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Unit 6: Severe Weather: What to look out for & how it forms

It's Weather Academy Wednesday! Join Meteorologist Corey Thompson for a talk on severe weather: what we look for and how it forms.

Posted by KCRG-TV9 First Alert Weather on Wednesday, April 29, 2020

Thunderstorms in the United States

  • Florida has the most days with thunderstorms in the country
  • They are closest to the Atlantic Ocean and Gulf of Mexico and creates a good stream of moisture which is needed for storms
  • The west coast has the least amount of storms
  • Iowa is somewhere in the middle: about 36-54 days in a given year

Thunderstorm ingredients:

  • Moisture
    • Moisture in the air primarily comes from evaporation from the oceans
    • During the summer, some of it comes from crops in the Midwest
    • Warm water evaporates easier
  • Instability
    • Determines if air rises when it’s given a push
    • Unstable: air continues to rise and forms clouds, then storms
    • Warm moist air at the surface and colder air above the surface. Warm air continues to go upwards until it hits that colder air and will stop rises
    • Stable: air stops rising and does not form clouds
  • Source of Lift
    • Something to push that air upward
    • Cold front and warm fonts can cause that lift
    • In the summer, outflows boundaries can cause lift. They are like a min cold front left behind from storms

Types of Thunderstorms

  • Single-cell: Storm forms up rains for bait and then dies. May rain for around 10 minutes before it dissipates
  • Multi-cell: Use their strength together. Single cells together and can cause heavy rain
  • Squall line:
    • Longline of storms that can stretch hundreds of miles
    • Can cause severe weather
    • New storms form on the leading edge keeping the line going
    • Can have damaging winds, large hail, and sometimes a tornado
  • Supercell: Most intense type of thunderstorm and can form severe weather
    • Actually they are single-cell storms with a twist’The storm updraft is rotating making it very strong and long-lasting.
    • Wind shear helps it spin
    • They form when there is both energy and wind shear
    • Can cause very large hail, damaging winds, and tornadoes
    • Most strong tornadoes come from supercell thunderstorms

The secret ingredient: Wind Shear

  • How the wind changes as you go up in the levels of the atmosphere
  • Directional shear: Wind direction changes the higher you go. This creates a natural rotation for air that is rising
  • Speed sheer: Wind gets stronger as you go higher and creates a rolling effect
  • The presence of wind shear makes a storm stronger and makes it last longer.
  • Most intense thunderstorms come when both wind shears work in combination

How do we know they’re coming:

  • Weather balloons called radiosondes. The NWS launches these balloons and measures weather like temperatures, moisture, and pressure as it goes up in the atmosphere
  • Computer models generate information that comes from weather balloons. It is a math equation simulating what weather could happen.
  • Surface observations. Taking actual measurements at the surface during these systems to look at data and determine if the ingredients are there for storms.
  • Storm spotters