Utility of 0-3 km Bulk Shear Vectors as a Predictor for Quasi-Linear Convective System (QLCS)

TornadoesMcKenna W. Stanford

University of South AlabamaMeteorology

Weather-Ready NationNational Weather Service, Springfield, MO

David Gaede, Jason Schaumann, & John Gagan

NOAA’s National Weather Service

Outline• Introduction/Objectives• Background• Methodology

Criteria & Recognition• Results

Statistical Analyses Application to Protection of Life & Property

• Next Steps• Summary• Acknowledgements• References

Introduction/Objectives• McKenna W. Stanford• University of South Alabama

Meteorology, Major Mathematics, Minor

• National Weather Service, Springfield, MO WFO• Weather-Ready Nation• Personal Motivation: My interest in severe convective storms

and aspirations to investigate them and improve warning strategies for destructive events aided in my selection of this project.

• Objective: Statistically verify identified predictors for QLCS tornadoes and improve Tornado Warning lead times in order to satisfy NOAA’s objective for “reduced loss of life, property, and disruption from high-impact events.”

Co-Collaborators

• Contributors to this project included:

David Gaede, Mentor, Science & Operations Officer

Jason Schaumann, Co-Mentor, Senior Forecaster

John Gagan, Co-Mentor, Senior Forecaster

QLCS Background• Quasi-Linear Convective Systems (QLCSs)

Produce large swaths of wind damage Descending rear-inflow jets (RIJs) Embedded microbursts &

macrobursts

Localized swaths of (E)F-0 to (E)F-1 wind damage can occur

Can contain embedded tornadoes Usually (E)F-0 to (E)F-1 damage Documented damage intensity up to

(E)F-4

QLCS vs. SupercellSunset Hills, MO – KLSX 31 December 2010 Moore, OK – KTLX 20 May 2013

Photo Courtesy of NWS St. Louis Photo Courtesy of FEMA

Motivation for Research• Much research has been conducted involving

environments and physical processes related to supercell tornadoes versus those of QLCSs

• Warning skill and lead times for QLCS tornadoes remains poor Most warning decision forecasters issue Tornado

Warnings after mesovortex development Recent studies have shown the average lead

time for this technique is only around 5 minutes Can also result in high False Alarm Rates (FAR)

- “ crying wolf ”

Additional Disadvantages to Current Tornado Warning Strategies

• Due to the quick nature of mesovortex genesis, mesovoritices can form in between radar volume scans

• Radar beam will overshoot features at distances greater than 40 nautical miles (nm) from the radar

Where does the 0.5° tilt reach 1 km AGL?

How do we resolve these issues?

Alternative Methodology to Anticipate QLCS Tornadogenesis

• Schaumann and Przybylinski (2012) examined several QLCS events to identify three co-existing ingredients, both physical properties and radar characteristics, that present an increased likelihood for mesovortex genesis and rapid intensification

(1) A portion of the QLCS in which the system cold pool and ambient low-level shear are nearly balanced or slightly shear dominant AND

(2) The 0-3 km line-normal bulk shear magnitudes are equal to or greater than 15 m s-1 (30 knots) AND

(3) A rear-inflow jet (RIJ) or enhanced outflow causes a surge or bow in the line

• The intent of this study is to verify this three-ingredients method and provide statistical significance to its practice

Methodology – Case Selection• Period of study: 2005-2011• 31 cases• Warm & cold season

Mesovortex Identification

GR2Analyst Software

Surge Identification

Surge on rear flank of leading convective line

Surge on forward flank of leading convective line

GR2Analyst Software GR2Analyst Software

Determining Balance Regime

Five Different Regimes

Shear Dominant Slightly Shear Dominant Balanced Slightly Cold Pool Dominant Cold Pool Dominant

Balanced & Slightly Shear Dominant are regimes necessary in three-ingredients method

Cold Pool Dominant

ShearDominant

Balanced

0.5° Z 0-3 km Bulk Shear Vectors

Determining 0-3 km Bulk Shear Vector Magnitude & Direction

4-Panels Courtesy of Chad Gravelle, Ph.D.

Determining 0-3 km Line-Normal Shear Magnitude

Δu = sin(θ)m

Δu

mΘ

Δu = line-normal magnitude of 0-3 km bulk shearΘ = angle between convective line and 0-3 km bulk shear vectorm = magnitude of 0-3 km bulk shear vector

Updraft-Downdraft Convergence Zone (UDCZ)

Performance of Three-Ingredients Method

• 67 Mesovortices• 64 Non-Mesovortex Surges

• 52% of identified mesovorticies produced at least one report of winds ≥ 50 knots and/or a tornado

• Verification for three-ingredients method Probability of Detection (POD) – 79% False Alarm Rate (FAR) – 23%

0-3 km Bulk and Line-Normal Shear for all Mesovortices

Mean Bulk Shear – 37 kts Mean Line-Normal Shear– 33 kts

0-3 km Line-Normal Shear for all Mesovortices & Non-Mesovortex Surges

Mean Line-Normal Shear for Mesovortices– 33 kts

Mean Line-Normal Shear for Non-Mesovortex Surges– 26 kts

Three-Ingredients Method for all Mesovortices

• Average Surge Genesis to Wind Damage Lead Time – 21 minutes

• Average Surge Genesis to Tornado Lead Time – 18 minutes

Tornado Warning Baseline

• Government Performance Requirements Act (GPRA) goals for 2013

Probability of Detection (POD) – 72%

False Alarm Rate (FAR) – 70%

Tornado Warning Lead Time – 13 minutes

Three-Ingredients Method for Mesovortex Tornadoes

Scenario: If a Tornado Warning is issued as soon as all three ingredients are met…

2013 GPRA Goals 3 Ingredients Method

Improvement

POD 72% 90% 22%

FAR 70% 65% 5%

Lead Time 13 minutes 18 minutes 5 minutes

New Warning Decision Strategy vs. Current 18 minute lead time is a substantial increase over the

average of 5 minutes currently offered by warning decision forecasters issuing Tornado Warnings upon the actual genesis of mesovortices

Future WorkSLS Manuscript and Poster Ernest F. Hollings Scholar Research• Formal Research• Conduct NOAA/NWS Training

– Interactive Webinars– Work with Warning Decision Training Branch

SummaryMesovortex genesis and strong intensification is favored…

1. In a portion of the QLCS in which the cold pool and ambient low-level shear are nearly balanced or slightly shear-dominant AND

2. Where 0-3 km line-normal bulk shear magnitudes are equal to or greater than 30 knots AND

3. Where a rear-inflow jet (RIJ) or enhanced outflow causes a surge or bow in the line.

Summary (cont)

• 52% of 67 identified mesovorticies produced at least one report of winds ≥ 50 knots and/or a tornado

• Utilization of the three-ingredients method for issuing Tornado Warnings would greatly exceed 2013 GPRA goals POD – 90% Lead Time – 18 minutes

Summary (cont)

• Results of utilizing the three-ingredients method offers a substantial and efficient means to reduce the loss of life, property, and disruption from high-impact events through the issuance of more accurate and timely warnings

Acknowledgements• Staff at Springfield WFO

• Chad Gravelle, Ph.D. for providing the 4-panel RUC files

• Ryan Kardell, Meteorological Intern, Springfield WFO for providing several programs used to collect and interrogate data

Questions?

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