1. Using Ensemble Probability Forecasts And High Resolution Models To Identify Severe Weather Threats Josh Korotky NOAA/NWS, Pittsburgh, PA and Richard H. Grumm NOAA/NWS, State College, PA

2. Overview <This study illustrates the operational value of combining diagnostic and probabilistic information from higher resolution models and SREF forecast products to better understand the nature of severe weather potential <SREF probability forecasts are examined for a vigorous severe weather event that occurred across much of the central Mississippi and lower Ohio Valleys on 2 April 2006 <Probabilities of exceedance for Convective Available Potential Energy (CAPE), Storm-Relative Helicity (SRH), mean shear, and the Energy Helicity Index (EHI) are examined <Single and combined probabilities of MUCAPE, effective shear, and 3 hr. convective precipitation are also considered

3. Overview <A forecast strategy is proposed which utilizes:  Ensemble data for assessing the likelihood of a severe weather event and the confidence level of NWP forecasts  Climatological anomalies for evaluating the historical context of a model forecast  High resolution model data for determining the timing, evolution, mode, and intensity of forecast convection, including important mesoscale structures and relevant forcing mechanisms

4. SREF Configuration <EMC runs a 21 member multi-model, multi-analysis mesoscale SREF system with enhanced physics. The SREF is run four times daily at 03, 09, 15, and 21 UTC, with forecasts to 87 hours <The current SREF configuration contains 10 NAM-Eta members, 5 Regional Spectral Model (RSM) members, and 6 WRF members

5. Introduction <A strong cold front brought severe weather to much of the central Mississippi and lower Ohio Valleys on 2-3 April 2006 <There were 871 severe weather reports and 85 tornadoes. 29 people lost their lives in this deadly early- spring tornado outbreak

6. SPC Day one Outlook 04/02/06 UTC valid 0402/12 UTC – 0403/12 UTC

7. SPC SREF Forecasts 0402/09 UTC Valid 04(02-03)/(21 – 00) UTC <Single and combined SREF probabilities - David Bright <http://www.spc.noaa.gov/exper/sref/

8. Prob Conv Precip ≥.01 in x Prob MUCAPE ≥ 1000 x Prob Eff Shr > 40 kts 04/02/09Z valid 0402/21 UTC04/02/09Z valid 0403/00 UTC Prob Conv Precip ≥.01 in x Prob MUCAPE ≥ 1000 x Prob Eff Shr > 40 kts Prob Effective Shear ≥ 40 kts, Mean Effective Shear ≥ 40 kts (yellow)

9. Prob Supercell Composite  3, Mean Supercell Composite = 3 (yellow) 04/02/09Z valid 0402/21 UTC Prob Sig Tor  3, Mean Sig Tor = 3 (yellow) 04/02/09Z valid 0403/00 UTC

10. SPC SREF Summary <The SREF single and combined probabilities illustrate an environment favoring supercells and tornadoes across the lower and central Mississippi Valley

11. SREF and Consensus Forecast Departure from Climatology 21Z01APR2006 valid 00Z03APR2006 <SREF Probabilities, composite probabilities, climate anomalies; Rich Grumm <http://nws.met.psu.edu/ensembles/index.html <http://eyewall.met.psu.edu/ensembles/

12. SREF init: 21Z01APR2006 valid 00Z03APR2006 Mean MSLP and Anomaly (shaded) SREF init: 21Z01APR2006 valid 00Z03APR2006 Mean PWAT and Anomaly (shaded a. b. SREF MSLP and PWAT Anomalies 21Z01APR2006 valid 00Z03APR2006 2 SD below normal over the upper Mississippi Valley <In the warm sector, moist air is surging poleward; PWAT anomalies are forecast to be 2 to 3 SDs above normal

13. SREF init: 21Z01APR2006 valid 00Z03APR2006 Mean CAPE (shaded) and EHI SREF init: 21Z01APR2006 valid 00Z03APR2006 SR-Helicity (shaded);1.5 km Shear (10 3 ) & vectors a. b. SREF CAPE, SRH, EHI, Shear 21Z01APR2006 valid 00Z03APR2006 Mean CAPE (shaded) and EHI Mean SRH (shaded) and 1.5 km Shear ≥.006 s -1 <Mean CAPE forecast 1200-2500 Jkg -1 and EHI values 1-3 from Illinois southward across much of the lower Mississippi Valley <Mean SRH in the warm sector 300-400+ m 2 s 2 from Indiana to Wisconsin, generally along and north of a strong warm front <SRH 200-300 m 2 s 2 extends southward into the lower Mississippi valley along and east of the cold front <1.5 km mean (normalized) shear is.009-.010+ s -1 (~30 kt) across the Mississippi Valley

14. SREF NARR 21Z01APR2006 valid 00Z03APR2006 Prob CAPE > 2000 Jkg -1; Mean CAPE ≥ 1200 Jkg -1 SREF 09Z01APR2006 valid 00Z03APR2006 Probability CAPE > 1000 Jkg -1 a. b. Prob CAPE ≥ 2000 m 2 s 2 (shaded) and Mean CAPE ≥ 1200 J/kg Prob CAPE ≥ 1000 J/kg SREF CAPE Probabilities 21Z01APR2006 valid 00Z03APR2006 <Exceedance probabilities indicate CAPE will exceed 2000 J/kg from TX to southern IL <…CAPE will likely exceed 1000 J/kg across much of the Mississippi Valley

15. SREF NARR 21Z01APR2006 valid 00Z02APR2006 Probability 1.5km shear >.006 s -1 SREF 09Z01APR2006 valid 00Z03APR2006 Probability SRH > 200 m 2 s 2 a. b. Prob 1.5 km shear ≥.006 s -1 (shaded) and mean shear ≥.006 s -1 Prob SRH ≥ 200 m 2 s 2 SREF Shear and SRH Probabilities 21Z01APR2006 valid 00Z03APR2006 <Mean shear will likely exceed.006 s -1 across entire outlook region …with mean values.012 -.018 s -1 <SRH will likely exceed 200 m 2 s 2 across entire warm sector

16. SREF Summary <SREF forecasts established a high likelihood of severe weather across much of the lower and central Mississippi Valley on 2 April 2006 <Probability forecasts indicated an environment favoring supercells, and SREF probabilities indicated a high degree of agreement among the ensemble members

17. NCEP Operational NAM-WRF Graphics <00 UTC and 1200 UTC:  http://www.emc.ncep.noaa.gov/mmb/mmbpll/nampll12_fullcyc_2mbtop/index.html <0600 UTC and 1800 UTC:  http://www.emc.ncep.noaa.gov/mmb/mmbpll/opsnam_offtime/index.html

18. NAM-WRF and SREF 60 0 F across central and southern Mississippi Valley <NAM-WRF illustrates details and magnitude of warm sector moisture content SREF Probability of the 2m Dew Point > 60 o F valid 0403/00 UTC NAM-WRF Dew point valid 0403/00 UTC

19. NAM-WRF Best CAPE <CAPE forecast 2000-3000J/kg along and ahead of the cold front

20. NAM-WRF Best LI <Best Li reveals addition structure of the unstable warm sector <NAM-WRF highlighted important forcing mechanisms in a forecast of significant low-level frontogenesis and moisture flux convergence along the frontal features (not shown)

21. Instant Pcp Rate - 36 H Forecast valid 0403/00 UTC Conv Pcp Rate - 36 H Forecast valid 0403/00 UTC NAM-WRF Instant and Convective Precipitation <NAM-WRF instant and convective precipitation shows convective potential along banded frontal structures and grid scale precipitation northwest of the surface cyclone

22. a. b. <Although it is not generally possible to make direct comparisons between actual and simulated radar, the simulated radar can reveal the nature of model-derived mesoscale forcing <The simulated radar indicates banded frontal and pre-frontal structures which correspond rather well with the actual radar, even though the actual radar shows much greater reflectivities in the convection <This information can be very useful to an analyst trying to understand how moisture, instability, and vertical wind shear are forecast to interact in a severe storm environment. NAM-WRF Simulated Radar Reflectivity

23. Summary <Ensemble data indicated the likelihood of a severe weather event with a high potential for supercells and tornadoes across the mid Mississippi Valley on 2-3 April 2006. Probability forecasts indicated a high degree of agreement between the 21 SREF members… increasing confidence in the forecast <Climate anomalies indicated the event would be associated with a an anomalous surface cyclone containing an anomalously moist warm sector <High resolution model data helped fill in the details of the mode, evolution, and intensity of forecast convection, and highlighted important mesoscale structures, including relevant forcing mechanisms