AN INTRODUCTION TO SEVERE WEATHER

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AN INTRODUCTION TO SEVERE WEATHER
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TOPICS

• Types of Thunderstorms
• Severe Weather Indices
• Severe Soundings
• Hodographs
• Basic Severe Radar

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Thunderstorm Ingredients

• Moisture
• Instability
• Shear
• Forcing (Lifting) Mechanism

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Stages of Development
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Single Cell or Pulse

• Single-Cell Storms
• Moves with mean environmental wind over 5-7 km.
• 30- 60 minute duration.
• Rainfall can be moderate to heavy, small hail is
  possible.
• Vertical wind shear is small.
• Form in very weak shear environments with new
  storms forming in an unorganized manner
• Can be severe at dissipation

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Pulse Storm Lifespan
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Multi Cell Clusters

• Cluster of 2-4 storms
• Feed off gust front
• Moderate unidirectional shear and med to large
  CAPE
• Follow mean 700 500 mb flow

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Mesoscale Convective Complex

• May Aug (86)
• Multi celled storms gone wild
• Oval shaped O.7 axis of eccentricity
• Cloud shield 50,000 km2
• Last at least 6 hours
• Usually night events
• Flash floods
• Hail

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Squall Lines

• Linear multi cell storms
• Frontal or pre-frontal
• Vertical tilt increases severity
• Moist low level S or SE inflow
• Mid level flow 25 knots and - to low level flow

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Squall Line Lifespan
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BOW ECHOS and DERECHOS

• Nasty Squall Line
• Strong downbursts
• Mid level winds are forced to the surface
• Axis 400 km
• Large area of winds 58mph
• Progressive
• Serial
• Bow Echoes

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Supercells

• 3 basic types
• LP
• Classic
• HP
• Rotating updraft
• Usually evolves from non severe strom
• Higher cape and higher shear more severe

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LP

• Updraft dominated
• Translucent precip core
• Outflow deficient
• Some large hail
• Very picturesque
• Dry Line

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HP

• Typical of eastern U.S.
• Most common
• Strong downdrafts
• Shapes can be undefined
• High reflectivity hooks on radar

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Classic

• Balanced updraft and downdraft
• Rear flank downdraft is stronger than LP
• Low level mesocyclogenesis more likely
• Moderate hook echo
• Nados are common

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CINH The Cap and CAPE

• What is CINH?CINH (Convective Inhibition in
  units of Joules per kilogram) is anti-CAPE
  (negative CAPE) in the lower troposphere.
• This is the region where a parcel of air if
  raised from the lower PBL would sink back down
  again.
• Anti Cape

• CINH
• 0 - 50 Weak Cap
• 51 - 199 Moderate Cap
• 200 Strong Cap
• CAPE
• 1 - 1,500 Positive
• 1,500 - 2,500 Large
• 2,500 Extreme

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Lifted Index

• LI (Lifted Index) is an index used to assess low
  level parcel (in)stability of the troposphere.
• LI formula Temperature of Environment at 500 mb
  - Parcel temperature at 500 mb
• If the LI is -6.2 the parcel of air raised from
  50 mb above the surface to the 500 mb level will
  be 6.2 degrees warmer (positively buoyant) as
  compared to the 500 mb actual (environmental)
  temperature.

• LIFTED INDEX
• Positive number Stable
• 0 to -4 Marginal instability
• -4 to -7 Large instability
• -8 or less Extreme instability

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Showalter Index

• The SI (Showalter Index) is an index used to
  assess 850 mb parcel (in)stability.
• SI formula Temperature of Environment at 500 mb
  - Parcel temperature at 500 mb (raised from 850
  mb)

• SHOWALTER INDEX
• Positive number Stable
• 0 to -4 Marginal instability
• -4 to -7 Large instability
• -8 or less Extreme instability

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KI

• The KI (K INDEX) is an index used to assess
  convective potential.
• The KI is a combination of the Vertical Totals
  (VT) and lower tropospheric moisture
  characteristics. The VT is the temperature
  difference between 850 and 500 mb while the
  moisture parameters are the 850 mb dewpoint and
  700 mb dewpoint depression.KI (T850 - T500)
  (Td850 - Tdd700)

• K INDEX
• 15-25 Small convective potential
• 26-39 Moderate convective potential
• 40 High convective potential

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Total Totals

• The TT (Total Totals) is an index used to assess
  storm strength.
• The TT is a combination of the Vertical Totals
  (VT) and Cross Totals (CT). The VT is the
  temperature difference between 850 and 500 mb
  while the CT is 850 mb dewpoint minus the 500 mb
  temperature.TT (T850 - T500) (Td850 - T500)

• TOTAL TOTALS
• 44-50 Likely thunderstorms
• 51-52 Isolated severe storms
• 53-56 Widely scattered severe
• 56 Scattered severe storms

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SWEAT

• SW (SWEAT Severe Weather Threat Index) uses
  several variables to determine the likeliness of
  severe weather and tornadoes.
• SWEAT 12(850Td) 20(TT - 49) 2(V850) (V500)
  125(sin(dd500 - dd850) 0.2)If TT less than
  49, then that term of the equation is set to zero
  If any term is negative then that term is set
  to zeroWinds must be veering with height or
  that term is set to zero850Td 850 mb dewpoint
  temperatureTT Total Totals IndexV850 850 mb
  wind speedV500 500 mb wind speeddd500 - dd850
  Directional veering of wind with height

• SWEAT
• 150-300 Slight severe
• 300-400 Severe possible
• 400 Tornadic possible

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Helicity

• Helicity (HEL) is a mathematical quantity derived
  from speed shear and directional shear and the
  strength of the low level wind directly into the
  speed and directional wind shear.

• HELICITY
• 150-300 Possible supercell
• 300-400 Supercells favorable
• 400 Tornadic possible

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Energy Helicity Index

• The Energy Helicity Index (EHI) is a combination
  of two indices.
• CAPE and SR Helicity.
• EHI (CAPE SR HEL) / 160,000

• EHI below 1.0Supercells and tornadoes unlikely
  in most cases,
• EHI  1.0 to 2.0Supercells and tornadoes are
  possible but usually tornadoes are not violent or
  long-lived.
• EHI  2.0 to 2.4Supercells more likely and
  mesocyclone-induced tornadoes possible.   
• EHI  2.5 to 2.9Mesocyclone-induced
  supercellular tornadoes more likely.
• EHI  3.0 to 3.9Strong mesocyclone-induced
  tornadoes (F2 and F3) possible.
• EHI over 4.0 Violent mesocyclone-induced
  tornadoes (F4 and F5) possible.

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Bulk Richardson Number (BRN)

• BRN (Bulk Richardson Number) is an index that
  assesses the balance between instability (CAPE)
  and wind shear (speed and directional shear with
  height) in a thunderstorm environment.
• The basic formula is CAPE / SHEAR. BRN CAPE /
  (0.5(shear differential)2)
• CAPE determines the updraft strength while the
  SHEAR determines storm character (supercell, MCS,
  pulse).
• Multi-cell storms (MCS) are favored in cases
  where good speed SHEAR is present but low level
  directional shear is weak.
• Pulse storms are favorable in cases with high
  CAPE and low shear.
• Supercells are favored when CAPE and SHEAR are in
  balance and both are significant.

• BRN
• 45 CAPE much higher than SHEAR- often pulse
  storms if CAPE is weak to moderate
• Teens Optimum for supercells

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BRN Shear

• BRN shear may be more useful than BRN in
  differentiating tornado environments
• BRN shear 0.5 (Uavg)2
• in m2/s2, where Uavg, the magnitude difference
  between the 0-6 km mean wind in the lowest 0.5
  km, is squared

• BRN shear 25 to 100 Associated with tornadic
  supercells

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Deep Convective Index (DCI)

• The DCI attempts to combine the properties of
  equivalent potential temperature (Qe) at 850 mb
  with instability.DCI T(850 mb) Td(850 mb) -
  LI(sfc-500 mb)in degrees C, where LI represents
  the lifted index value from the surface to 500
  mb.

• DCI values of roughly 30 or higher indicate the
  potential for strong thunderstorms.

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Wet Bulb Zero (WBZ)

• Wet bulb temperature (Tw) represents the lowest
  temperature a volume of air at a constant
  pressure can be cooled to by evaporating water
  into it.
• Its value falls between the dry bulb (actual air)
  temperature and dewpoint.
• The height of the wet bulb zero is that level on
  the sounding whereby the lowest temperature
  attainable is zero degrees C, i.e. Tw 0 C at
  this level.

• In general, WBZ heights from 5,000 to 12,000 ft
  AGL are associated with hail at the ground.
• The potential for large hail is highest for WBZ
  heights of 7,000 to 10,000 ft AGL, with rapidly
  diminishing hail size below 6,000 and above
  11,000 ft AGL.

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Inverted V (Dry Microburst)

• Named Inverted-V since the dewpoint depression
  decreases significantly with height
• High based convection from high CCL

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Wet Microburst

• Mid-level dry air
• Similar to goal post sounding but with more
  moisture (higher precipitable water in sounding)
• The dry air aloft will entrain into the downdraft
  and cause evaporational cooling.
• This increases the negative buoyancy and can
  result in microbursts

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Goalpost (Loaded Gun)

• Severe weather sounding (large CAPE, very
  unstable LI) (mT air in boundary layer capped by
  cT air)
• There must be an inversion above mT air
• If speed /directional wind shear and strong
  low-level jet are present on sounding, severe
  weather chances are enhanced

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Basic Hodograph

• The primary purpose of a hodograph is to reveal
  vertical wind shear
• Vertical wind shear is a description of how the
  velocity of the horizontal wind changes with
  height.
• The hodograph is based on wind vectors, rather
  than wind barbs.
• The total magnitude of vertical wind shear over a
  particular depth is an important factor in
  anticipating possible storm structure and
  evolution.

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Severe WX Types
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Severe Wx Types