Severe Weather Prediction - posted 6/21/12

I. Moisture

• Precipitable Water - depth of the water if the entire sounding were to support heavy rain
• Theta-E - the temperature of an air parcel if all its water condensed as it was brought down "adiabatically" (without external heating) to 1000 mb
• Moisture Advection - transport of moisture by the wind, usually calculated using dew point, Theta-E, or - in rare cases - Wet-Bulb
• Lifted Condensation Level (LCL) - the height at which an air parcel will reach 100% humidity. Heights below 2 km will increase the likelihood of tornadoes and severe storms. More moisture, lower LCL.

II. Instability: a saturated air parcel that's warmer than its surroundings is unstable and will ascend.

• Level of Free Convection (LFC) - the height at which an air parcel becomes positively bouyant
• Convective Available Potential Energy (CAPE) - the area on a sounding between the level of free convection (LFC, where a saturated air parcel first meets the environmental temperature) and the equilibrium level (EL, where the ascending parcel meets the environmental temperature at higher altitudes). Large values indicate high instability.
• Lifted Index (LI) - the temperature of the environment at 500 mb minus the temperature of the ascending air parcel at 500 mb. Large negative values indicate high instability.
• Lapse Rate - the rate of decrease with height for an atmospheric variable, usually temperature. Negative lapse rates above the LCL may indicate instability.

III. Capping: a lid that prevents storms from forming too early will allow instability to increase. It is usually a layer of warm-air at the top of the boundary layer (the lowest mile of the troposphere).

• Convective Inhibition (CIN or CINS) - the amount of energy required to overcome the negatively buoyant energy the environment exerts on an air parcel. CIN usually is found between the top of the boundary layer/ LCL to the LFC. CIN between -30 and -60 J/kg is optimal for t'storms forming later in the day.
• Inversion - increase in temperature aloft. This is a common type of cap: warm air aloft decreases instability by warming the environment past the parcel's temperature; it is too dense (cool) and will fall as a result.

IV. Forcing Mechanism: something to push air parcels past the cap during the afternoon once stability decreases. Storms cannot form otherwise.

• Surface fronts (boundaries between air masses) create upward motion near the surface.
• Dry lines & cold fronts are the most common boundaries
• Dying thunderstorms produce outflow boundaries that produce localized convergence zones, leading to rising motions and new t'storms to form.
• Moisture Advection in the region between the LCL and LFC will help to bring the height difference to zero, bringing CIN to zero.
• Divergence above the LND is air spreading outwards or increasing speed. The mass must decrease. When air rushes away at high altitudes, air from the surface zooms upward - lift.

V. Wind Shear: a change in the direction and/ or speed of the wind with changing height. Mostly this can be an indicator of how organized the storms will be.

• Storm-Relative Helicity (SRH) - the extent to which helix-like motion occurs (For reference, DNA is in the shape of a double helix.). Horizontal velocity minus storm motion and horizontal relative vorticty (spin) are taken into account from the surface to a few kilometers (up to two miles). This measures the rotation potential in a storm.
• Shear Vector - subtract the wind velocity at one height from another, usually from the surface to six kilometers. This is better for speed shear (increasing or decreasing speed with changing height).

Soundings and Hodographs are great tools in visualizing severe weather scenarios. I'll cover combined indices like STP, Craven-Brooks, and EHI next time.

6/1/12 Outbreak: Only instability was missing, but forcing combined with moisture created the storms. Directional shear was a huge factor as well, leading to tornadoes in some parts of the Mid-Atlantic States.