation forecasts are given by NWP but not exactly in the same areas  How could forecasters have improved their short range forecasts (+12 to +30h): -interpreting deterministic models (using other models, other NWP data,...) ? -improving their physical knowledge on the weather mechanisms (conceptual models) ? NB: up to now ensemble forecasts are not exploited for our short range forecasts

2. OBSERVATIONS Sampling of the weather event : SYNOPS (cumulated precipitation on 12 hours) + teledetection: satellite [ VIS – IR cold cloud tops (colour scale) – Air Mass)] radar (PPI : intensities (mm/h)) and lightning densities t (u.t.c.) 00h0006h00 12h00 18h0000h0006h00 7/10/098/10/09

Cumul of precipitation on 24h (mm) estimated by the radar of Wideumont from 7/10/09 at 6h00 u.t.c. to 8/10/09 at 6h00 u.t.c. Cumul of precipitations on 12h (mm) at synoptic stations : 7/10/09 between 06h00 and 18h00 u.t.c.(above) 7>8/10/09 between 18h00 and 06h00 u.t.c.(below)

3. ANALYSIS ECMWF model analysis : run date 7/10/2009 at 00h00 u.t.c. Area = Western Europe (centred over Belgium) A selection of diagnosis model fields to characterize the synoptic situation is proposed : -MSLP+surface wind -MSLP+Z500+Wind speed 300 hPa -Z500+Wind 500+Relative VORTicity 500 hPa -Z+T+Wind 850 hPa -Z+Relative Humidity+Wind 850 hPa -Z+Theta’w 850 hPa -Z+Thetae 850 hPa -Z+Wind 925 and Thickness hPa OBJECTIVE : forecasters are building a 3D “mental image” of the atmosphere at synoptic scale at the starting point of their forecasts (analysis date). To elaborate this “mental image” they can proceed as follows: -extraction of the main elements (fields, CMs,...) which characterize the analysis -confrontation with observations (teledetection (mainly satellites), synops) These elements (fields, CMs,...) will be identified and used in the forecast to facilitate the elaboration of a deterministic scenario. A conceptual models approach will be useful to estimate the impact on the surface parameters.

Model analysis : ECMWF – 7/10/09 00h00 u.t.c. MSLP – surface wind Belgium Synoptic confluent pattern : - perturbed quasi zonal flow - S/SW-ly flow +meso Low : SW of England L West of spain + orographic l Bay of Biscay

Belgium Model analysis: ECMWF – 7/10/09 00h00 u.t.c. MSLP (- - -) Z500 Wind speed main confluent upper-air streams + jet streams + short wave upper trough (southwest of England) L+troughs west of Spain +jet stream In-between (France-Italy): flat ridge

Model analysis: ECMWF – 7/10/09 00h00 u.t.c. Z 500Wind 500Rvorticity 500 Belgium Upper-air troughs (500hPa) RVORT (500 Pa): -different scales of cyclonic vorticity cores -vorticity advection at 500 hPa

Model analysis: ECMWF – 7/10/09 00h00 u.t.c. Belgium Z 850T 850Wind 850 Temperature at low levels (850 hPa) -waving cold front (Sweden - UK) -frontal system (NW spain) ? -strong thermal advection at 850 hPa

Model analysis: ECMWF – 7/10/09 00h00 u.t.c. Belgium Z 850 RH 850Wind 850 Relative Humidity at low levels (850 hPa) - advection over SE of UK, NW of Spain - waving cold front (Sweden-UK) and dry air behind cold front

Model analysis: ECMWF – 7/10/09 00h00 u.t.c. Belgium Z 850 THETA’W 850 Detection of air masses (850 hPa): -waving cold front (Sweden-UK) -frontal system (NW Spain)?

Model analysis: ECMWF – 7/10/09 00h00 u.t.c. Belgium Z 850 THETAE 850 Potential instability of air masses: -Theatae ridge (Bay of Biscay) -frontal system (NW Spain)? -waving cold front (UK-Sweden)

Model analysis: ECMWF – 7/10/09 00h00 u.t.c. Belgium Z 925Thickness Wind 925 Low level thickness ( hPa): - high thickness SW France : lee orographic effect /thermal trough ?

To summarize briefly forecasters’ analysis A confluence synoptic pattern between two main streams:  W/SW-ly perturbed strong stream with embedded waving cold front from SW of England to Sweden and significant meso Low associated to the frontal wave (SW UK – West Channel)  S/SW-ly warm and humid (potentially unstable) rather strong stream from Portugal to France and Germany between a Low centred NW of Spain and a flat ridge (H) over the Alps and southern of France Embedded upper troughs (Bay of Biscay) and NW of Spain + frontal system (warm front - thermal trough or lee orographic effects) not easy to analyze

4. FORECASTS A- ECMWF model forecast: run date 7/10/2009 at 00h00 u.t.c. Area = Western Europe (centred over Belgium) Exploitation of the deterministic model for short range forecasts: a selection of model fields to forecast the evolution of the weather situation at synoptic scale is proposed : -MSLP+3h cumulated precip.+surface wind -MSLP+Z500+Wind speed 300 hPa -Z500+Wind 500+Relative VORTicity 500 hPa -Z+T+Wind 850 hPa -Z+Thetae 850 hPa -Z500+Omega 700 hPa OBJECTIVE : forecasters are building a 4D “mental image” of the atmosphere at synoptic scale starting at the analysis date. To elaborate this “mental image” (deterministic short term scenario) : -they try to use the main elements (or CMs) already identified in the analysis -they try to estimate their evolution and their impact on the surface parameters (main mechanisms responsible for the severity of the sensible weather) t (u.t.c.) 00h0006h0012h0018h0000h0006h00 7/10/20098/10/2009 F+12F+18F+24F+30 ANA

7/10/09 12h00 u.t.c. (F+12) : MSLP+3hR+W10 7/10/09 18h00 u.t.c. (F+18) 8/10/09 00h00 u.t.c. (F+24)8/10/09 06h00 u.t.c. (F+30) ECMWF forecasts : run date 7/10/09 at 00h00 u.t.c.

7/10/09 12h00 u.t.c. (F+12): MSLP (- - -)+Z500+W Speed 300 7/10/09 18h00 u.t.c. (F+18) 8/10/09 00h00 u.t.c. (F+24)8/10/09 06h00 u.t.c. (F+30) ECMWF forecasts : run date 7/10/09 at 00h00 u.t.c.

7/10/09 12h00 u.t.c. (F+12) : Z500+RVORT500+W5007/10/09 18h00 u.t.c. (F+18) 8/10/09 00h00 u.t.c. (F+24) 8/10/09 06h00 u.t.c. (F+30) ECMWF forecasts : run date 7/10/09 at 00h00 u.t.c.

7/10/09 12h00 u.t.c. (F+12) : Z+T+Wind 850 7/10/09 18h00 u.t.c. (F+18) 8/10/09 00h00 u.t.c. (F+24)8/10/09 06h00 u.t.c. (F+30) ECMWF forecasts : run date 7/10/09 at 00h00 u.t.c.

7/10/09 12h00 u.t.c. (+12h) : Z+THETAE 850 7/10/09 18h00 u.t.c. (+18h) 8/10/09 00h00 u.t.c. (+24h) 8/10/09 06h00 u.t.c. (+30h) ECMWF forecasts : run date 7/10/09 at 00h00 u.t.c.

7/10/09 12h00 u.t.c. (F+12) : Z500 + OMEGA 7007/10/09 18h00 u.t.c. (F+18) 8/10/09 00h00 u.t.c. (F+24)8/10/09 06h00 u.t.c. (F+30) ECMWF forecasts : run date 7/10/09 at 00h00 u.t.c.

ALARO-0 FORECASTS (Limited Area Model) B- ALARO-0 model forecast: run date 7/10/2009 at 00h00 u.t.c. ALARO-0 is a Limited Area Model with a choice of two resolution : 4 and 7 km Area = part of Western Europe (centred over Belgium) Here we focus on a selection of surface model fields (high resolution of 4 and 7 km): MSLP+3h cumulated precip.+surface wind and on the Z500 + Omega700 fields OBJECTIVE : contribution of a Limited Area Model (resolution of 4 and 7 km) for the severe precipitation forecasts (and thundery activity) during the evening of the 7/10/2009, over Belgium and the surrounding areas ALARO and ECMWF forecasts are compared for the same run and verifying dates (F+12, +18, +24h) t (u.t.c.) 00h0006h0012h0018h0000h0006h00 7/10/20098/10/2009 F+12F+18F+24F+30 ANA

MSLP+3h-Precip.+Wind 10m : 7/10/09 18h00 u.t.c. (F+18h) ALARO 7km ALARO 4km ECMWF 25 km

MSLP+3h-Precip.+Wind 10m : 7/10/09 21h00 u.t.c. (F+21h) ALARO 7 km ALARO 4 km ECMWF 25 km

MSLP+3h-Precip.+Wind 10m : 8/10/09 00h00 u.t.c. (F+24h) ALARO 7 km ALARO 4 km ECMWF 25 km

Z500+OMEGA700 forecasts for 7/10/09 : run date 7/10/09 at 00h00 u.t.c. Forecast for 7/10/09 at 21h00 u.t.c. (F+21h) Forecast for 8/10/09 at 00h00 u.t.c. (F+24) ALARO 7 km ECMWF 25 km ECMWF 25 km

To summarize briefly forecasters’ interpretation of NWP data (4D “mental” image)  Short term forecast scenario - derived from selected analysis and forecasts (deterministic) fields -significant precipitation area moving E/NE in a low level convergence area (surface trough) just south of a quasi stationary cold front waving from the Channel to North Germany and Sweden. -possible but rather limited development of convective precipitation in the unstable (warm and humid) air stream advected from France. A conjonction of a short wave upper trough and a thetae ridge could have triggered enhanced convective systems and (thundery) precipitations, in the evening of the 7 th over the belgian areas  LAM and ECMWF models are consistent -similar MSLP, surface wind and precipitation fields over the northern part of Belgium (a little bit more precipitations are forecasted with ALARO 4 km) -similar upper-air geopotential and vertical motion patterns

5. CONCLUSIONS and RECOMMENDATIONS  The severe precipitations and thundery event has been underestimated and not rightly positioned by forecasters so what could be recommended to forecasters ? Global and LAM models helpful enough to elaborate a realistic weather scenario ? Are there missing relevant model fields in the forecasters’ scenario (wind shear, instability indices, helicity,...)? Real time NWP verification with observations useful Use of Regional SATREPS analyses (using a NWP background) ? (documented on the next slide) Which benefits from a more physical approach (mechanisms, conceptual models) ? Which benefits from a short range EPS forecast for these situations ?

Regional Satreps charts (KNMI) /10/2009 respectively at at 9h00 – 14h45 and 21h00 u.t.c. CMs and surface weather are documented on IR satellite pictures 9h00 u.t.c. 14h45 u.t.c.21h00 u.t.c.