Thunderstorms and Tornadoes

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Understanding the WeatherEAS-107

• Chapter 14
• Thunderstorms and Tornadoes

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Understanding the WeatherEAS-107

• Chapter 14 Overview
• Ordinary Cell Thunderstorms
• Severe Thunderstorms
• Mesoscale Convective
  Complexes
• Lightning
• Tornadoes

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Understanding the WeatherEAS-107

• Thunderstorms
• A thunderstorm contains thunder and lightning.
• Come in many different shapes and sizes.
• Warm, moist air rises in a conditionally unstable
  environment.
• As long as parcel is warmer than environment then
  it will continue to rise, it is buoyant.
• Greater the temp difference, the faster the air
  will rise.
• Rising air must be triggered/forcing mechanism.
• Unequal heating, terrain, lifting of air along
  shallow boundaries of converging winds.
• Frontal lifting.
• Large scale divergence aloft.

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Understanding the WeatherEAS-107

• Ordinary Cell Thunderstorms
• Afternoon storms develop away from fronts.
• Form in a region with limited wind shear wind
  speed and direction do not change with height.
• Form along shallow convergence zones.

• Topography, sea-breezes, cold outflow from a
  prior thunderstorm (differences in temp) can be
  locations for development.

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Understanding the WeatherEAS-107

• Ordinary Cell Thunderstorms (Cumulus/Growth
  Stage)
• Warm and humid parcel of air rises, cools, and
  condenses into one cloud or a cluster of clouds.
• Gradually moistens the dry air aloft.

• Latent heat release keeps the parcels warm.
• Must have continuous source of rising air, the
  cloud needs to be constantly fed rising air from
  below.
• Updrafts suspend the liquid until it grows large
  enough to fall.
• This usually does not happen until ice is
  present.

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Understanding the WeatherEAS-107

• Ordinary Cell Thunderstorms (Mature Stage)
• The entrainment of drier air at the edges of the
  storm evaporates some of the liquid which cools
  the surrounding air. Evaporative cooling.

• The air, now heavier and cooler than the air
  around it, begins to descend a downdraft. This
  can be enhanced by falling precip.
• When this occurs the storm is mature.
• The updraft and downdraft now constitute the
  cell.

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Understanding the WeatherEAS-107

• Ordinary Cell Thunderstorms (Mature Stage)
• The most intense stage. Lightning, thunder, rain,
  and occasionally small hail.
• Top of cloud reaches a stable region (tropopause)
  and spreads out into anvil, up to 40,000 ft (12
  km).
• Some updrafts are so strong they penetrate the
  stable air, a condition know as overshooting.

• The cold downdraft forms a gust front.
• The gust front can act to force more warm, humid
  air into the storm.

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Understanding the WeatherEAS-107

• Ordinary Cell Thunderstorms (Dissipating Stage)
• Once the storm enters the mature stage, it begins
  to dissipate after 15 30 mins.
• Downdrafts dominate through much of the cloud and
  cut off the supply of warm, moist air.

• Deprived of the rich supply of warm, humid air,
  cloud droplets no longer form.
• As the storm dies, the lower-level cloud
  particles evaporate rapidly.

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Understanding the WeatherEAS-107

• Severe Thunderstorms
• Criteria A thunderstorm that produces hail at
  least ¾ diameter (penny/dime), and/or surface
  wind gusts of 50 kts, and/or produces a tornado.
• The longer a storm survives, the more likely it
  will become severe.
• Forms in same way as an ordinary storm, only the
  environment features vertical wind shear.
• The lack of wind shear in an ordinary
  thunderstorm causes the precip to fall into the
  updraft.
• Increased winds aloft push the precip away from
  updraft.
• The downdraft can still undercut the updraft
  without the precip falling into it. This produces
  a long-lasting multicell storm.

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Understanding the WeatherEAS-107

• Severe Thunderstorms (Multicell Storm)
• Most ordinary thunderstorms are multicell storms
  (storms with a cluster of cells at various stages
  of their life cycles), however many multicell
  storms are severe.
• The gust front initiates new storms (cells).
• This process may repeat over and over, producing
  a long lasting storm.

• Each cell in a different state of development.

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Understanding the WeatherEAS-107

• Severe Thunderstorms (Supercell)
• When the upper level winds are even stronger and
  also change direction with height
• The storm can move in such a way that the
  downdraft never undercuts the updraft.
• Can create horizontal spin which may be tilted
  into a rotating updraft.
• The result is a supercell.
• A supercell modifies its own environment.
• The vertical wind shear creates a storm structure
  that allows the storm to continually move towards
  the area of warm moist air.
• Supercells move to the right of the mean flow.

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Understanding the WeatherEAS-107

• Severe Thunderstorms (Supercell)
• The supercell thunderstorm is typically a large
  thunderstorm that consists primarily of a single
  rotating updraft.
• The internal structure is organized in a way so
  that the storm may maintain itself as a single
  entity for hours on end.

• Storms of this magnitude can produce updrafts
  that exceed 90 kts, hail the size of grapefruit,
  damaging surface winds, and large, long lasting
  tornadoes.

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Understanding the WeatherEAS-107

• Mesoscale Convective Complex
• A large, convectively driven system that is made
  up of many individual thunderstorms. Many times,
  as much as 1000 times larger than an individual
  thunderstorm cell.
• Within the MCCs, the individual thunderstorms
  work together to generate a long-lasting weather
  system that moves slowly and quite often for
  periods exceeding 12 hrs.

• Typically about the size of Ohio.
• Often forms during summer nights.
• Major source of rainfall for Plains states in the
  summer.

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Understanding the WeatherEAS-107

• Lightning
• Lightning is simply a discharge of electricity
  (giant spark) which usually occurs in mature
  thunderstorms.
• Cloud to cloud, cloud to ground, cloud to air,
  within cloud.
• Can heat the air to 54,000F!! (5 times hotter
  than sun)
• The extreme heating causes the air to rapidly
  expand, initiating a shock wave that becomes a
  sound wave, called thunder.
• Sound travels 330 m/sec vs. 300 million m/sec for
  light.
• Sound travels faster in warm air than in cold
  air.
• It takes sound about 5 seconds to travel 1 mile.
• If you see lightning and hear thunder 15 seconds
  later, the lightning stroke occurred 3 miles
  away.

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Understanding the WeatherEAS-107

• Lightning (Electric Charge)
• In normal fair weather, the electric field of the
  atmosphere is characterized by a negatively
  charged surface and a positively charged upper
  atmosphere.
• For lightning to occur, separate regions
  containing opposite electrical charges must
  exists within the cloud.
• Separation of charge is brought about by
  precipitation processes.
• There is a net transfer of positive ions (charge)
  from a warmer object to a colder object.
• Ice crystals are colder than hailstones
  (graupel).
• Hailstones become negatively charged, ice
  crystals positively.
• Charge is then distributed by weight.
• Ice and a strong updraft are necessary for
  lightning to occur.

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Understanding the WeatherEAS-107

• Lightning (Exchange of Charge)
• When the negative charge near the bottom of the
  cloud is large enough to overcome the airs
  resistance, a flow of electrons (stepped leader)
  rushes towards the earth.
• As the electrons approach the ground, a region of
  positive charge moves up into the air through any
  conducting objects (trees, buildings, even
  humans).

• When the downward flow of electrons meets the
  upward surge of positive charge, a strong
  electric current (bright return stroke) carries
  the positive charge up into the cloud.
• Takes 3 or 4 return strokes to fully exchange the
  charge.

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Understanding the WeatherEAS-107

• Tornadoes
• A tornado is a rapidly rotating column of air
  that blows around a small intense area of low
  pressure.
• Circulation must reach the ground as either a
  cloud or area of swirling debris.
• Funnel cloud is a tornado that does not reach the
  ground.
• The majority of tornadoes rotate
  counterclockwise.
• Diameter 300 2000 ft, some gt 1 mi.
• Usually move NE at 20 40 kts, up to 70 kts.
• Most tornadoes usually last only a few minutes
  and have an average path length of 4 miles.
• But some can last many hours and travel hundreds
  of miles.
• Most are found in the Plains (tornado alley).

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Understanding the WeatherEAS-107

• Tornadoes (Occurrence)
• Top number Number of tornadoes in 25 years.
• Bottom number Number of tornadoes per 10,000
  square miles.
• Tornado alley is from north Texas through
  Nebraska.
• Can occur in any state.

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Understanding the WeatherEAS-107

• Tornadoes (Watch vs Warning)
• Tornado watch. Tornadoes are likely to form
  within the next 4 to 6 hours somewhere in the
  region (issued by Storm Prediction Center).
• Tornado warning. A tornado has been spotted or
  indicated on Doppler radar (issued by the local
  weather service office).

• Take shelter in the basement in an interior room.
  In a structure without a basement, the safest
  place is in an interior room on the lowest floor.
• In a dorm or apartment building, move to an
  interior hallway and lie flat with your head
  covered.

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Understanding the WeatherEAS-107

• Tornadoes (Life Cycle)
• 1. Dust Whirl Stage
• 2. Organizing Stage
• 3. Mature Stage
• 4. Shrinking Stage
• 5. Decay Stage

1.
2.
3.
4.
5.
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Understanding the WeatherEAS-107

• Tornadic Thunderstorms
• Not everything is known about the formation of a
  tornado.
• It is known that tornadoes tend to form with
  intense thunderstorms and that a conditionally
  unstable atmosphere is essential for their
  development.
• Tornadoes form with both supercell thunderstorms
  and non-supercell thunderstorms.
• The most intense tornadoes form with supercell
  thunderstorms. Thunderstorms with a strong,
  single rotating updraft that develop in a region
  of strong vertical wind shear.

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Understanding the WeatherEAS-107

• Tornadic Thunderstorms (Mesocyclones)
• The rotating updraft of a supercell thunderstorm
  is called a mesocyclone.
• Change of wind speed and direction with height is
  responsible for the rotating updraft.
• Creates horizontally rotating vortex tubes.
• The strong updraft will tilt the tube and draw it
  into the storm.
• This creates a mesocyclone 5-10 km wide.

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Understanding the WeatherEAS-107

• Tornadic Thunderstorms (Bounded Weak Echo Region)
• The updraft is so strong in a supercell that
  precipitation cannot fall through it.
  Southwesterly winds aloft usually help to blow
  the precipitation northeastward.
• Large hailstones that have remained in the cloud
  for some time, usually fall just north of the
  updraft.
• If a mesocyclone is strong and persistent, the
  precip can be wrapped around the updraft.

• This swirling area of precip shows up on the
  radar, where the area inside the mesocyclone does
  not.
• This is called the bounded weak echo region, an
  area that is bounded by precip. On radar, it can
  appear as a hook on the southern side of the
  storm.

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Understanding the WeatherEAS-107

• Tornadic Thunderstorms (Tornado Formation)
• The mesocyclone can be compared to a small low
  pressure system, like those that we have studied.
• At this point, the updraft, counterclockwise
  swirling precip, and the surrounding air interact
  to form the rear flank downdraft (southwestern
  side of supercell).

• When the downdraft strikes the ground, it may
  interact with the region of inflow to help
  initiate the tornado.

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Understanding the WeatherEAS-107

• Tornadic Thunderstorms (Tornado Formation)
• As air rushes into the low-level core of the
  mesocyclone, the air expands, cools, and if
  moist, condenses into a cloud (funnel cloud).
• As the air beneath the funnel cloud is drawn into
  its core, the air cools rapidly and condenses,
  and the funnel descends to the surface.
• As the funnel reaches the ground, it usually
  picks up dirt and debris, making it appear dark.
• While the air along the outside of the funnel is
  spiraling upward, in the most violent tornadoes,
  the air is descending towards the extreme low
  pressure at the ground (sometimes 100 mb lower
  than surrounding air).

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Understanding the WeatherEAS-107

• Tornadic Thunderstorms (Final Thoughts)
• Not all supercells produce tornadoes in fact,
  only around 15 do.
• However, studies reveal that supercells are more
  likely to produce tornadoes when they interact
  with a pre-existing boundary.
• Outflow boundary from previous convection.
• Warm front
• When supercells interact with these boundaries,
  low-level wind shear is enhanced. This can lead
  to a stronger and deeper mesocyclone.

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Understanding the WeatherEAS-107

• Fujita Scale
• Classifies wind speed based on damage. Tornado
  winds are estimated based on the damage caused by
  the storm.
• Studies indicate that the majority of tornadoes
  are F0 and F1, and only a few are above F3.
• On average, only 1 or 2 F5s occur a year.

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Understanding the WeatherEAS-107

• Fujita Scale (F0 and F1)

F0
F1
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Understanding the WeatherEAS-107

• Fujita Scale (F2 and F3)

F2
F3
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Understanding the WeatherEAS-107

• Fujita Scale (F4 and F5)

F4
F5