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THERMODYNAMIC IMAGES

METEOROLOGIST JEFF HABY

There are 4 charts which will be discussed in this section which are extremely important to severe weather forecasting. The charts include: CAPE, Helicity, Energy Helicity Index, and the 850-500 relative humidity/ Lifted Index plot. An example of each chart is shown along with operational interpretation. One location these plots are available are on UNISYS weather at:

http://weather.unisys.com/nam/index.php

CLICK THE TITLE TO SEE THE CURRENT ONLINE IMAGE FOR EACH


Online images: CAPE

*Largest CAPE will occur in the warm sector of a mid-latitude cyclone
*Interpretation of values:
*1 to 1,500 Positive CAPE
*1,500 to 2,500 Large
*2,500+ Extreme
*High values of CAPE will result in high values of upward vertical velocity in the updraft region of a thunderstorm
*CAPE is increased by low level warm air advection, daytime heating, low level moisture advection (high low level dewpoint), upper level cold air advection (cooling temperature in mid-levels)
*CAPE values tend to be highest in the warm season (especially late Spring)


Online images: HELICITY

*Low level inflow, speed shear and directional shear maximize helicity values
*Helicity values tend to be highest ahead of cold fronts and near warm fronts within the warm sector of a mid-latitude cyclone
*Interpretation of values:
*150-300 Possible Supercell
*300 to 400 Supercell Severe Favorable
*400+ Tornadic Thunderstorms Favorable
*The Low Level Jet can amplify low level inflow. Higher winds lead to a more rapid rotation of air if shear exists
*Differential advection and surface friction cause speed and directional shear
*Helicity is the amount of streamwise vorticity available for ingestion into thunderstorms. Streamwise vorticity is the amount of parallel inflow to horizontal vorticity


Online images: ENERGY HELICITY INDEX

*Combines CAPE and Helicity into a single index
*Formula is (Helicity CAPE)/160,000
*EHI increases as CAPE and/or Helicity increases
*Interpretation of values:
*Greater than 1: Supercells likely
*1 to 5: Possibility of F2, F3 tornadoes
*5+: Possibility of F4, F5 tornadoes
*Tornadic development often initiates in region of EHI max (especially if EHI max is 5 or greater). In extreme situations, EHI max will be near 10.


Online images: 850-500 RELATIVE HUMIDITY/ LIFTED INDEX PLOT

*Thunderstorms need moisture, especially in the low levels. RH gives an indication to how much moisture is available to be lifted. Only look at 850-500 relative humidity values in the warm sector of a mid-latitude cyclone because this is where severe thunderstorms will develop. Actually, the PBL dewpoints are even more important to analyze since it is low level moisture and warm PBL air that fuels supercells.
*Lifted Index indicates how unstable the atmosphere is as a whole
*RH greater than 80% indicates troposphere is near saturation
*LI interpretation:
*0 to -4: Positive Thermodynamic Buoyancy Potential
*-4 to -7: Large Thermodynamic Buoyancy Potential
*-8 or less: Extreme Thermodynamic Buoyancy Potential

Conclusion: Best case scenario for supercells thunderstorms is high CAPE, high helicity, plenty of low level moisture, lifting mechanisms and a capping inversion that will break in the afternoon.