Download Severe Thunderstorms

Severe
Thunderstorms
AOSS 102:
Extreme Weather
20 November 2015
Multiple Choice
Thunderstorms
All thunderstorms need three basic ingredients:
1) Moist surface air
2) A conditionally unstable or absolutely unstable atmosphere
3) A mechanism “trigger” that forces air to rise
The thunderstorm classifications are:
• 
Ordinary cell (Air mass) thunderstorm
• 
Multicell thunderstorm
• 
Squall-line thunderstorm
• 
Mesoscale Convective Complex (MCC)
• 
Supercell Thunderstorm
Characteristics of a Multicell
Thunderstorm: Cells at all stages
Updrafts and downdrafts are tilted, supplying additional energy.
Image Quiz
Image Quiz
Multicell Thunderstorms
Gust Front
—  leading edge of the cold outflowing air
—  around 1000 feet deep with average speeds of 10-30 mph,
wind gusts (called straight line winds) around 60 mph
—  May form shelf cloud (if atmosphere is very stable at the
ground) and roll cloud (spinning around horizontal axis)
—  Many gust fronts can merge into an ‘outflow boundary'
Microbursts
—  localized downburst (less than 2.5 miles wide) that hits the
ground and spreads horizontally in a radial burst of wind
—  Winds as high as 150 mph, severe damage possible
Downdrafts are mainly created by evaporating rain drops that cool the
air.
Downdrafts reach the
surface, cool
air spreads out, forming a gust front.
Fig. 11.6, p. 303
Multiple Choice
Outflow Boundary from Multicell Storm System
Radar image of a
outflow boundary,
often triggering new
thunderstorms
Outflow
Dust and sand can be picked up by the wind at the outflow boundary.
New thunderstorms might form along the outflow boundary.
Outburst Winds of a Microburst
Forming a Dust Cloud
Fig. 11.11, p. 305
Downburst Characteristics
Microbursts are small downbursts: Winds do not extend more than 2.5 miles
Virga
Precipitation evaporates
before reaching the ground, cools air
Downburst Characteristics:
Stationary & Traveling Microbursts
Stationary Microburst:
Damage Pattern
Traveling Microburst:
Damage Pattern
Microbursts and Aviation
Downburst Characteristics
Microbursts: An Aviation Hazard
Fig. 11.12, p. 306
Thunderstorm Categories:
Squall-Line Thunderstorms
Fig. 11.13, p. 306
—  Line of multicell thunderstorms
with strong gusty
winds directly
along or ahead
of a cold front
Radar image of a
pre-frontal squall
line from Indiana to
Arkansas (red:
most intense)
Causes severe weather over much
of the length of the
squall line
Most common: Pre-frontal squall line
thunderstorms ahead of cold front
Triggered by upper-air gravity waves that originate at the cold front
Squall-Line Thunderstorms
Squall-line thunderstorm with a trailing stratiform cloud layer and
tilted updrafts:
•  tilt promotes development of new cells as storm system moves
•  rear inflow jet may exceed 100 mph
Fig. 11.15, p. 307
Squall Line Thunderstorms
Side view of the lower half of the squall-line thunderstorm:
If the strong straight-line winds extend horizontally for a considerable distance, the wind storm is called “Derecho”
Fig. 11.16, p. 308
Bow-Shaped Squall Line: Bow Echos
Preferred area where tornadoes develop: northern
side of bow
Fig. 11.17,p. 308
Damaging straight-line winds along the squall line: Derecho
Bow-Shaped Squall Line: Bow Echos
Small bows (mini bows)
embedded in squall line
Fig. 11.18, p. 309
Downbursts: Derecho Wind Storms
•  Thunderstorm-induced (non-
rotating) straight-line winds
•  wind damage extends for more than 240 miles
•  includes wind gusts > 57 mph
along most of its length
•  embedded several 75 mph or greater (hurricane-strength, Derecho may be associated with a
up to 130 mph) wind gusts
single bow echo or with multiple bows.
Damaging Effect of a “Derecho”
Straight-Line Windstorm (80 mph)
Hurricane-strength wind speeds are often observed
Fig. 11.19, p. 309
Derecho Climatology in the U.S.
Predominantly from April to August,
11.19, arise
p. 309
within minutes, extremely high windsFig.can
June 2012 North American Derecho
Fig. 11.19, p. 309
June 2012 North American Derecho
Red: tornado warnings
Yellow: severe thunderstorm
warnings, Green: flash flood warnings
Tornado / Hurricane-like damage
https://en.wikipedia.org/wiki/June_2012_North_American_derecho
Fig. 11.19, p. 309
Thunderstorm Categories:
Mesoscale Convective Complexes
(MCCs)
—  During the summer a number of individual multicell
thunderstorms may occasionally grow in size and organize
into a large circular convective weather system
—  Can be 1000 times larger than an air mass thunderstorm
—  Can cover entire states, an area bigger than 100,000 km2
—  Move slowly (< 20 mph) and last for over 12 hours
—  Provide significant portions of the growing season rainfall in
the Central U.S., reach their max intensity at night
—  Fueled by moist low-level jet from the south (brings moisture)
—  Severe weather threat: hail, high winds, flash floods &
tornadoes
Fig. 11.20, p. 310
Satellite image showing
the cold cloud tops (dark
red) of an MCC in Kansas
Radar image of an MCC moving SSEward into Kentucky. Red: strong
thunderstorm activity
Fig. 11.21, p. 311
Radar Image
Short Answer
What Makes a Thunderstorm
Severe ?
A thunderstorm is considered “severe” if it contains
one of the following:
•  Hail with a diameter of three-quarter inch or greater
•  Wind damage or gusts of 50 knots (58 mph) or greater
•  A tornado
Supercell Thunderstorm:
A large, long-lasting thunderstorm with a single rotating updraft
Fig. 11.23, p. 313