Severe Weather Forecasting

1
Severe Weather Forecasting

• What will the weather be like in the future?
• To begin we need to take observations
• Surface
• Upper Air
• Satellite
• Profiler
• Radar
• Once the data is collected, we need to analyze
  the data by creating maps and charts.

2
Severe Thunderstorm Requirements

• One of the first things we look for is
  conditional instability.
• We look at soundings to see if there is a
  potential for severe weather later in the day.
• In Oklahoma, the morning sounding is launched at
  6 AM CDT.

3
Sample Morning Sounding
Tparcel Tenvironment Large potential instability
Tparcel 4
19APR99OKC
No potential instability Tp CCL
Nocturnal Inversion
LCL
5
Sample Evening Sounding
Large instability with afternoon cooling.
Cap essentially gone
6
Stability Indices

• It would take a long time to look through many
  soundings to find a location for severe weather
  in the U.S.
• A simpler way would be to create a single number
  that would represent the potential instability
  for the entire sounding.
• These numbers are called stability indices.

7
Showalter Index

• A measure of the potential instability of the
  atmosphere in the 850 mb to 500 mb layer.
• Lift from the 850 mb layer to an 850 mb LCL.
• T500 is the measured air temperature at 500 mb.
• Tp is the temperature of a parcel lifted moist
  adiabatically from the 850 mb LCL.
• Showalter index requires a deep layer of moisture
  -- deep enough to reach 850 mb.

S T500 - Tp
8
Showalter Index -- OUN
-20
-10
0
S T500 - Tp S -19 - (-21) S 2
T500
Tp
500
10
600
700
20
LCL from 850 mb
800
Lift from 850 mb
900
1000
Note Below 850 mb the atmosphere is very dry!
9
Showalter Index

• What do these numbers mean?

S Values Possible Event 3 to
1 Rain Showers 1 to -2 Thunderstorms -3 to
-6 Severe Thunderstorms
10
Showalter Index

• A measure of the potential instability of the
  atmosphere in the 850 mb to 500 mb layer.
• Lift from the 850 mb layer to an 850 mb LCL.
• T500 is the measured air temperature (ºC) at 500
  mb.
• Tp is the temperature (ºC) of a parcel lifted
  moist adiabatically from the 850 mb LCL.
• Showalter index requires a deep layer of moisture
  -- deep enough to reach 850 mb.

S T500 - Tp
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1998 Example Showalter Index -- OUN
S T500 - Tp S -16 - (-25) S 9
T500
Tp
LCL from 850 mb
Lift from 850 mb
12
Zoom In
-20
-10
-30
S T500 - Tp S -16 - (-25) S 9
500
LCL from 850 mb
Lift from 850 mb
800
1000
13
1998 Example Showalter Index -- JAN
S T500 - Tp S -12 - (-9) S -3
Tp
LCL from 850 mb
T500
Lift from 850 mb
14
Zoom In
-20
-10
0
S T500 - Tp S -12 - (-9) S -3
500
10
LCL from 850 mb
Lift from 850 mb
800
1000
15
Showalter Index -- OUN
-20
-10
0
S T500 - Tp S -19 - (-21) S 2
T500
Tp
500
10
600
700
20
LCL from 850 mb
800
Lift from 850 mb
900
1000
Note Below 850 mb the atmosphere is very dry!
Monday
Dry Adiabat
Moist Adiabat
Mixing Ratio
16
Showalter Index -- OUN
-20
-10
0
S T500 - Tp S -12 - (-11.8) S -0.2
Tp
500
T500
10
600
700
LCL from 850 mb
20
800
Lift from 850 mb
900
1000
Wednesday
Dry Adiabat
Moist Adiabat
Mixing Ratio
17
Showalter Index

• What do these numbers mean?

S Values Possible Event 3 to
1 Rain Showers 1 to -2 Thunderstorms -3 to
-6 Severe Thunderstorms
18
Lifted Index

• A measure of the potential instability of the
  atmosphere between the surface and 500 mb.
• A fine tuned version of the Showalter index.
  Accounts for shallow layers of moisture.
• Lift a parcel with average mixing ratio and dry
  adiabat in the lowest 100 mb.
• T500 is the measured air temperature (ºC) at 500
  mb.
• Tp is the temperature (ºC) of an average parcel
  lifted moist adiabatically from the surface.

LI T500 - Tp
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Lifted Index -- OUN
LI T500 - Tp LI -16 - (-30) LI 14
Tp
T500
Average the lowest 100 mb
20
Lifted Index -- JAN
LI T500 - Tp LI -12 - (-8) LI -4
Tp
T500
Average the lowest 100 mb
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Lifted Index -- OUN
-20
-10
0
LI T500 - T'p LI -19 - (-24) LI 5
T500
Tp
500
10
600
700
20
800
900
1000
Lowest 100 mb
Monday
Dry Adiabat
Moist Adiabat
Mixing Ratio
22
Showalter Index -- OUN
-20
-10
0
LI T500 - Tp LI -12 - (-10.3) LI -1.7
Tp
500
T500
10
600
700
20
800
900
Lowest 100 mb
1000
Wednesday
Dry Adiabat
Moist Adiabat
Mixing Ratio
23
Lifted Index

• What do these numbers mean?

LI Values Possible Event LI 4 Rain Showers LI -4 Severe Weather
24
Total-Totals Index
TT T850 Td850 - 2(T500)

• T850 is the temperature (ºC) of the atmosphere at
  850 mb.
• Td850 is the dew point temperature (ºC) of the
  atmosphere at 850 mb.
• T500 is the temperature (ºC) of the atmosphere at
  500 mb.

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Total-Totals Index

• Total-Totals Index
• TT 44 Thunderstorms
• TT 48 Severe Thunderstorms
• TT 50 Tornadoes

TT T850 Td850 - 2(T500)
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Total-Totals Example
T500 -14.3ºC T850 13.4ºC Td850 13.1ºC
T500
TT 13.4 13.1 - 2(-14.3) TT 55
T850
Td850
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K Index
K T850 Td850 - (T700 - Td700) - T500

• T850 is the temperature (ºC) of the atmosphere at
  850 mb.
• Td850 is the dew point temperature (ºC) of the
  atmosphere at 850 mb.
• T700 is the temperature (ºC) of the atmosphere at
  700 mb.
• Td700 is the dew point temperature (ºC) of the
  atmosphere at 700 mb.
• T500 is the temperature (ºC) of the atmosphere at
  500 mb.

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K Index

• K Index
• K
• K 20 Scattered thunderstorms
• K 35 Numerous thunderstorms
• Not good for severe weather but is good for
  estimating how widespread the convection will be.

K T850 Td850 - (T700 - Td700) - T500
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K Index Example
T500 -14.3ºC T700 9.2ºC Td700 -11.8ºC T850
13.4ºC Td850 13.1ºC
T500
K 13.4 13.1 -(9.2 - (-11.8)) -(-14.3) K 20
Td700
T700
T850
Td850
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CAPE

• Convective Available Potential Energy
• A measure of the size of the positive area
  between the level of free convection (LFC) and
  the equilibrium level (EL).

CAPE Values Potential
Weather 1000 - 3000 J kg-1 Deep
Convection 5000 - 7000 J kg-1 Maximum Values
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CIN

• Convective Inhibition (CIN)
• A measure of the size of the negative area
  between the surface and the level of free
  convection (LFC).
• Often called negative CAPE.
• A measure of the strength of the cap.

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CAPE and CIN
EL
CAPE
LFC
33
CAPE and CIN
CIN