January10thSevereWx

January 10th Severe Weather in Wildey, Barbados: 

January 10th Severe Weather in Wildey, Barbados Tornado or Microburst???

The ingredients for thunder showers and possible severe weather: 

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;

Thunder showers in the “Dry Season”: 

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?

Synoptic Conditions: 

Synoptic Conditions

Visible Image11:45 UTC (7:45 local): 

Visible Image 11:45 UTC (7:45 local) H Clouds indicate low level moisture

850 mb winds: 

850 mb winds A

Upper Level 200 mb : 

Upper Level 200 mb C

IR image – deep convection: 

IR image – deep convection Note Cold top near -30°C ~300 mb (30000 ft) H

Deep convection over SE Barbados: 

Deep convection over SE Barbados 2 km IR image at 12:15 (8:15 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

Cb Propagation: 

Cb Propagation 11:15 (7:15) 12:15 (8:15) 1. Mass of convection over northern Barbados; general motion east to west 2. Note a cell appears to form over Central Barbados; on the left flank of deep convection to the north 3. Possible Cb over SE Barbados: 11:45 (7:45)

Possible triggers: 1. Moisture Gradient: 

Possible triggers: 1. Moisture Gradient Moisture gradient as ITCZ moisture from South America is advected northwards, converging with drier air over the Caribbean Basin

2. Moist and unstable soundings : 

2. Moist and unstable soundings Low level Easterly wind Maximum 31kt at 700 mb (3013 m) Upper level Jet (westerly) at 79 kt at 162 mb (12147 m) Winds ‘veering’ with height

How unstable was the atmosphere?K Index - General Thunderstorm Predictor : 

How unstable was the atmosphere? K Index - General Thunderstorm Predictor @ TBPB 12: UTC sounding ---- K index: 35.00

Observed Wx Time Series at Grantley Adams Airport: 

Observed Wx Time Series at Grantley Adams Airport

Estimating Wind Gust: 

Estimating Wind Gust Rule of Thumb 2/3 of observed low level wind max + current surface wind speed At 12:00 UTC ~ 2/3 (30kt at 700 mb) + 17 kt = 37 knots

Stages of development: 

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

Theories: 

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

Theories of Tornado Formation: 

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.

Fujita’s wind damage scale: 

Fujita’s wind damage scale

Microbursts: 

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.

Dangers of Microburst: 

Dangers of Microburst

Microburst with vortex rolls: 

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.