Rare Tornado Event Part Two: What Transpired on February 28, 2017

In Part One of this series, I discussed the role of the National Weather Service in warning situations.  I thought this would be good topic to cover in light of the rare and tragic tornado outbreak that occurred February 28, 2017.  Note all data were provided by the National Weather Service Chicago.

Background on the event

  • In total, there have been at least six tornadoes confirmed across all of northern Illinois.
  • The strongest were two tornadoes rated EF-3 on the Enhanced Fujita Scale — one impacting the communities of Naplate and Ottawa, and the other Washburn, both causing significant damage.
  • The Naplate to Ottawa tornado killed two persons and injured 14.
  • The largest hail reported in the area was in Ottawa, to the size of baseballs
    ef_scale.png

    Enhanced Fujita Scale (EF) measures the intensity of tornadoes. Data sources: National Weather Service

    Meteorological background

    On this day, several ingredients came together to create a volatile situation.  Thunderstorms need three ingredients to develop: moisture, instability, and some sort of focus or lifting mechanism.  There was an abundance of moisture moving into the area.  This is called moisture transport.  You know the days when you can either feel or smell the moisture in the air?  Well than that occurs, you know plenty of moisture is in place to fuel thunderstorms.

However storms need more than moisture. They also need a focus to lift air parcels above the surface so they will continue to rise and eventually condense into cumulonimbus (thunderstorm clouds).  On this day, there was a warm front nearby.  This front served as the lifting mechanism.

For a parcel to continue to rise, the air has to be unstable.  That is the surrounding air has to be colder than the air parcel. Sure enough this day was characterized by unusually warm air near the surface and cold air in the middle and upper levels of the atmosphere.  In addition wind fields at the surface to the middle levels of the atmosphere varied in direction and speed.  This different creates wind sheer, which can lead to severe thunderstorms and sometimes tornadoes.

Determining tornado intensity

Meteorologists from the National Weather Service Chicago looked at the damage from these tornadoes.  When they do this, it is called a damage survey.  This is the best way meteorologists can tell the strength and path length of tornadoes.  They analyze the debris (where and how far the debris is located).  They also look at the trees and structures that were damaged and/or destroyed.  Questions such as how sound was the structure before it was damaged, the materials the structures were made of, how large the tree was, and if the tree was alive or dead before getting uprooted must be answered to determine the strength of the winds.  Damage surveys need to be done as soon as possible before the victims of the storms start cleaning up the damage. Below are the findings of the damage survey conducted by National Weather Service offices affected by this tornado outbreak:

Naplate-Ottawa Storm

Tornado #1 (EF-3):

  • Naplate (LaSalle)
  • Ottawa (LaSalle)

Tornado #2 (EF-1):

  • Marseilles Area (LaSalle)

Possible other area north of Seneca and northwest of Morris

Washburn-Rutland Storm

Tornado #3 (EF-3):

  • Washburn (Woodford):
  • Rutland (LaSalle)

Tornado #4 (EF-2):

  • Near Long Point (LaSalle/Livingston)

Oregon Storm

Tornado #5 (EF-1):

  • Oregon (Ogle)

Conclusion

This tornado outbreak was rare for late February since the peak tornado season for northern Illinois does not really start until May and continue into June.  However as we recently saw, tornadoes can develop outside of this window.  While tornadoes are a fascinating phenomena, they are dangerous and you need to heed any tornado watch and warning seriously. If you have any pictures of these storms (or any other pictures of storms) and would like to share them, please contact me.  I would love to see them as would any of my viewers who are fascinated by convective hazardous weather.

 

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