Severe Weather Potential May 27th 2013

Low pressure is going to be over eastern Colorado and extending southward across the TX Panhandle will be the dryline. A line of storms could fire along this dry line. The stationary front further to the north will be the focus for severe weather as well..

Lets look at the severe parameters first...starting with the temperatures which will be warm to hot from north to south with 70's to 90's

There will be no lack of convective available potential energy with CAPE as high as 4000.

There will also be ample lift in the atmosphere.. with the lift at around negative 10.

Wind Shear will be sufficient with 40 knots or slightly greater..

A New index yet we have not used this severe season so far is SWEAT .. To explain what SWEAT is..

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:

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. 

SWEAT INDEX CHART

150-300 =Slight Severe

300-400 = Severe Possible

+400 =Tornadic Possible 

Now according to the SWEAT index chart anything over 400 means tornadic is possible..

So you can see that places like Western OK ..Most of KS and then isolated locations further NE into Dakota region and Western Nebraska have a chance at tornado potential.

This pretty much goes along with the EHI index as well..

Also the Helicity is in the same general area...

Now what may be the saving factor in these places not seeing the tornado's despite the parameters is that the GOM return flow is basically shut off but as you can see the parameters are in place. 

One final piece of information is the dew points which are in the 60s and 70s..

So with all that explained above we have put together this outlook ..

The main threats should be winds and hail but if we can get some return flow from the GOM going then the places mentioned above would have a shot at tornados. At this point the locations furthest north has the greatest shot for the tornado potential as there will be more moisture available...