Storm Relative Helicity

1
Storm Relative Helicity

• SREH -- A measure of the potential for a
  thunderstorm updraft to rotate.
• Helicity is typically measured over a depth in
  the atmosphere
• 1 to 3 km
• 0 to 4 km
• A good helicity estimate depends on accurate
  winds and storm motion data.

2
Storm Relative Helicity
180
This area represents the 1-3 km helicity
3 km
4 km
2 km
5 km
7 km
6 km
1 km
Storm Motion
SFC
270
SREH Potential Tornado Strength 150 - 300 m2
s-2 Weak 300 - 500 m2 s-2 Strong gt 450 m2
s-2 Violent
3
Severe Weather Forecasting

• Idealized Classic conditions for severe weather.

• Low level moisture
• Instability -- Cold air aloft associated with the
  upper level trough
• Inversion (Cap) -- Warm air near 800 mb
• Strong Wind Shear
• Lifting Mechanism -- Cap Breaker

4
Forecasting Example

• Setting the stage -- Surface

Td 60oF
Moist
6 PM CST -- 2 April 1974
5
Forecasting Example

• Setting the stage -- 850 mb

LLJ
Moist
Dry
6 PM CST -- 2 April 1974
6
Forecasting Example

• Setting the stage -- 500 mb

Cold
6 PM CST -- 2 April 1974
7
Forecasting Example

• Setting the stage -- 300 mb

DIV
6 PM CST -- 2 April 1974
8
Forecasting Example

• Setting the stage -- Lifted Index

LI lt -4
6 PM CST -- 2 April 1974
9
Forecasting Example

• By the morning of 3 April, the storm had moved
  into the central Plains.
• The 850 mb LLJ was now tapping the moisture from
  the Gulf.
• The upper level trough was initiating convection
  over the northern Plains.
• A cap over the southeastern U.S. held back
  convection during the morning.

10
Forecasting Example

• 3 April -- 6 AM -- 850 mb

Moist
LLJ
11
Forecasting Example

• 3 April -- 6 AM -- 300 mb

DIV
DIV
12
Forecasting Example

• Radar Summary -- 525 AM -- 3 April 1974

13
Forecasting Example
JAN Jackson, MS 12Z 4 April 1974
Evening Sounding (No Cap)
Morning Sounding (Capped)
14
Forecasting Example
Severe Storm Setup
15
Forecasting Example
Severe Storm Setup -- Lifted Index
CVG
-4
600 AM 3 April 1974
-8
6 AM -- 3 April 1974
16
Forecasting Example

• By the afternoon, severe weather had broken out
  over a large portion of the U.S. from the Ohio
  River to New Orleans.

535 PM CST 3 April 1974
17
Forecasting Example

• The Cincinnati, Ohio (CVG) radar indicated
  numerous tornadic supercells.

18
Forecasting Example

• 148 Tornadoes occurred in a two day period 3-4
  April 1974.

19
Severe Storm Forecasting Basics

• Strong Instability
• Instability Enhancement
• Increase low level temperature
• Decrease upper level temperature
• Increase low level moisture
• Decrease upper level moisture
• Weak cap or inversion around 900 to 850 mb
• Cap inhibits convection until later in the day

20
Severe Storm Forecasting Basics

• Strong low level (surface to 800 mb) in excess of
  10 to 15 m s-1
• Strong shear (wind speed increases with height
  and wind direction changes with height
• Clockwise shear for supercells
• Strong upper level (300 to 200 mb) winds greater
  than 25 m s-1
• Jet streaks can enhance rising motion -- left
  front or right rear quadrants.

21
Severe Storm Forecasting Basics

• Forcing mechanism to provide lift to remove cap
• Boundaries
• Surface convergence
• Surface Heating
• Addition of moisture in the lower levels