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 SKEW-T: A LOOK AT SW

METEOROLOGIST JEFF HABY

1. What is SW?

SW (SWEAT= Severe Weather Threat Index) uses several variables to determine the likeliness of severe weather and tornadoes.

2. How is SW determined?

SWEAT= 12(850Td) + 20(TT - 49) + 2(V850) + (V500) + 125(sin(dd500 - dd850) + 0.2)

*If TT less than 49, then that term of the equation is set to zero
*If any term is negative then that term is set to zero
*Winds must be veering with height or that term is set to zero

850Td = 850 mb dewpoint temperature
TT = Total Totals Index
V850 = 850 mb wind speed
V500 = 500 mb wind speed
dd500 - dd850 = Directional veering of wind with height

The sounding below gives a SWEAT value of 148. Here is how that value was found:

850Td = 9 C
TT = 44
V850 = 15 knots
V500 = 10 knots
dd500 - dd850 = SSW (200 degrees) - SW (220 degrees) = sin(-20) = term set to zero since wind is not veering
Since TT is less than 49, that term is set to zero

SWEAT = 12(9) + 2(15) + 10 = 148

Although the sounding has good low level moisture and a deep layer of instability, it currently lacks significant speed and directional wind shear. This significantly reduces the tornado potential.

3. Operational significance of KI:

 SWEAT
 150-300 Slight severe 300-400 Severe possible 400+ Tornadic possible

The SWEAT index assess low level moisture, convective environment (via TT), and changes in wind speed and direction with height (low level and middle level jet, horizontal vorticity). When all these factors occur together, the severe weather threat and tornado threat is enhanced.

4. Pitfalls:

a. Always use index along with examining the actual sounding and forecast soundings. The sounding environment can change rapidly over time.