Stability and Tornado Forecasting

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Stability and Tornado Forecasting
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Tornado Forecasting
Forecast of Tornadic Potential (Soundings)
Identification of pre- tornadic storms (Nowcast)
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Convective Outlooks (SPC)

• Assess likelihood of severe weather threats for
  the day
• Tornado
• Hail
• Wind
• Graphically represents where convection and/or
  severe weather is possible

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Day 3 convective outlook
Day 2 convective outlook
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Day 1 Outlook
Day 1 Tornado chance (Probability of tornado
within 25 miles of a point)
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• SPC AC 211247
• DAY 1 CONVECTIVE OUTLOOK
• NWS STORM PREDICTION CENTER NORMAN OK
• 0747 AM CDT
• MON APR 21 2008 VALID 211300Z - 221200Z
• ...THERE IS A SLGT RISK OF SVR TSTMS OVER
  PARTS OF OK...KS...AND MO... A LARGE UPPER TROUGH
  IS PRESENT TODAY OVER THE WESTERN STATES... WITH
  SEVERAL SMALLER-SCALE FEATURES ROTATING THROUGH
  BASE OF TROUGH INTO THE SOUTHERN/CENTRAL ROCKIES.
  ONE SUCH FEATURE IS CURRENTLY ROTATING ACROSS THE
  SOUTHERN ROCKIES AND WILL MOVE INTO WEST TX/OK
  AND PARTS OF KS LATER TODAY AND TONIGHT. LOW
  LEVEL MOISTURE IS RAPIDLY RETURNING NORTHWARD
  ACROSS THE PLAINS...WHILE A COLD FRONT SURGES
  SOUTHWARD ACROSS WESTERN KS AND THE TX/OK
  PANHANDLE. DESPITE SEVERAL FAVORABLE PARAMETERS
  FOR SEVERE STORM DEVELOPMENT THIS EVENING OVER
  KS/OK/TX...A STRONG CAPPING INVERSION RESULTS IN
  A HIGHLY CONDITIONAL SEVERE OUTLOOK TODAY OVER
  PARTS OF KS/OK.
• ...OK... MORNING SOUNDINGS AT OUN/FWD SHOW A
  PRONOUNCED CAP THAT IS LIKELY TO BE MAINTAINED
  THROUGH TODAY/TONIGHT. MEANWHILE...LIMITED
  VERTICAL

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Watches/Warnings

• Issued only as needed
• More specific in regards to timing, location, and
  expected types of weather
• Warnings issued by local NWS offices

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Approaches to Tornado Forecasting

• Synoptic pattern recognition
• Super outbreak example
• Checklists
• Soundings, Indices
• Climatology

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Using Soundings To Predict Thunderstorms/Tornadoes

• Key Terms
• Lifting Condensation Level (LCL)
• Level of Free Convection (LFC)
• Equilibrium Level (EL)
• Convective Available Potential Energy (CAPE)
• Convective Inhibition (CIN)
• Helicity

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LCL

• Level that the cloud base will form due to
  lifting by an outside force
• Intersection of dry adiabat from the surface
  temperature and the mixing ratio line from the
  surface dew point

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LFC

• Level at which parcel becomes warmer than
  environment and can continue to rise on its own
• Start at LCL, trace moist adiabat until it
  intersects actual temperature trace that is the
  LFC

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EL

• Point where the saturated parcel becomes cooler
  than the environment
• Take moist adiabat from LFC until it intersects
  the temperature trace
• Typically the anvil height

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CAPE

• Positive energy between the LFC and EL
• Internal energy that the parcel has to aid its
  rising motion (buoyancy)
• 500 means convection favorable
• 1000-3000 favors strong convection
• Can be related to the updraft velocity in
  thunderstorms

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CIN

• Energy required to get the parcel to the LFC
• Values
• Values 200 inhibit convection

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Helicity

• Measure of the low-level wind shear
• Higher the better
• More shear more rotation more likely
  tornadoes
• Values 300 indicate heightened risk of tornadoes

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Stability Indices

• Lifted Index From LCL, go up moist adiabat to
  500 mb, compare parcel temp to environmental temp
  (actual env.)
• Anything
• Showalter Index Same as LI, but start at 850 mb
  instead of the LCL
• Anything

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Stability Indices

• Total totals Index based on actual lapse rate
  and low-level moisture
• Higher the better for severe weather (generally
  62 indicates possible tornadoes)
• SWEAT Index Esp. for severe storms, see text
  for details
• K Index Esp. for air mass storms, see text for
  details

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How To Forecast For Tornadoes?

• Synoptic situation will determine the
  larger-scale risk
• Examine soundings
• High CAPE severe storms likely, but not
  necessarily tornadoes
• Need strong shear to create supercells, which are
  responsible for most violent tornadoes
• Mesoscale factors very important
• 90 of tornado days/year are not evident through
  synoptic analysis (Doswell et al. 1993)

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Verification of Severe Weather Forecasts

• Contingency Table
• Matrix of categorical forecasts and verification

0 No 1 Yes (e.g. tornado)
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Contingency Table

• Example 100 trials (thunderstorms). Tornado
  warning was issued 50 times. Of those 50, there
  were 42 confirmed tornados. Of the 50 times
  where no warning was issued, there were 2
  confirmed torndoes

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2
48
42
8
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2
48
Missed Forecast
Success
42
8
False Alarm
Success
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Verification Measures - POD

• Probability of Detection (POD) The percentage
  of tornadoes (or whatever event you are
  forecasting) are detected
• POD d / (bd)

b
a
Tornado POD?
c
d
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Verification Measures - FAR

• False Alarm Rate (FAR) Percent of forecasts
  that resulted in a false alarm
• FAR c / (ac)

b
a
Tornado FAR?
c
d
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Critical Success Index (CSI)

• Combines POD and FAR into one measure of skill
• Ranges from 0 (worthless) to 1 (perfect)

Tornado CSI?