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

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

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

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

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

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

SPC SREF Forecasts 0402/09 UTC Valid 04(02-03)/(21 – 00) UTC <Single and combined SREF probabilities - David Bright <

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)

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

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

SREF and Consensus Forecast Departure from Climatology 21Z01APR2006 valid 00Z03APR2006 <SREF Probabilities, composite probabilities, climate anomalies; Rich Grumm < <

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 00Z03APR 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

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 Jkg -1 and EHI values 1-3 from Illinois southward across much of the lower Mississippi Valley <Mean SRH in the warm sector m 2 s 2 from Indiana to Wisconsin, generally along and north of a strong warm front <SRH 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 s -1 (~30 kt) across the Mississippi Valley

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

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 s -1 <SRH will likely exceed 200 m 2 s 2 across entire warm sector

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

NCEP Operational NAM-WRF Graphics <00 UTC and 1200 UTC:  <0600 UTC and 1800 UTC: 

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

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

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)

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

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

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 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