Download Chapter 11: Thunderstorms and Tornadoes

Chapter 15: Thunderstorms
and Tornadoes
 40,000 thunderstorms occur
everyday somewhere in the world.
 Thunderstorms produce
 Heavy rain
 High winds
 Lightning & Thunder


Hail
Tornadoes
 Thunderstorms occur as
cumulonimbus clouds.
Two kinds of thunderstorms
1. Air Mass
2. Severe
1. Air Mass Thunderstorm
 most common
 least destructive
 “self-extinguishing”
 fairly upright
 downdrafts cut off warm moist
air to updraft that initially created
thunderstorm
Characteristics
 Usually live less than one hour
 Form within air mass
 Form away from frontal
boundaries
Some definitions…
Cell – a single, individual thunderstorm
Updraft – a column of warm, moist,
rising air in a thunderstorm
Downdraft – a column of cold,
descending air in a thunderstorm
Air Mass Thunderstorm Life Cycle
3 stages
I. cumulus stage
II. mature stage
III. dissipating stage
Cumulus Stage
 Unstable, warm parcels rise
 Cu clouds form
 Cu 7-9 km deep
 Updraft speeds up to 20ms-1
(45 mph)
 Cu stage ends when
precipitation begins to fall.
I.
Mature Stage
 Begins when precipitation starts
to fall
 Falling precipitation drags on air
around it to initiate downdraft
 Updrafts and downdrafts, both
distinct, dominate cloud
 Lightning, thunder,
precipitation most intense
 Anvil forms, giving the
thunderstorm its characteristic
appearance.
Dissipating Stage
 An increasing amount of the
cloud ends up precipitating
 So…an increasing amount of
the cloud is represented by
downdrafts
 Dominant downdraft cuts off
inflow to updraft
 Bottom half precipitates out of
evaporates
 Top may persist a while
Severe Thunderstorm
A thunderstorm that produces
 Winds of at least 58 mph
(50 knots, 93 km/h)
 Hail of at least 3/4" (1.9 cm)
 Tornadoes
This is the official definition of a
severe thunderstorm…
***No accounting for rainfall***
Extreme flash flooding can eliminate
a town or city - but it's not "severe."
Any thunderstorm can become severe
Criteria may be met for only a few
minutes---hail, winds, even small
tornadoes (“gustinadoes”)
BUT…
Severe thunderstorms usually have
more organization that the average air
mass thunderstorm.
Updraft and downdraft typically work
together in a severe thunderstorm
Supercell Thunderstorm - a thunderstorm with a single, rotating updraft.
Distinguished from air mass thunderstorm by organized updraft
Diameters: 20-50 km (12-30 miles)
(Air mass thunderstorms
typically smaller)
Smaller than organized thunderstorm
systems (squall lines, mesoscale
convection systems)
Lifetime: 2 to 4 hours
(Air mass thunderstorms
last < 1 hour)
Organized Thunderstorm Systems
Far larger than single air mass or
supercell thunderstorm
Mesoscale Convective Systems (MCS) groups of individual thunderstorms
clustered together
Individual thunderstorms come from a
common origin (warm, moist, unstable air)
But consider when…
These favorable condition to form
thunderstorms often occurs over large
areas (10-1000 km)
In these regions, thunderstorms often lead
directly to the formation of others
Two primary MCS types:
1. Squall line
(line-type MCS)
2. Mesoscale Convective Complex
(circular MCS)
1. Squall Line - Large number of
individual thunderstorm cells
arranged in a band.
 500 km (300 mi) long band
(average)
 10 hour life span (average)
 usually parallel to and 300-500
km (180-300 mi) ahead of cold
front.
Gust front - a cold, dense wedge of
air that surges ahead of the squall
line.
 Formed by spreading out of
downdraft air
 Can initiate new or maintain
current updrafts
2. The Mesoscale Convective Complex
(MCC) - An MCS having certain
satellite-based size & shape criteria
Shape - not a line---criteria require
round or oval shape
Structure - more complex than squall
Line
Downburst - a strong downdraft
which induces an outburst of
damaging winds on or near the
ground
 may be straight or curved, but
always highly divergent
Macroburst - large downburst with
outburst winds extending in excess
of 4 km (>2.5 miles) in horizontal
dimension.
 Intense macrobursts can cause
widespread, tornado-like damage
 Damaging winds last 5 to 30
minutes
 Can be as high as 60 m/sec (134
mph).
Microburst - small downburst with its
outburst, damaging winds extending
only 4 km (≤ 2.5 miles) or less.
 intense microburst  winds as
high as 75 m/sec (168 mph)
From
UIUC
Microbursts pose a distinct danger to
aircraft (esp. on landing)
From UIUC
Tornadoes
rotating
Def - a rapidly
column of air with extreme
horizontal winds
o most often beneath a
Cb cloud
Average Characteristics
Diameter
Path Length
Lifespan
Wind Speeds
Forward Speed
100 yards
2.5 miles
A few minutes
40 – 280 mph
30 mph
Waterspouts
Similar to a tornado, but over a
water body
-
Usually in tropics, over warm
water body
-
Usually smaller than tornadoes
 Diameter - 17-330 feet
 Wind speeds ~ 90 mph
-
Develop from Towering Cu
-
Tropical waterspouts usually
develop up from water surface
Funnel Cloud
A tornado that has not yet reached
the surface
Tornadoes: When and Where
Everywhere in the world, all months,
all hours, BUT…
- More in the U.S. than anywhere
**Due largely to topography**
- > 1,000 tornadoes in U.S. annually
Florida and Oklahoma are the tornado
capitals of the U.S.
Florida  Hurricane induced
May  largest number of tornadoes
Late afternoon generally preferred
Missouri Tornadoes
 Late May – June
 Western MO more prone
 Average 7 per year
 La Nina year, positive phase of
Pacific Decadal Oscillation 
up to 12 per year
Tornado Formation
Gustinadoes – outflow driven;
bottom up
Supercells – well-developed from
wind shear changes;
top down
Remember wind shear???
55
15
5000 feet
Rotation from shear becomes tilted
into the vertical
Mesocyclone – large vortex up to
12 miles in diameter
 deep inside cloud
 several km above surface
 can precede tornado
development up to 30
minutes
Tornado Damage
“Equalizing pressure” by opening
windows is a
MYTH
Fujita Scale
- a damage scale used to rank
tornado intensity
-
most applicable to damage to
residential homes
“F-4 tornado”
 no such thing
“A tornado that does F-4 damage”
 that I’ll buy…
Severe Weather Safety
Watch – a statement from the
National Weather Service (NWS)
that says weather conditions are
right for the formation of severe
weather
-
covers large areas
-
issued by the Storm Prediction
Center
Q: What’s your job?
A: Keep an eye on the sky--keep a radio or TV on.
Severe Weather Safety
Warning – a statement from the
National Weather Service that
says severe weather has
developed or is imminent.
-
Usually covers one or several
counties
-
Issued by your local NWS
Warning and Forecast Office
(WFO)
***FIND SHELTER***
Severe Weather Safety
- Don’t open windows
- Go to an internal room on
the lowest floor
 Basement
 Bathroom – pipes form a
“cage” around you
- In a car, don’t try to outrun
the storm
 A ditch is probably ideal
 Underpasses are NOT
necessarily safe