Class #26: Friday October 30 Thunderstorms 1Class #26: Friday, October 30, 2009

2 Conditions for thunderstorm formation (continued) For the more severe thunderstorms, vertical shear of the horizontal wind – The westerly (west to east) part of the wind increasing as height increases – Clockwise turning of the direction from which the wind blows as height increases For example, southeasterlies at the surface, southerlies at 850mb, southwesterlies at 700mb, and westerlies at 500mb

Class #26: Friday, October 30, 20093

An ordinary or air mass thunderstorm Class #26: Friday, October 30, 20094

Life cycle of a thunderstorm cell/air mass thunderstorm Class #26: Friday, October 30, 20095

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7 Lifted Index Is a way to describe stability with one number Is not a perfect measure of stability Is useful for forecasting, but not the only criterion forecasters use Is a difference in temperature at 500mb T(environment) – T(air parcel) Negative values are unstable

Class #26: Friday, October 30, Lifted Index (continued) Where do the temperatures come from For the environment, use a sounding For the parcel – Start with surface conditions of temperature and humidity – Next, lift the parcel dry adiabatically (10°C/km) until it is saturated – Finally, raise the parcel moist (saturated) adiabatically to 500mb

Satellite-derived values of the lifted index Class #26: Friday, October 30, 20099

An environment that favors the formation of severe thunderstorms At the highest levels there is a jet stream with divergence aloft At 500mb there is a trough to the west At 700mb there is a warm dry air mass Between 700mb and 850mb there is convective or potential instability Just above the surface there is a low level jet of southerlies with warm moist air Overall, a clockwise turning wind in the warm sector Class #26: Friday, October 30,

Class #26: Friday, October 30, Drylines Are not true fronts because there is no temperature contrast across the front. Resemble fronts because there is a boundary between air masses, cT and mT. Resemble fronts because there is a wind shift, pressure trough, convergence, and often convective clouds along the dryline. Occur in West Texas in spring and early summer, and severe weather can occur.

Class #26: Friday, October 30,

Class #26: Friday, October 30, Drylines (continued) Have a strong contrast in humidity and wind direction across the front Can persist for several days. Can move westward at night (called the dew- point front) Move from west to east during the day Are a powerful source of convergence The moister air is lighter, and rises

Schematic view of a multicell thunderstorm Class #26: Friday, October 30,

The shelf cloud signifies the arrival of the gust front Class #26: Friday, October 30,

Composite radar imagery of a squall line Class #26: Friday, October 30,

Satellite image of the southern portion of the squall line Class #26: Friday, October 30,

Infrared image of a supercell thunderstorm and squall line Class #26: Friday, October 30,

Photograph of an overshooting thunderstorm cloud The updraft of the thunderstorm is so strong that its top pokes into the tropopause. The overshooting cloud updraft is decelerating and the updraft speed decreasing. Cloud top occurs when the updraft slows to a stop. Class #26: Friday, October 30,

Visible satellite image of the same storms Class #26: Friday, October 30,

Life cycle of the multicell thunderstorm A squall line would extend into the slide This storm is moving from right to left on the slide Class #26: Friday, October 30,