Two critical ingredients for significant tornadoes include Helicity and CAPE. Helicity increases as the wind direction changes with height and the wind speed increasing with height. When it comes to tornado forecasting, it is the lower troposphere that is examined the most critically such as between the surface and 3 kilometers. A significant wind direction change with height, veering of wind with height, strong wind speeds and wind speeds increasing with height between the surface and 3 km in the lower troposphere lead to high values of Storm Relative Helicity in the lower troposphere. CAPE gives an indication of the updraft strength potential. The region of CAPE that is most important to tornado forecasting is the amount of CAPE between the surface and 3 km above the surface (lower troposphere). This is termed the 0 to 3 km CAPE. Larger CAPE values contribute to stronger updraft speeds. Significant tornadoes can be more likely when there is a combination of both significant Helicity and significant CAPE. The delicate balance between these two factors helps determine if significant tornadoes occur. An index has been created that helps show the balance between total storm relative humidity and total CAPE. This index is called the Energy Helicity Index (EHI). A general guide to interpret EHI values is given below keeping in mind that this is just a general guidance based on accurate data and an accurate analysis of the severe weather situation. As with any index value, there are exceptions and pitfalls: EHI > 1: Supercell potential EHI from 1 to 5: strong tornadoes possible EHI > 5: Violent tornadoes possible The diagram below shows a visual of how Helicity and CAPE can be examined on a Skew-T sounding. Note that both the Helicity and CAPE are only being examining in the lower troposphere since this is the location that is most critical for tornado development. Note on this diagram that there is a significant amount of CAPE between the surface and 3 km. This helps ensure the strong updraft strength will begin closer to the surface where there is also better access to heat and moisture. The strong winds and the turning of the wind with height from the surface to 3 km contribute to strong Helicity values. Of significant importance is the overlap between the turning of the strong wind speed with height along with the CAPE in the lower troposphere. This will contribute to a twisting motion imparted upon the strong updraft. This increases the likelihood of significant tornadoes in the updraft region of a supercell.  |