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Skew-T Analysis and Stability
Indices to Diagnose Severe
Thunderstorm Potential
Mteor 417 – Iowa State University
– Week 6
Bill Gallus
Basic features on a skew-T chart
Moist
adiabat
isotherm
isobar
Mixing
ratio line
Dry adiabat
Parameters that can be determined
on a skew-T chart
• Mixing ratio (w)– read from dew point
curve
• Saturation mixing ratio (ws) – read from
Temp curve
• Rel. Humidity = w/ws
More parameters
• Vapor pressure (e) – go from dew point up an
isotherm to 622mb and read off the mixing ratio
(but treat it as mb instead of g/kg)
• Saturation vapor pressure (es)– same as above
but start at temperature instead of dew point
• Wet Bulb Temperature (Tw)– lift air to saturation
(take temperature up dry adiabat and dew point
up mixing ratio line until they meet). Then go
down a moist adiabat to the starting level
• Wet Bulb Potential Temperature (θw) – same as
Wet Bulb Temperature but keep descending
moist adiabat to 1000 mb
More parameters
• Potential Temperature (θ) – go down dry
adiabat from temperature to 1000 mb
• Equivalent Temperature (TE) – lift air to
saturation and keep lifting to upper
troposphere where dry adiabats and moist
adiabats become parallel. Then descend
a dry adiabat to the starting level.
• Equivalent Potential Temperature (θE) –
same as above but descend to 1000 mb.
Meaning of some parameters
• Wet bulb temperature is the temperature air
would be cooled to if if water was evaporated
into it. Can be useful for forecasting rain/snow
changeover if air is dry when precipitation starts
as rain. Can also give a rough idea of
temperature within the cold pools of
thunderstorms
• Equivalent Potential temperature – gives an idea
of total energy in the atmosphere (sensible heat
plus latent heat)
Conserved quantities
• For unsaturated lift, potential temperature and
mixing ratio are conserved (which is why you
move along dry adiabats and mixing ratios when
lifting/sinking parcels on a skew-T chart)
• For satured lift, θE or θW are conserved so parcels
follow a moist adiabat (line of constant θE or θW ),
and thus dew point drops faster than for
unsaturated lift, and temperature cools more
slowly due to latent heat release
Other parameters to compute
• Tc – Convective temperature – temperature air
has to be heated to for it to rise on its own and
form deep convection
• CCL – Convective condensation level – level at
which air becomes positively buoyant when Tc is
reached
• LCL – Lifted Condensation Level – level where
air first becomes saturated after lifting
• LFC – Level of Free Convection – level where
air becomes positively buoyant after being lifted
past LCL
More parameters
•
•
•
•
EL – Equilibrium Level – level where parcel no
longer is positively buoyant and acceleration
would cease
MCL – Mixing Condensation Level – level
where saturation occurs due to mixing of layer
near ground
CAPE – Convective Available Potential Energy
– positive area in region where parcel is
warmer than environment
CIN – Convective Inhibition – negative energy
where parcel is cooler than environment
More…
• Cap – informal term for warm layer of air,
usually between 700 and 850 mb, which
creates CIN and prevents convection from
easily forming
• Overshooting top – go beyond EL to the
point where it appears you’ve created
enough negative area to balance the
positive area in the CAPE region
Overshooting top
Negative area
EL
CAPE
CIN
LFC
LCL
Stability changes
• Diabatic (radiation, evaporation,
condensation)
• Advection of different lapse rate
• Differential advection due to wind shear
• Vertical motion (“potential instability” can
exist if currently stable lapse rates would
become unstable due to lift – this usually
happens if lower part of layer is moist and
upper part is dry)
Lift this layer
100 mb and see
how lapse rate
changes
Stability Indices
• LI (Lifted Index) = T(env@500mb) –
•
•
•
•
•
T(parcel@500 mb).
Lift an air parcel from the surface (requires use
of skew-T)
LI < 0 significant cu convection possible
-4 < LI < 0 showers
-6 < LI < -4 thunderstorms
LI < -6 severe thunderstorms (SPC has -6 to -9
is very unstable; < -9 extremely unstable)
LI here is
about -4 since
parcel temp at
500 is -12 and
environ. Temp
is -16.
SI (Showalter Index)
• Same as LI but lift the parcel from 850 mb
• Index is mostly used in Midwest but not so much
elsewhere. Why? (elevated convection at night
dominates the Midwest – so lifting from 850
makes more sense than from surface)
• SI 0 – 3 Thunderstorm possible but need trigger
• SI -3 to 0 unstable, T.storm probable
• SI -4 to -6 very unstable, svr.storm potential
• SI < -6 extremely unstable, svr.storms likely
K Index
• K = (T850 – T500) + Td850 – DD700
• Because DD (dew point depression is
subtracted out at 700 mb, and dry air there
is helpful for severe weather (thus big DD),
this index works best for general
thunderstorm prediction and NOT severe
storm prediction
K guidelines
K value
< 15
Thunderstorm
probability
0%
15-20
<20%
21-25
20-40%
26-30
40-60%
31-35
60-80%
36-40
80-90%
>40
Near 100%
TT (Total Totals)
• TT = T850
+Td850 2*(T500)
• Works for
severe storm
prediction
East
44
Central/ Wx
West
Iso. Storms (winds < 35
48
46
52
Scatt. Storms (winds
35-50 knts, hail .5-1 in.)
48
55
Scatt. MDT, iso. SVR
50
58
Scatt. MDT, few SVR,
iso Tornadoes
52
61
Scatt-numerous MDT,
Scatt. SVR, few
Tornadoes
56
64
Numerous MDT, Scatt.
SVR, Scatt. Tornadoes
knts, hail < .5 inch)
SWEAT (Severe WEAther Threat)
• SWI = 12*Td850 +20*(TT-49) +2*fff850
+fff500 +125*(S + 0.2)
• fff is wind speed in knots
• S = sin (dd500 – dd850) where dd is
direction in degrees, and S= -0.2 if
dd850 < 130 or > 250
dd500 < 210 or > 310
dd500 - dd850 < 0
Both 850 and 500 speeds < 15 knots
SWI Interpretation
• This is a good index for helping with
rotating storm/tornado forecasting since it
includes wind and wind shear information
• SWI > 300: high severe storm potential
• SWI > 400: high tornado potential
HWBZ (Height of the Wet Bulb
Zero)
• This is a standard procedure for
forecasting large hail probability
• HWBZ between 2.1 – 2.7 km is ideal
• HWBZ between 1.5 – 3.4 km is possible
range for large hail
• HWBZ > 3.4 km can still yield large hail IF
very large CAPE exists
HWBZ
calculation is
an interative
procedure. Try
a point like 650
mb and
compute Tw
there. It looks
like -8C. So,
obviously
HWBZ is lower
down than that.
Try new point,
like 700mb.
700mb Tw
still is colder
than 0
(about -6 C),
so we need
to try further
down –
maybe 740
mb?
Tw at 740
mb looks
like exactly
0C, so this
is HWBZ
level.
Need to
convert to
a height
value to
apply the
rule.
Usually,
740 mb will
be around
2.6 km, so
this is
probably a
good large
hail
sounding
BRN (Bulk Richardson Number)
• BRN is used to predict storm type, and in
particular supercell potential
• BRN = CAPE/(.5*∆U2) where
∆U = Ave U in 0-6km layer – Ave U in 0-500 m
layer ( U here is full wind, so components are
needed for calculation)
BRN < 10 – too much shear and storms are ripped
apart
BRN 15-35 – Severe weather likely with supercells
favored
BRN > 50 – Multicells likely (less tornado potential)
EHI (Energy-Helicity Index)
• EHI = CAPE * SRH /166000
where SRH is storm relative helicity
EHI > 2.5 supercells with tornado potential
Sounding from around time of tornadic thunderstorms
Sounding from near tornadic storms that hit Waterloo area