January 10th Severe Weather in Wildey, Barbados

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January 10th Severe Weather in Wildey, Barbados

• Tornado or Microburst???

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The ingredients for thunder showers and possible
severe weather

• Convectively unstable atmosphere
• Warm and humid boundary layer (layer near the
  surface)
  
• Strong vertical shear, in the form of upper level
  jet (strong winds in excess of 50 kt or 25
  km/hr)
  
• Strong veering (clockwise turning) winds
• Mesoscale or Synoptic scale lifting (moisture
  boundary, frontal boundary, tropical wave, etc.)
  
• Lower level jet can add to convective
  development
  

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Thunder showers in the Dry Season

• Strong winds associated with thunder showers
  (deep convection) as experienced on January 17th
  , 2007 is rear but not unheard of that time year.
  
• What lead up to the severe weather?
• Was the winds produced associated with a weak
  tornado or microburst?

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Synoptic Conditions
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Visible Image1145 UTC (745 local)
H
Clouds indicate low level moisture
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850 mb winds
A
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Upper Level 200 mb
C
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IR image deep convection
H
Note Cold top near -30C 300 mb (30000 ft)
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Deep convection over SE Barbados
2 km IR image at 1215 (815 am) UTC centred on
Barbados Isolated cold top over SE Barbados
1 km VIS image centred image on Barbados note
shadow of Cb located over SE Barbados
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Cb Propagation
1. Mass of convection over northern Barbados
general motion east to west
1115 (715)
2. Note a cell appears to form over Central
Barbados on the left flank of deep convection to
the north
1215 (815)
1145 (745)
3. Possible Cb over SE Barbados
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Possible triggers 1. Moisture Gradient
Moisture gradient as ITCZ moisture from South
America is advected northwards, converging with
drier air over the Caribbean Basin
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2. Moist and unstable soundings
Upper level Jet (westerly) at 79 kt at 162 mb
(12147 m)
Winds veering with height
Low level Easterly wind Maximum 31kt at 700 mb
(3013 m)
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How unstable was the atmosphere?K Index -
General Thunderstorm Predictor
_at_ TBPB 12 UTC sounding ---- K index 35.00
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Observed Wx Time Series at Grantley Adams Airport
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Estimating Wind Gust

• Rule of Thumb
• 2/3 of observed low level wind max current
  surface wind speed
  
• At 1200 UTC 2/3 (30kt at 700 mb) 17 kt 37
  knots

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Stages of development
Developing (cumulus) stage
Mature (cumulonimbus) stage
Dissipating
Stage of severe wx development
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Theories

• Just Tornado
• What are the necessary conditions?
• Just Microburst
• What are the necessary conditions?
• Microburst and vortex?
• View the damage

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Theories of Tornado Formation

• Rotating updraft region as there is rapid
  increase of wind speed with height (vertical wind
  shear).
  
• Ice skate analogy arms outstretch, low
  rotation, arms pulled inward, fast rotation.
  
• Low level moisture.
• A mesocyclone forms and with strong horizontal
  winds with considerable curvature and vertical
  shear will eventually grow into a tornado.

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Fujitas wind damage scale
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Microbursts

• Microburst a downburst of cold air that extent
  for only 4 km or less.
  
• Microburst can be responsible for heavy damage to
  tree and buildings.
  
• Often damage caused by microburst as mistaken as
  tornado damage.
  
• Microburst debris is often blown flat and not
  twisted as in the case of tornadoes.
  
• Wind shear accompanying microburst as there is a
  rapid change in wind speed and direction and is
  often responsible of airline crashes.
  
• Doppler radars are now used to observe the advent
  of microburst.

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Dangers of Microburst
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Microburst with vortex rolls

• The images and eye witness accounts suggest that
  there was a microburst with resultant vortex
  rolls.
  
• The best indication of this was a tent from
  Luther Thorne Primary was blown 100 m north of
  its original position and was seen rotating in
  mid-air.