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 ›Review a vigorous and a marginal severe weather event  Examine diagnostic fields from the NAM-WRF  Examine single and combined probability forecasts from the SREF ›Examine threshold probabilities for:  CAPE  Storm-Relative Helicity (SRH)  Mean shear (total shear divided by shear depth)  The Energy Helicity Index (EHI)  The Supercell Parameter  The Significant Tornado Parameter ›Examine the combined probability of MUCAPE > 1000 J/kg, effective bulk shear > 30/40 kts, and 3 hr. convective precipitation >.01 inch

Overview ›This study illustrates the value of using diagnostic information from higher resolution models with probability information from SREF forecasts to better understand the nature of a severe weather threat potential ›A forecast strategy is proposed:  Use ensemble data for assessing the likelihood of a severe weather event and the confidence level of NWP forecasts Uncertainty information is critical for forecast users  Use climatological anomalies for evaluating the historical context of a model forecast Important for users to appreciate possible impacts  Use high resolution model data for determining the timing, evolution, mode, and intensity of forecast convection, including important mesoscale structures and relevant forcing mechanisms The devil is in the model details

SREF Configuration ›EMC runs a 21 member multi-model, multi-analysis 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:  10 NAM-Eta members  5 Regional Spectral Model (RSM) members  6 WRF members ›Why SREF?  SREF designed to address both initial state and model uncertainties

Introduction – Case 1 ›A deep cyclone and frontal system brought severe weather to much of the central Mississippi and lower Ohio Valleys on 2 April 2006 ›871 severe weather reports ›85 tornadoes ›29 deaths

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

SPC SREF Forecasts April 02/09 UTC valid April (02-03)/(21 – 00) UTC ›Single and combined 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) Prob Supercell Composite ≥ 3, Mean Supercell Composite = 3 (yellow) Prob Sig Tor ≥ 3, Mean Sig Tor = 3 (yellow) 04/02/09Z valid 0403/00 UTC

SREF Departures from Climatology and Probability Forecasts April 01/21Z valid April 02/00Z ›Climate anomaly: SREF forecasts assessed relative to seasonal climatology; 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 valid 00Z 03APR2006 ›Deepening surface cyclone ›Central pressure forecast > 2 SD below climate normal over the upper Mississippi Valley ›Moist air surging poleward ›PWAT anomalies forecast 2 to 3 SDs above climate normal ›CAPE anomaly SD (not shown)

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. Mean CAPE, SRH, EHI, Shear Forecasts valid 00Z 03APR2006 Mean CAPE (shaded) and EHI Mean SRH (shaded) and 1.5 km Shear ≥.006 s -1 ›Mean CAPE Jkg -1 ›Mean EHI 1-3 from Illinois to lower Mississippi Valley ›Mean SRH m 2 s -2 along and north of a strong warm front ›Mean SRH m 2 s -2 along and east of the cold front ›Mean 1.5 km mean shear 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) Mean CAPE ≥ 1200 J/kg Prob CAPE ≥ 1000 J/kg SREF CAPE Threshold Probabilities valid 00Z 03APR2006 ›CAPE forecast to exceed 2000 J/kg from southern IL (>30%) to TX (> 90%) ›CAPE forecast to exceed 1000 J/kg (70% - 90%) 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); mean shear ≥.006 s -1 Prob SRH ≥ 200 m 2 s 2 SREF Shear and SRH Threshold Probabilities valid 00Z 03APR2006 ›Mean shear will likely exceed.006 s -1 (> 90%) across entire outlook region …ranging from s -1 ›SRH will likely exceed 200 m 2 s 2 (50% - 70+%) along and east of cold front

SREF Summary ›SREF departures from climatology indicate an anomalously deep cyclone with atypical moisture and instability in the warm sector ›SREF forecasts indicate a high likelihood of severe weather… including a potential for supercells with significant tornadoes… across much of the lower and central Mississippi Valley on 2 April 2006 ›Additional SREF products (not shown) indicated considerable agreement among the ensemble members ›Users need to know that forecast confidence is high for a high impact event

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

NAM-WRF and SREF ›SREF: Greater than 90% probability of the surface dew point > 60 0 F across central and southern Mississippi Valley SREF Probability of the 2m Dew Point > 60 o F valid 0403/00 UTC NAM-WRF Dew point valid 0403/00 UTC ›NAM-WRF illustrates distribution and magnitude of warm sector dew points

NAM-WRF Best CAPE ›CAPE forecast J/kg along and ahead of the cold front ›NAM-WRF illustrates distribution and magnitude of warm sector instability

NAM-WRF Forcing and Vertical Wind Shear (not shown) ›NAM-WRF highlighted important forcing mechanisms in a forecast of significant low-level frontogenesis and strong moisture flux convergence along the frontal features ›NAM WRF substantiated the SREF probabilities of a highly sheared environment and added details to magnitude and distribution of shear

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 valid to make direct comparisons between actual and simulated radar at 12 km resolution… simulated radar can reveal important details about the mode of convection and evolution of a severe storm environment ›Indicates banded frontal and pre-frontal structures with a potential for cellular elements…which correspond rather well with the actual radar, even though the actual radar shows much greater reflectivities in the convection NAM-WRF Simulated Radar Reflectivity Mosaic of radar base reflectivity valid 0001 UTC 3 April 2006 NAM-WRF 24 hr forecast of simulated refl at 1 km AGL – valid 04/03/0000 UTC

a. b. Mosaic of radar base reflectivity valid 0001 UTC 3 April 2006 WRF-NMM4 24 hr forecast of simulated refl at 1 km AGL – valid 04/03/0000 UTC Mosaic of radar base reflectivity valid 0001 UTC 3 April 2006 NAM-WRF 24 hr forecast of simulated refl at 1 km AGL – valid 04/03/0000 UTC

Summary – Case 1 ›SREF graphics indicated the likelihood of a severe weather event with a high potential for supercells and tornadoes across the mid Mississippi Valley on 2 April EPS forecasts also indicated substantial agreement between the 21 SREF members… increasing confidence in the forecast ›Climate anomalies indicated the event would be associated with an uncharacteristically deep surface cyclone and 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

Overview – Case 2 ›A marginal severe weather event occurred across parts of the Ohio Valley and Pennsylvania on 4 October 2006

October 04, 2006 Ohio Valley Severe Weather ›A frontal system moved southward across the Ohio Valley and PA during the late afternoon/evening of 10/04/06 ›Significant heating/destabilization were questionable because morning convection was expected across OH and PA ›35 to 45 kt mid-level flow was expected across the region ahead of front ›SPC highlighted locally-damaging winds and some hail… with storms organized linearly along/ahead of the front

SPC SREF Oct 04/09Z Valid Oct (04-05)/21Z – 00Z

10/04/09Z valid 10/04/21Z Prob Conv Precip ≥.01 in x Prob MUCAPE ≥ 1000 x Prob Eff Shr > 40 Prob Conv Precip ≥.01 in x Prob MUCAPE ≥ 1000 x Prob Eff Shr > 30 Craven Brooks Mean Sig Svr and Standard Deviation Mean MUCAPE (shaded/contour), Eff shear vectors, 3km SRH (green)

10/04/09Z valid 10/04/21Z Prob Supercell Composite > 1 and Mean Supercell Comp = 1Prob Craven Brooks Sig Svr > and Mean Sig Svr = 20000

Mean Precipitable Water Forecast and Normalized Climate Anomaly Mean MSLP Forecast and Normalized Climate Anomaly 10/04/09Z valid 10/04/21Z ›PWAT 2-3+ SD above climate norm….MSLP 1-2 SD above climate norm

10/04/09Z valid 10/04/21Z ›Mean CAPE 1200J/kg OH and IN ›Mean EHI ≤ 1 IN/OH CAPE (shaded) and EHI

SRH (shaded), 5km Shr >.003 s km Shr >.003 s -1 10/04/09Z valid 10/04/21Z ›Mean SRH < 150 m 2 /s 2 PA; m 2 /s 2 along/south of front ›Mean deep (5 km) Shear ≤.003 s -1 (30 kt) along front ›Mean low-level (1.5 km) Shear ≤.006 s -1 (18kt) along front

10/04/09Z valid 10/04/21Z ›Prob CAPE > 1200 J/kg 30% PA to 70+% OH ›Mean EHI < 1 ›Prob EHI > % Prob CAPE > 1200 J/kg Prob EHI > 1

SREF Summary ›SREF single and combined probabilities illustrate an environment marginally favoring severe weather from organized convection across the parts of the Ohio Valley and Pennsylvania on 10/04/06 ›Isolated supercells are possible but not probable. Tornadoes are unlikely ›Main threat: damaging winds

AWIPS NAM-WRF Oct 04/12Z Valid Oct (04-05)/21Z

10/04/12Z valid 10/04/21Z CAPE Pre-frontal mconv Supercell composite EHI 3+, 0-3km Shear >.007s -1 Front

10/04/12Z valid 10/04/21Z Significant Severe SRH

10/04/12Z valid 10/04/21Z mb Frontal Omega mb Frontal Omega mb Frontal Omega 200 mb – 300 mb Divergence 250 mb 850 mb 500 mb

10/04/12Z valid 10/04/21Z 250 mb 850 mb 500 mb

NCEP Operational NAM-WRF Graphics

10/04/09Z valid 10/04/21Z (L) and 10/05/00Z (R)

NAM-WRF Summary ›NAM-WRF depicts more unstable environment with greater shear than SREF ›It appears high resolution model provides valuable additional information …especially when a marginal severe event is expected

10/04/09Z valid 10/04/21Z (L) and 22Z (C and R) Simulated Radar Reflectivity, Real-time Satellite and Radar