Soundings and Adiabatic Diagrams for Severe Weather Prediction and Analysis Continued - PowerPoint PPT Presentation

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Soundings and Adiabatic Diagrams for Severe Weather Prediction and Analysis Continued

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Title: Soundings and Adiabatic Diagrams for Severe Weather Prediction and Analysis Continued


1
Soundings and Adiabatic Diagrams for Severe
Weather Prediction and Analysis Continued
2
Shear vs. CAPE
  • Need a balance between Shear and CAPE for
    supercell development
  • Without shear single, ordinary, airmass
    thunderstorm which lasts 20 minutes
  • If shear is too strong multicellular t-storms

(gust front moves too fast)
3
Shear Classification
  • Two Main types Directional and Speed
  • Bulk Shear The Boundary Layer through 6 km (or
    higher) above ground level shear vector denotes
    the change in wind throughout this height.
  • Usually given in units of knots
  • Thunderstorms tend to become more organized and
    persistent as vertical shear increases.
    Supercells are commonly associated with vertical
    shear values of 35-40 knots and greater through
    this depth
  • Doesnt take into account elevated parcels
  • Effective Shear (kts)
  • Bulk Richardson Number shear (m2/s2)

4
CAPE and Shear
5
Shear Just Right
  • 2-D equilibrium squall line develops
  • 3-D equilibrium right moving and left
    moving supercells

A
B
A
B
L
Left Mover
L
Right Mover
6
Bulk Richardson Number
  • BRN CAPE
  • ½ (Uz2)
  • Where Uz the vertical wind shear
  • (averaged over 3-6km layer)
  • In general 15-40 favors supercell development
  • gt40 favors multicellular type storms
  • Explains the balance between wind shear and
    convective energy

7
Bulk Richardson Number (BRN)
BRN CAPE 1/2Uz2 (where Uz is a
measure of the vertical wind shear)
8
Hodographs
V
South
  • Draw wind vectors in direction they are going
  • This is opposite of how the wind barbs are drawn

U
East
West
Wind speed
North
9
Example
10
Straight Line Shear
500
700
  • Storm Splitting
  • R and L storm cells move with mean wind but drift
    outward

850
900
1000
11
Curved Hodograph
  • Emphasizes one of the supercells
  • Veering (clockwise curve)
  • right moving supercells
  • warm air advection in northern hemisphere
  • Backing (counter clockwise curve)
  • left moving supercells
  • warm air advection in southern hemisphere

700
300
500
850
900
1000
12
Straight Line Hodograph
Curved hodograph
13
Helicity
  • Can be thought of as a measure of the corkscrew
    nature of the winds.

H velocity dotted with vorticity V ?
u (dyw - dzv) - v (dxw - dzu) w (dxv - dyu)
  • Higher helicity values relate to a curved
    hodograph.
  • large positive values--gt emphasize right cell
  • large negative values--gt emphasize left cells
  • Values near zero relate to a straight line
    hodograph.

14
CAPE and Helicity
  • Plainfield, IL tornado
  • CAPE7000
  • Helicity165
  • Energy Helicity

15
Tornado OutbreakMissouri, Illinois, Indiana
  • 3/08/09

16
Synoptic Setup300mb
17
500mb
18
700mb
19
850mb
20
925mb
21
Surface
22
SPC really jumps on the bandwagonYee Haw!!!
23
SPC really jumps on the bandwagonYee Haw!!!
24
Springfield, Missouri
25
Lincoln, Illinois
26
1600z SPC Mesocale Discussion
27
Radar Loopshttp//vortex.plymouth.edu/nids.html
28
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29
Radar Loopshttp//vortex.plymouth.edu/nids.html
30
(No Transcript)
31
(No Transcript)
32
What to take away from this event?
  • Dynamics/Shear dominate heat energy!
  • Strong mid/upper level shortwave trough
  • Strong dynamical forcing/frontal forcing
  • Strengthening surface low
  • Cold temperatures lt70F
  • Very small CAPE values lt1000J/kg
  • Bulk Shear 80 kts !!!
  • Helicity 500 m2/s2 !!!
  • Clockwise Hodographs favoring right moving cells
  • Perfect situation for low topped discrete
    supercells capable of producing tornadoes near
    triple point.

33
Stability Indices
34
K Index
  • This index uses the values for temperature (t)
    and dew point temperature (td), both in oC at
    several standard levels.
  • K t850 - t 500 td850 - t700 td700

K value T-Storm Probability
lt15 0
15-20 lt20
21-25 20-40
26-30 40-60
31-35 60-80
36-40 80-90
gt40 gt90
35
Vertical Totals
  • VT T850 - T500
  • A value of 26 or greater is usually indicative of
    thunderstorm potential.

36
Cross Totals
  • CT T d850 - T500

CT T-Storm Potential
18-19 Isolated to few moderate
20-21 scattered moderate, a few heavy
22-23 scattered moderate, a few heavy and isolated severe
24-25 scattered heavy, a few severe isolated tornados
26-29 scattered to numerous heavy, few to scattered severe, a few tornados
gt29 numerous heavy, scattered showers, scattered tornadoes
37
Total Totals (TT)
  • TT VT CT
  • T850 T d850 - 2 T500

TT T-Storm Potential
44-45 Isolated to few moderate
46-47 scattered moderate, a few heavy
48-49 scattered moderate, a few heavy and isolated severe
50-51 scattered heavy, a few severe isolated tornados
52-55 scattered to numerous heavy, few to scattered severe, a few tornados
gt55 numerous heavy, scattered showers, scattered tornadoes
38
SWEAT (severe weather threat) Index
  • SWI 12D 20(T - 49) 2f8 f5 125(S 0.2)
  • where D850mb dew point temperature (oC)
  • (if Dlt0 then set D 0)
  • T total totals (if T lt 49 then set entire term
    0)
  • f8speed of 850mb winds (knots)
  • f5 speed of 500mb winds (knots)
  • S sin (500mb-850mb wind direction)
  • And set the term 125(S0.2) 0 when any of the
    following are not true
  • 850mb wind direction is between 130-250
  • 500mb wind direction is between 210-310
  • 500mb wind direction minus 850mb wind direction
    is positive
  • 850mb and 500mb wind speeds gt 15knots

39
SWEAT (severe weather threat) Index
  • SWI 12D 20(T - 49) 2f8 f5 125(S 0.2)

lt300 Non-severe thunderstorms
300-400 Severe thunderstorms possible
gt400 Severe thunderstorms, including possible tornados
40
Lifted Index (LI)
  • Compares the parcel with the environment at
    500mb.
  • LI (Tenv-Tparcel)500

Lifted Index Thunderstorm Potential
gt2 No convective activity
0 to 2 Showers probable, isolated thunderstorms possible
-2 to 0 Thunderstorms probable
-4 to 2 Severe thunderstorms possible
lt -4 Severe thunderstorms probable, tornados possible
41
  • Best Lifted Index
  • Uses the highest value of qe or qw in the lower
    troposphere.
  • Use the highest mixing ratio value in combination
    with the warmest temperature.
  • SELS Lifted Index
  • Use the mean mixing ratio and mean q of the
    lowest 100mb
  • If using a 12z sounding add 2o
  • Start parcel at 50mb above the surface

42
Showalter Index (SI)
  • Compares a parcel starting at 850mb with the
    environment at 500mb.
  • SI (Tenv-Tparcel)500

SI Thunderstorm Possibility
gt 3 No convective activity
1 to 3 Showers probable, isolated thunderstorms possible
-2 to 1 Thunderstorms probable
-6 to 2 Severe thunderstorms possible
lt -6 Severe thunderstorms probable, tornados possible
43
Supercell Index
  • Weights various parameters which are indicative
    of possible supercell development

44
Important Points to Remember
  • Severe weather is more dependent on dynamical
    forcing than instability!
  • No one parameter tells the full tale!
  • 12z soundings usually predict afternoon
    convection better than 00z soundings predict
    evening convection.

45
Links
  • http//www.geocities.com/weatherguyry/swx2.html
  • http//avc.comm.nsdlib.org/cgi-bin/wiki.pl?Severe_
    Weather_Indices
  • http//www.theweatherprediction.com/severe/indices
    /
  • http//www.theweatherprediction.com/habyhints/315/
  • http//www.spc.noaa.gov/exper/mesoanalysis/
  • http//mocha.meteor.wisc.edu/table.12z.html
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