Tornado Warning Skill as a Function of Environment National Weather Service Sub-Regional Workshop Binghamton, New York September 23, 2015 Yvette Richardson Alexandra Anderson-Frey

 Warm colors = high false alarm rates  Circle size shows number of warnings issued by that WFO in 2003—2013  Note: low FAR in Great Plains region (“textbook” tornado situations)  Higher FAR along Gulf Coast where QLCS storms are more common Motivation

 Supercell tornadoes are most likely when there is  Sufficient CAPE  Strong deep-layer shear  Strong low-level shear  Low LCLs  However, these do not discriminate perfectly—if they did, we’d have perfect skill! Tornadic Storm Environment Discriminators Brooks et al. 2003

 False alarms are higher than we would like, but reducing them means more misses—unless we can increase our skill  Where in the environmental parameter space is the skill poor?  Are high false alarms limited by our knowledge or by forecaster training in applying that knowledge? Warning skill by environment Brooks (2004) Forecast Skill

Distribution of Tornadoes by Environment using Kernel Density Estimation 90% 75% 50% 25% All tornadoes from all storm types and all times of day and year

Supercell QLCS Supercell QLCS (E)F1+ (E)F0 (E)F1+ (E)F0

POD and FAR by Environment (0-6km shear and CAPE) for all Events

POD and FAR by Environment (0-1 km helicity and MLLCL) for all Events

Warnings Events

Warnings Events

Warnings Events

Regions in this Study

All Events Vs. Northeastern Events Best-discrimination curve (Brooks 2003) All Northeast All Northeast

Warnings Events

Warnings Events

Conclusions  For the most part, tornado warning distributions are similar to the tornado event distributions, particularly if (E)F0 tornadoes are excluded  Best warning performance (high POD with low FAR) occurs in the high CAPE and strong 0-6 km shear part of the parameter space (expected)  For 0-1 km SRH > 200 m 2 s -2, POD is similar for all LCLs, but FAR decreases with increasing LCL (unexpected)  Future work will seek to develop a better parameter for QLCS tornadoes and look at warning skill as a function of the heterogeneity of the environment

Questions?

Extra Slides

MUCAPE Thompson et al. 2012

0 – 6 km Bulk Shear (EBS in black) Thompson et al. 2012

MLLCL

0 – 1 km SRH (ESRH in black) Thompson et al. 2012

0 – 1 km Bulk Shear Thompson et al. 2012

STP = (MLCAPE/1000 J/kg )(SHR6/20 m/s) (SRH1/100 m 2 /s 2 ) [( MLLCL)/1500 m] Supercell tornado conditions are captured in the Significant Tornado Parameter: Needed for supercell Needed for tornado, given a supercell Thompson et al. (2012) Supercells QLCS Nontor EF0 EF2+ Performs Well for Distinguishing Nontor and EF2+! Performs Not so Well!

Thompson et al. (2012) Supercells QLCS 0-1 km Bulk Wind Difference Nontor EF0 EF2+

Trapp et al. (2005) Given that a fair number of tornado days come from QLCS systems, can we use the SPC storm environment database to determine a better discriminator for these tornadoes? For example, they seem to have a different relationship with CAPE then that for supercells Supercells QLCS NontorEF0 EF2+ Nontor EF0 EF2+ OBJECTIVE 1: EVALUATE THE PERFORMANCE OF CURRENT TORNADO DISCRIMINATORS AND DEVELOP NEW ONES BETTER SUITED TO THE EASTERN REGION AND QLCS TORNADOES

In what part of the environmental parameter space do the most false alarms and misses occur? OBJECTIVE: IDENTIFY THE PORTION OF THE PARAMETER SPACE WITH THE LOWEST SKILL Also assess whether environmental heterogeneity, time of day, or geographical region affect skill % Misses in each bin

 It is important to know when to stop warning a storm because it is unlikely to produce another tornado  Is this a source of many false alarms? Does skill vary when a storm or system of storms has multiple warnings? OBJECTIVE 3: DEVELOP A CLIMATOLOGY FOR STORMS WITH MULTIPLE AND/OR LONG-LIVED TORNADOES

 Radar signatures for tornadoes  Supercells  QLCS (possibly with embedded supercells)  Near-storm environment  Favorable for supercells:  Most-unstable CAPE (converted to a speed via W max = √2*MUCAPE)  Bulk shear 0 – 6 km  Favorable for significant tornadoes:  0 – 1 km storm-relative helicity  Strongly correlated with 0 – 1 km bulk shear (r 2 = 0.84)  Mixed-layer LCL height (warm, moist boundary layer) Tornado Warning Basics

MLLCL vs. 0 – 1 km Vector Shear Magnitude and SRH