Severe Weather Soundings and Wind Shear Environments

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Severe Weather Soundingsand Wind Shear
Environments
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Typical Synoptic Severe Weather Pattern
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Severe Weather Soundings

• Type A or Inverted V Sounding
• Most commonly found in the High Plains, Great
  Basin, and Desert SW
• Low Humidity at most levels, but especially at
  lower levels
• Produces Storms with very high cloud bases
• Mostly Virga
• Largest Severe Weather Threat
• Severe Straight-Line Winds
• Dry Microbursts
• Due to evaporational cooling of precipitation
  falling out of the cloud base

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Severe Weather Soundings

• Type A Sounding Cont.
• Downdrafts are normally much colder than the
  environment and therefore more dense through a
  deep layer of the atmosphere
• Rate of cold air production is proportional to
  the amount of evaporation taking place
• Ratio of evaporated rain (Virga) to rain reaching
  the surface increases with the increasing height
  of the cloud bases

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Inverted V Sounding
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Inverted V Sounding
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Inverted V Sounding
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Severe Weather Soundings

• Type B or Loaded Gun / Goalpost Sounding
• Moist air in the boundary layer with dry air
  aloft
• Typically found in the Central Southern Plains
• Large supply of moisture in the boundary layer
  provided by a low-level southerly flow (mT air)
• Low-level moist convergence on the nose of a
  low-level jet (LLJ) (850mb)

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Severe Weather Soundings

• Very warm and dry air at mid-levels (cT air) off
  of the Mexican Plateau from an elevated region
  known as the EML or Elevated Mixed Layer
• Provides a strong capping inversion which will
  inhibit the premature release of the convective
  instability
• Most commonly associated with the warm sector of
  Spring Fall mid-latitude cyclones

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Loaded Gun Sounding
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Loaded Gun Sounding
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Loaded Gun Sounding
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(No Transcript)
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A severe weather environment that includes an EML
usually results in high-end convection. Why?

• The EML prevents deep, moist convection until
  high potential instability is achieved (bottom of
  EML acts as a lid or cap).
• In the absence of deep, moist convection, warm,
  moist low level air can flow northward in an
  unimpeded manner (underrunning).
• Tendency to keep storms from becoming overly
  widespread (the exception is for severe MCSs).

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A severe weather environment that includes an EML
usually results in high-end convection. Why?

• Prevention of deep vertical mixing. Generally
  does not allow SFC dewpoints to mix out.
• Very steep lapse rates in mid levels enhances
  CAPE fast updraft accelerations.
• DCAPE (Downdraft CAPE) enhancement

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Elevated Heating Steeper Lapse Rates
Cold
Cold
Warm
Cool
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In the absence of widespread diabatic processes,
EMLs are advected downstream without changing
much character at all
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Confirming the sounding does contain an EML
trace back to source region
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Backward trajectory analysis
SSM
ALB
LBF
ELP
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ELP 12z 25Aug73
LBF 12z 26Aug73
SSM 12z 27Aug73
ALB 12z 28Aug73
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Severe Weather Soundings

• Type C or Humid/SE Sounding
• Found in Midwest SE U.S. in the late Spring and
  Summer
• Associated with a barotropic environment
• Non-advection environment in which isotherms
  parallel isoheights/isobars
• Very deep layer of moist air (mT), generally
  extends from sfc to at least 700mb
• Very small amount of evaporation, so generally
  light to only moderate downdrafts
• Greatest Threat Heavy/Flooding Rains
• Convection is initiated from differential surface
  heating and a lack of a capping inversion

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Humid / Rain Sounding
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Humid / Rain Sounding
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Humid / Rain Sounding
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Launched into Convection
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Launched into Convection
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Severe Weather Soundings

• Wet Microburst Sounding
• Similar to Type C sounding in that there is a
  deep layer of moist air
• However, there is significant drying aloft
• Most common in the Southern Plains, Midwest, and
  Southeastern U.S.
• Deep layer of moisture begins at sfc and extends
  to approximately 700mb
• Moist air is capped by a dry layer that begins at
  700mb 600mb
• Dry air provides evaporative power
• Get the production of negative CAPE (B-)or
  Downdraft CAPE (DCAPE)
• This leads to intense downdrafts and downbursts

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Wet Microburst Sounding
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Wet Microburst Sounding
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Severe Weather Soundings

• Low-Level Jet Sounding
• The low level jet is a high speed return of warm
  and moist air from the south or southeast
  moisture source is the Gulf of Mexico
• Most common and intense over the Plains states
  and Southeast states
• The low level jet occurs in the warm sector of a
  developing mid-latitude cyclone in the Central
  and Eastern U.S. occurs generally ahead of the
  cold front boundary
• Intensity of low level jet is increased due to
  temperature gradient between cooler high
  elevations in the high plains compared to warmer
  East Great Plains at night. Can also intensify by
  the warm sector of a mid-latitude cyclone being
  east of the cold sector.

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Severe Weather Soundings

• Low-Level Jet Sounding Cont.
• Low level jet adds heat, mass and momentum to
  developing thunderstorm and produces low level
  speed and directional shear (results in very high
  Helicity values)
• Produces abundant WAA (warm air advection) that
  may break a weak to moderate cap. WAA produces
  broad synoptic scale uplift
• Strongest low level jet winds are generally at
  the top of Planetary Boundary Layer due to less
  friction than at the surface
• Advection may well be over 65 miles per hour

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Low-Level Jet Sounding
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Severe Weather Soundings

• Elevated Convection Sounding
• Most common in the cool season on the north side
  of a frontal boundary (in the cool air)
• Parcels do not rise from surface during elevated
  convection. Parcel lapse rate on skew-T from
  surface is useless when the boundary layer is
  very stable.
• Parcel will generally rise from top of
  temperature inversion during elevated convection.
  On the sounding below, a parcel rising from the
  700-mb level will be much less stable than a
  parcel rising from the surface.

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Elevated Convection Sounding
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Elevated Convection Sounding
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Elevated Convection Sounding
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Wind Shear Environments

• What is shear?
• The rate of change of the wind in both the
  horizontal and the vertical
• Organizational capacity of the wind
• Two parts Speed and Direction
• Strong speed shear is detrimental to the growth
  of small or weak storms
• Large cells are typically enhanced by wind shear

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Wind Shear Environments
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Wind Shear Environments

• Speed Shear Change in speed with height

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Wind Shear Environments

• Directional Shear Change in wind direction
  with height

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Wind Shear Environments

• No Shear
• Little cell movement
• Downdraft will pool equally in all directions
• Convergence along the outflow may initiate new
  and weaker cells if uplift and B are great
  enough
• New cells will die quickly as a result of stable
  air behind the gust front

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Wind Shear Environments

• Moderate Shear
• New cells growing along outflow will move
  downshear
• Therefore having a better chance at long life
• Increase in storm relative inflow
• Magnitude of the inflow is better matched to the
  magnitude of the updraft
• Good match between inflow and updraft strength
  leads to redevelopment of the updraft and may
  force new cells to form on the right flank of
  original cells due to enhanced convergence
• New cells on right flank is known as Discrete
  Propagation

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Wind Shear Environments

• Strong Shear
• Production of updraft rotation
• Takes place through the tilting of horizontal
  vorticity
• Rotation produces a pressure gradient
• HPG produces a very strong vertical jet
• Horizontal vorticity tube can be stretched in the
  updraft and produce a rotating updraft
• Possible tornado production
• Cell rotates and propagates to the right of the
  mean flow called Continuous Propagation

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Wind Shear Environments

• Unidirectional Shear
• Weak
• Short-lived cells with a gust front that may
  produce short-lived secondary convection directly
  downshear
• Strong
• Splitting cells
• Caused by forcing that splits the updraft into
  two separate storms
• Anticyclonically rotating storm will continue to
  the left of the environmental winds
• Dissipates after a short period of time
• Cyclonically rotating storm will continue either
  along or to the right of the mean environmental
  flow

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Wind Shear Environments

• Curved Shear
• Veering of the winds on a sounding
• Clockwise with height
• Weak
• Weak, short-lived cells with short-lived
  regeneration of new storms along the gust front
• Strong
• Hydrodynamic Forcing only on the right flank of
  the parent cell (storm)
• Produces a single quasi-steady cyclonically
  rotating updraft
• Supercell thunderstorms
• If it becomes a right mover, the storm can
  sustain itself for long periods of time due to
  enhanced inflow