Here are some conditions favorable to severe weather and an explanation of each DRY AIR IN THE MID-LEVELS OF THE ATMOSPHERE: 1) Produces convective instability 2) Produces a large negative buoyancy in association with thunderstorm downdrafts. The dry air entrains into the moist air of the cloud causing intense evaporation, negative buoyancy, and a strong downdraft. 3) Evaporative cooling reduces the amount of melting hail experiences as it falls. HIGH INSTABILITY: High CAPE, unstable LI, unstable KI and TT; Strength of updraft is determined by amount of positive buoyancy in the atmosphere. Large instability produces large updrafts. A main determinate of hail size is the strength of the updraft. High CAPE also causes the stretching necessary to produce tornadogenesis (wind shear must also be present). PBL WIND SHEAR: Speed shear (wind speed increasing with height in the PBL); directional shear (wind veering, turning clockwise more than 45 degrees in the PBL); Average PBL wind greater than 20 knots (It has been found that for tornadoes to develop the PBL inflow needs to be greater than 20 knots, the higher the better) STRONG UPPER LEVEL WINDS: Causes tilting of storms, displaces updraft from downdraft; Creates a vacuum affect at the top of storms; helps sustains the intensity and verticality of the updraft. STRONG UPPER LEVEL TROUGH: Generates strong positive vorticity advection; creates differential temperature advection (i.e. upper level and low level fronts) HIGH DEWPOINTS IN PBL: Must have moisture in low levels or storm development will be very limited. Low level moisture increases latent instability. DYNAMIC TRIGGER MECHANISMS: Without a trigger mechanism, such as when a strong cap is present, storms may not form. Here are examples of dynamic trigger mechanisms: 1. dryline 2. cold or warm front 3. outflow boundary 4. jet streak 5. strong upper level vorticity 6. orographic lifting 7. low level warm air advection (strong gradient of warmer temperature moving toward a fixed point) 8. Low level jet 9. Gravity waves 10. Meso-lows |