Chapter
18
Thunderstorms
ATOC
1050;
Spring
2011;
MWF
2:00-‐2:50pm
Airmass
Thunderstorms
• Where
do
airmass
thunderstorms
typically
form?
• What
triggers
an
airmass
thunderstorm?
• What
are
the
winds
like
in
the
environment
where
airmass
thunderstorms
form?
Anvil
–
the
flat
top
of
a
thunderstorm
that
forms
as
the
thunderstorm
updraJ
hits
the
tropopause
and
spreads
out
horizontally
Strong
shear
No
shear
www.spc.noaa.gov
Why
do
we
care
about
MCSs?
What
area
can
MCSs
cover?
What
is
the
typical
life
cycle
of
an
MCSs?
Mesoscale
ConvecNve
Systems
(MCSs)
4pm-‐11pm
LT
4
July
2003
Squall
line
–
a
long
line
of
thunderstorms
in
which
adjacent
thunderstorm
cells
are
so
close
together
that
the
heavy
precipitaNon
falls
in
a
long
conNnuous
line
Bow
echo
–
porNon
of
a
squall
line
that
bows
outward
from
MCS
Trailing
straNform
region
–
area
of
less
intense
precipitaNon
on
rear
side
of
a
squall
line
Gust
front
–
leading
edge
of
evaporaNvely
cooled
air
Rear
inflow
jet
Gust
front
–
leading
edge
of
evaporaNvely
cooled
air
Derecho
–
a
widespread
thunderstorm
generated
severe
windstorm
Key
features
of
an
MCS
squall
line
Frontal
Squall
Lines
What
are
the
key
features
of
a
frontal
squall
line?
OvershooNng
top
–
the
porNon
of
the
updraJ
that
penetrates
the
tropopause
Supercell
Thunderstorms
• Supercell
thunderstorm
–
a
rotaNng
thunderstorm
• Supercell
thunderstorms
are
responsible
for
creaNng
the
majority
of
the
most
dangerous
severe
thunderstorm
weather
and
strongest
tornadoes
that
form
in
the
United
States.
CAPE
–
convecNve
available
potenNal
energy
(thunderstorms
form
when
>1500
joules/kg)
Low
level
jet
–
strong
southerly
low
level
winds
that
extend
from
the
surface
to
a
maximum
alNtude
of
3
km
hcp://weather.uwyo.edu/upperair/sounding.html
Supercells
typically
first
form
where
airmass
boundaries
intersect.
14
April
16
April
14
April
15
April
15
April
2300
UTC
1800
UTC
1800
UTC
1000
UTC
1400
UTC
Key
features
of
a
supercell
thunderstorm:
Mesocyclone
(5-‐10
km),
Tilted
updraJ,
OvershooNng
top,
Anvil,
Rain
free
base,
Bounded
weak
echo
region,
Wall
cloud,
Mammatus,
Virga,
Rear
flanking
lin
Wall
cloud
–
a
lowered
cloud
base
in
the
vicinity
of
the
updraJ
that
is
oJen
observed
to
be
rotaNng
Bounded
weak
echo
region
–
an
area
of
low
reflecNvity
on
a
radar
image
of
a
supercell
thunderstorm
UD
–
updraJ
FFD
–
forward
flank
downdraJ
RFD
–
rear
flank
downdraJ
Gust
fronts
mark
the
leading
edge
of
FFD
and
RFD
air
at
the
surface.
New
thunderstorm
cells
form
along
the
RFD
gust
front.
Which
of
the
following
is
least
responsible
for
how
thunderstorms
organize?
a) verNcal
wind
shear
b) amount
of
moisture
present
c) degree
of
instability
in
the
atmosphere
d) liJing
mechanism
that
triggers
the
storms
Which
of
the
following
is
required
for
severe
thunderstorm
development?
a) verNcal
wind
shear
b) a
source
of
moisture
c) a
mechanism
to
trigger
an
updraJ
d) a
condiNonally
unstable
atmosphere
e) All
of
the
above
are
required
Where
and
when
(Nme
of
day)
would
airmass
thunderstorms
be
most
likely
to
form?
a) just
ahead
of
a
cold
front;
8:00
AM
b) far
from
any
frontal
boundaries;
8:00
AM
c) far
from
any
frontal
boundaries;
4:00
PM
d) just
ahead
of
a
cold
front;
4:00
PM
e) along
a
dry
line;
10:00
PM
What
is
the
primary
mechanism
that
triggers
the
updraJ
of
an
airmass
thunderstorm?
a) fronts
b) jetstream
curvature
c) solar
heaNng
of
the
surface
d) jetstreak
divergence
