Summer Tornadoes – NWA 2015 Statistical Severe Convective Risk Assessment Model (SSCRAM) (Hart & Cohen, 2015) SPC Mesoanalysis Data Every hour from CG Lightning Data Severe Reports System that relates current meteorological conditions near thunderstorms to short-term forecasts of severe weather occurrences. Sample size of over 3.8 million data points.

Conclusions… The parameters, ingredients, and respective threshold values that are typically used to predict significant tornadoes in the Spring show little value in the Summer. We have not identified the reason. Significant tornadoes in the Summer often occur in weaker shear environments, resulting in poor predictability. Plainfield, IL Aug 28, 1990 College Park, MD Sep 24, 2001 Summer Tornadoes – NWA 2015 Summer = [ JUL, AUG, SEP]

Tornado Climatology - Spring vs Summer Summer Tornadoes – NWA 2015 Spring = March, April, May Summer = July, August, September ,795Tornadoes 67 EF2-EF5 Tornadoes Fewer tornadoes in summer. Summer significant tornadoes are rare (only 5-8 per year). 5,201 Tornadoes 757 EF2-EF5 Tornadoes

Very little forecast utility Spring significant tornadoes follow the rules… Summer significant tornadoes don’t… Excellent utility as a forecast parameter Summer Tornadoes – NWA 2015 Minimum 25 environment hours. Increasing STP MAR, APR, MAY JUL, AUG, SEP 40.8% probability If a thunderstorm is present in an environment of (STP=10), there is a 40.8% probability of an EF2+ tornado immediately downstream of that location in the next 2 hours. 6.5% probability If a thunderstorm is present in an environment of (STP=10), there is a 6.5% probability of an EF2+ tornado immediately downstream of that location in the next 2 hours. Virtually flat-line probability distribution for STP values below 5.

Summer Sample Size ( ) Summer Tornadoes – NWA 2015 STP Environment Hours > 3.0 (11,398) (4,765) ENOUGH TO DEVELOP A ROBUST DATABASE. There are thousands of SSCRAM data points with high STP in the Summer. ENOUGH TO DEVELOP A ROBUST DATABASE. (848) SUMMER SPRING WINTER Spring vs. Summer Comparison SPRING SUMMER ~2 1 / 2 :1 ratio Spring to Summer

Summer Sample Size ( ) Summer Tornadoes – NWA 2015 STP Environment Hours > 3.0 Verifying w/ sig tor SUMMER SPRING WINTER Spring vs. Summer Comparison SPRING SUMMER 16:1 ratio Spring to Summer High STP environments result in significant tornadoes MUCH less often in the Summer. Vast majority of data points are null cases.

STP - Individual Components Summer Tornadoes – NWA 2015 Minimum 50 environment hours. SPRINGSUMMER Spring vs. Summer Comparison

STP - Individual Components Summer Tornadoes – NWA 2015 Minimum 50 environment hours. SPRINGSUMMER Spring vs. Summer Comparison

STP - Individual Components Summer Tornadoes – NWA 2015 Minimum 50 environment hours. SPRINGSUMMER Spring vs. Summer Comparison

STP - Individual Components Summer Tornadoes – NWA 2015 Minimum 50 environment hours. SPRING SUMMER Spring vs. Summer Comparison

Minimum 25 environment hours. SPRING Significant Tornado Parameter (STP) - The “Cream of the crop” Environments Remarkable forecast value! Summer Tornadoes – NWA 2015 Effective Shear > 50kt, Effective Helicity > 300 m 2 /s 2, and CIN > -25 J/kg

Significant Tornado Parameter (STP) - The “Cream of the crop” Environments Summer Tornadoes – NWA 2015 Effective Shear > 50kt, Effective Helicity > 300 m 2 /s 2, and CIN < 25 J/kg SPRING SUMMER Minimum 25 environment hours. We have not found ANY diagnostic parameter that shows substantial skill for Summer significant tornado events. Substantial seasonal variation Virtually no signal in the Summer.

Does this signal show in other datasets? Effective Helicity Effective Shear STP Summer significant tornadoes occur in weaker shear environments. Plots based on data documented in Thompson et. al., 2013 Spring Summer Summer Tornadoes – NWA 2015

1km AGL Wind Speed (kt) Summer Non-tornadic Sig-tornadic SPRING Summer Tornadoes – NWA 2015 Substantial separation of IQRs between non-tornadic and sig-tornadic cases. Spring threshold candidate value ~ 30 knots

1km AGL Wind Speed (kt) Summer Non-tornadic Sig-tornadic SUMMER Summer Tornadoes – NWA 2015 Substantial separation of IQRs between non-tornadic and sig-tornadic cases remains. Probability distribution (not shown) is similar to all others – no useful signal. Values are substantially lower than in Spring. Use of Spring threshold fails in the Summer.

1km AGL Wind Speed (kt) Spring non-tornadic cases occur in environments similar to summer sig-tornadic cases. Summer Non-tornadic Sig-tornadic SPRINGvs. SUMMER THIS PATTERN REPEATS FOR VIRTUALLY ALL FORECAST PARAMETERS!!! Summer Tornadoes – NWA 2015

July 13, 2015 Over 20 discrete, persistent thunderstorms. Most were in a favorable environment for tornadoes (STP > 1) Numerous severe reports, but only 2 small, brief, weak tornadoes. Only 1 significant tornado (High-end EF3) Over 20 discrete, persistent thunderstorms. Most were in a favorable environment for tornadoes (STP > 1) Numerous severe reports, but only 2 small, brief, weak tornadoes. Only 1 significant tornado (High-end EF3)

Conclusions… The parameters, ingredients, and respective threshold values that are typically used to predict significant tornadoes in the Spring show little value in the Summer. We have not identified the reason. Significant tornadoes in the Summer often occur in weaker shear environments, resulting in poor predictability. Plainfield, IL Aug 28, 1990 College Park, MD Sep 24, 2001 Summer Tornadoes – NWA 2015 Summer = [ JUL, AUG, SEP]

So far, we have tried… - STP- SCP- MUCAPE - EFFS- Sfc-1km Shear- MLCAPE - MLCIN- Eff Helicity- Sfc-1km SRH - MLLCL Height- Sfc-MLLCL RH%- Theta-E Diff - Bunkers Dir- Bunkers Spd- LCL-LCL+1.5km RH - Sfc-3km LR mb LR mb LR - BRN Shear- Sfc-3km Shear- Sfc-6km Shear - Low-6km Shear- 6km AGL Wind Speed- 2km SR Wind Dir - 2km SR Wind Spd- 2km Mean Wind Dir- 2km Mean Wind Spd - Surface Wind Dir- Surface Wind Spd- 2km Layer Shear Dir - 2km Layer Shear Spd- PMSL- 500mb T - Surface Td- Surface T- 1km AGL Wind Speed - 3km AGL Wind Speed- Precipitable Water Summer Tornadoes – NWA 2015

Further Work Continue to verify that the SSCRAM technique is correct. “Pixie Dust” Search for the “Pixie Dust”. Examine individual cases Convective mode details? Increased mesoscale variability in the summer? Something about synoptic-scale forcing? Model analysis error? Solving this mystery may SIGNIFICANTLY increase our understanding of tornado environments year-round. Summer Tornadoes – NWA 2015 pixie dust noun | pix·ie dust The variables and processes that result in significant tornadoes, that are not contained within our current state of understanding. John A. Hart Ariel E. Cohen Storm Prediction Center, Norman, Oklahoma

Thank you! John A. Hart Ariel E. Cohen NWS Storm Prediction Center Norman, OK John A. Hart Ariel E. Cohen NWS Storm Prediction Center Norman, OK Summer Tornadoes – NWA 2015

SUMMER ParameterAprilAugust POD Tornadoes in tornado watches75%26% FAR tornado watches (1+ tornado)58%52% * 46% (13 of 28) of summer EF2+ tornadoes during the period occurred with an SPC Day1 Tornado Outlook probability of 2% or lower.