Introduction to Severe Weather Analysis

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Introduction to Severe Weather Analysis

• A Sounding Perspective
• Clark Evans
• Current Weather Discussion
• 5 March 2008

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What is meant by severe weather?

• Tornadoes of any intensity
• Significant winds (gusts gt 58 mph)
• Significant hail (diameter gt 0.75, or
  penny-sized)
• Each requires a thunderstorm of sufficient
  organization to occur and be counted as severe
• Winds can gust above severe thresholds due to
  pressure gradients, etc. but are not
  thunderstorm-based and thus not severe

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Severe Weather Ingredients

• For thunderstorms, a checklist of three
  ingredients
• Moisture
• Instability
• Lift (often referred to as a trigger)
• For severe weather, a fourth ingredient is
  needed
• Vertical wind shear

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Moisture and Instability

• Contribute to CAPE (Convectively Available
  Potential Energy)
• Provide positive buoyant energy that can energize
  updrafts
• Reflected at the surface (e.g. dewpoint) as well
  as in upper-level temperatures and lapse rates

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Lift/Trigger

• Oftentimes comes in the form of a boundary
• Cold/warm front
• Dry line
• Sea breeze
• Outflow boundary, etc.
• Can also be found on larger-scales
• E.g. Q-G Lift from positive vorticity advection
• Necessary to allow updrafts to access CAPE and
  help overcome any potential capping

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Vertical Wind Shear

• Allows for the organization of thunderstorms into
  cellular structures
• Weaker wind shear multicell structures
• Stronger wind shear discrete/semi-discrete
• Allows for the development of rotation within the
  storm
• Mesocyclones
• Rotating updrafts, etc.
• Less vertical wind shear can be made up for by
  higher CAPE and vice versa!
• Important in summer/winter events respectively

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Random Obligatory Tornado Image
(courtesy numerous sources)
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Severe Weather Diagnosis

• On the synoptic-scale, upper air charts, surface
  maps, and instability analyses are often used as
  a first guess toward severe potential
• 300/500 hPa wind, height, vorticity fields
• 700/850 hPa wind, height, moisture, temperature
  fields
• Surface pressure, temperature, dewpoint, wind
  fields
• Surface-based CAPE, CIN (Convective Inhibition)
  fields
• This is not meant to be an exhaustive listing

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Severe Weather Diagnosis

• On the mesoscale, however, sounding analysis
  becomes much more critical
• Soundings allow for the diagnosis of instability,
  capping, vertical wind variations, storm motion,
  and much more
• Well focus on dissecting severe weather
  soundings in this lecture today

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Sounding Example
(courtesy Storm Prediction Center)
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Parcel Parameters

• Surface based upon currently-observed surface
  data
• Mixed layer based upon averaged temperature and
  mixing ratio in lowest 100 hPa
• MU most unstable based upon parcel in lowest
  300 hPa with highest theta-e value

• Fcst Surface same as mixed layer, except using a
  surface temperature from the observed 850 hPa
  temperature
• Drop dry adiabatically to surface and add 2C

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Which to use?

• Surface and mixed layer views give a good
  approximation to what surface-based storms have
  available to them
• Recall tornadic storms must be surface-based
• Forecast surface has significant limitations that
  must be understood
• Why the temperature may not get to the forecast
  temperature, effects of moisture on instability,
  etc.
• Most unstable is best when convection is likely
  to be elevated (e.g. atop a warm front)
• Hail is the predominant severe threat in such
  situations

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Stability-Related Parameters

• CAPE available buoyant energy
• Integrated area where T(parcel)gtT(sounding)
• CIN convective inhibition
• Integrated area where T(parcel)ltT(sounding)
• LCL lifting condensation level
• Saturation via lifting
• Cloud base height estimate

• LI lifted index
• T(environment)-T(parcel) at 500 hPa
• LFC level of free convection
• Level at which CIN ends and CAPE begins

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Stability Parameters

• For severe weather, high CAPE and low to minimal
  CIN values are required
• CIN is not always a bad thing, though (discrete
  storm modes, etc.)
• Lower LCLs suggest lower storm bases and deeper
  storms
• More negative LIs suggest an increased hail
  potential and often correlate to high CAPE values
• Lower LFCs suggest freely buoyant convection at
  lower levels and often also correlate to high
  CAPE values

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Sounding Example
(courtesy Storm Prediction Center)
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Moisture/Thermal Parameters

• We wont be considering all of these parameters,
  just the most significant/less esoteric ones
• Primarily relate to moisture content, downdraft
  potential, hail size, and thermal buoyancy

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Moisture Parameters

• PW precipitable water integrated water content
  in the lowest 400 hPa
• K K-Index comparison of mid-level lapse rates
  to low-level moisture (30-40 deep convection)
• MidRH mid-level relative humidity lower
  suggests enhanced evaporative cooling/downdraft
  potential
• LowRH low-level relative humidity higher
  suggests lower LCLs (and potentially lower LFCs
  and higher CAPE)

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CAPE Parameters

• 3CAPE CAPE in the lowest 3 km higher is better
  and suggests low LCLs and large lapse rates
• DCAPE downdraft CAPE factor of dry air and
  evaporative cooling potential in lowest 400 hPa
• Higher values, particularly gt1000 J/kg, suggest
  enhanced downdraft potential

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Thermal Parameters

• WBZ height of the wet bulb 0C isotherm lower
  values suggest less hail melting
• FZL height of the environmental 0C isotherm
• ConvT convective temperature temperature parcel
  must warm to for surface-based inhibition to be
  overcome
• MaxT estimated maximum temperature similar to
  forecast surface value from before except from
  100 hPa above ground level

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Sounding Example
(courtesy Storm Prediction Center)
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Lapse Rate Parameters

• Note First and third/second and fourth listings
  are similar to one another

• All represent the average environmental
  temperature lapse rate in the given layer
• Higher lapse rates environment cools more
  quickly with increasing height potential for
  greater CAPE values
• Lower lapse rates moister column possibly
  indicative of capping concerns

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Sounding Example
(courtesy Storm Prediction Center)
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Hodograph Analysis

• Take the wind vectors from the sounding, starting
  at the surface.
• Draw arrows pointing toward where wind is blowing
• Length given by wind speed
• Connect the ends of those arrows

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Hodograph Analysis

• What does the hodograph tell us?
• It graphically shows how winds vary with height
• It can be used to infer storm motion
• More importantly, however, it allows us to
  calculate and obtain helicity!

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Helicity Calculation

• Helicity is simply the area underneath the curve
  of the hodograph!
• Different curves are obviously selected for the
  desired helicity levels (0-1 km, etc.)
• Storm-relative helicity is similar but uses
  projected storm motion as its basis

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Sounding Example
(courtesy Storm Prediction Center)
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Helicity and Shear Parameters

• Thunderstorm organization relies on vertical wind
  shear, particularly in the lowest 6-8 km
• Mesocyclone development relies on lower-level
  vertical wind shear
• 0-1, 0-3 km ? perhaps even lower

• Storm motion (storm-relative flow) must be taken
  into account, however.

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Helicity and Shear Parameters

• Most important value SFC 6 km Shear
• Determines predominant storm mode
• Weak (lt20 kt) pulse convection
• Moderate (lt50 kt) multicellular storms
• Strong (gt50 kt) supercell structures possible

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Helicity and Shear Parameters

• Next most important value storm motion estimate
• Used in helicity calculations
• Most concerned with right moving motion about
  95 of the time

• Obtained from the SFC-6 km shear vector and the
  SFC-6 km wind estimate

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Helicity and Shear Parameters

• What exactly is helicity?
• Helicity is a measure of vertical vorticity
• Contrast ?, relative vorticity, is the vertical
  component of the horizontal vorticity
• Why is it important?
• It is a measure of the strength of the vertical
  environmental circulations

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Helicity and Shear Parameters

• But wait, arent mesocyclones and tornadoes
  horizontal features?
• This is where the updraft becomes important!
• The updraft tilts the vertical circulations into
  the horizontal
• thus allowing for mesocyclone and potentially
  tornadic formation!
• Another example of why we need both
  buoyancy/instability and vertical wind shear for
  severe thunderstorm formation.

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Helicity and Shear Parameters

• SFC 1 km storm-relative helicity (SRH) in the
  lowest 1 km
• SFC 3 km SRH in the lowest 3 km
• Eff Inflow Layer SRH in the effective inflow
  layer

• Effective inflow layer is determined as the layer
  where all parcels, starting at the ground, have gt
  100 J/kg CAPE and gt -250 J/kg CIN

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Additional Parameters

• On soundings, you may find any number of other
  parameters
• Total Totals (TT)
• Bulk Richardson Number (BRN)
• Supercell Composite Index
• Significant Tornado Index (SigTor), etc.
• Most of these involve some combination of shear
  (whether vertical shear or something like
  helicity) and CAPE to estimate severe potential

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Summary

• There are four ingredients needed for severe
  convection moisture, instability, lift, and
  vertical wind shear
• These ingredients need to be assessed and
  diagnosed on both the synoptic and mesoscales
  when forecasting or analyzing severe weather

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Summary

• On the mesoscale, sounding analysis is a very
  effective means of analyzing the ingredients
  necessary for severe thunderstorms
• Caution must be exercised, though, as soundings
  can and do change
• Air mass changes (e.g. temperature advection)
• Entrance or exit of jet streaks, etc.

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Whats Next?

• Despite all that we went into today, there is a
  lot still left to be covered
• Synoptic-scale severe weather forecasting
• Radar analysis
• Severe convection storm modes
• Linear, discrete, MCS/MCVs, QLCS, etc.
• Left splitting vs. right splitting cells
• More?
• Well get into these topics over the coming weeks.