ThunderstormsConvective Storms, form with rising air.Warm, moist air rises in a conditionally stable environment.Catalyst may be unequal heating at surface, the effect of terrain, or lifting of air along shallow boundaries of converging surface winds.Warm air rising along a frontal zone; diverging upper-level winds and converging surface winds.Ordinary Cell Thunderstorms, scattered thunderstorms that often form in summer and develop away from significant weather fronts.Supercell Thunderstorms, much larger than the ordinary cell thunderstorm. Consist of a single rotating updraft and are capable of producing much more severe weather.Severe Thunderstorm is defined of having large hail (3/4 inch), and/or surface wind gusts of 50 knots, or produces a tornado.Ordinary Cell ThunderstormsTend to form in regions of limited wind shear (where the wind speed and direction do not abruptly change).Many appear to form as rising parcels of air.Tend to form along shallow zones where surface winds converge. Topographic irregularities, sea-breeze fronts.Cumulus Stage, as a parcel of warm, humid air rises, it cools and condenses into a single cumulus cloud. Grows taller through a successively higher condensation height. Energy comes from the transformation of water vapor into a liquid or solid, releasing latent heat. As the storm grows, the particles grow larger until they are too heavy for the updraft to support, and fall.Entrainment, the mixing of environmental air into a preexisting air current or cloud so that the environmental air becomes part of the current or cloud.This entrainment of drier air causes the existing air to chill (and become heavier) causing a downdraft. This downdraft is only enhanced by downward falling precipitation.Mature Stage, most intense stage. The anvil is formed. Updrafts and downdrafts reach their greatest strength. Overshooting may occur if updrafts intrude above the cloud top. Thunder and lightning may be present.Gust Front, caused by the cold downdraft reaching the surface and spreading out horizontally. Forces warm, humid air up into the storm, enhancing the updraft.Dissipating Stage, occurs 15 to 30 after the storm enters its maturity. Occurs when the updrafts weaken as the gust front moves away from the storm and no longer enhances the updrafts. Downdrafts dominate, and cut off the storm’s fuel supply (updrafts).The drop in temperature during a storm is caused by the downdrafts arrival at the surface. After the storm has ended, the air temperature usually rises due to the evaporation of all the moisture.Severe Thunderstorms and The SupercellLikelihood that a thunderstorm will become severe increases with the length of its survival.Multicell Storm, a storm with a cluster of cells at various stages of their life cycles.Severe updrafts that are not so easily suppressed by overwhelming downdrafts are the cause of severe thunderstorms. Violent updrafts keep hailstones and particles suspended long periods of time, allowing their size to increase exponentially.Strong winds aloft may change direction allowing the storm to move so that the downdraft never undercuts the updraft. This wind shear may cause a horizontal spin to form.Supercell storm, has the ability to last several hours. A thunderstorm that consists of one violently rotating updraft.During the morning, stable air caps humid air at the surface, and only small cumulus clouds are able to form. However, as the day progresses, the surface warms and rising blobs of air are able to break through the stable layer. This venting of humid air is the catalyst for supercells.Squall Lines and Mesoscale Convective ComplexesSquall Line, a line of various thunderstorms comprising a multicell thunderstorm. Located right along a cold front or in the warm air directly ahead of it. Out ahead of a coldfront, the rising motion of the wave may be the trigger that initiates the development of cumulus clouds and a pre-frontal squall line.Mesoscale Convective Complex, a circular cluster of thunderstorms. A phenomenon driven by convection. Responsible for much of the growing season’s rainfall and tend to form during the summer where upper-level winds are weak.Dryline ThunderstormsDryline, a narrow zone where there is a sharp horizontal change in moisture.Intense thunderstorms often form along a dryine.Gust Fronts, Microbursts, and DerechoesGust Front, the leading edge of cold air originating inside a thunderstorm. Characterized by a sharp drop in temperature, the rising of surface pressure, and strong wind. This gust front may produce a shallow area of high pressure called a mesohigh.Haboob, Gust fronts can pick up loose dust and soil and lift them into a cloud.Shelf Cloud, may form when warm, moist air rises out along the forward edge of the gust front. Attached to the base of the storm.Roll Clouds, ominous clouds that form just behind the gust front. Appear to spin slowly about a horizontal axis.Outflow Boundary, the leading edge of a gust front may force the warm, moist air upward eventually forming a huge gust front.Downburst, a localized downdraft.Microburst, a smaller localized downdraft. Winds extend only 4 km or less. Responsible for several airplane crashes.Straight Line Winds, thunderstorm-generated winds that are not associated with rotation.Derecho, stronger manifestation of straight line winds. Typically form in the early evening and last throughout the night.Bow Echo, when thunderstorms on a radar screen appear in the shape of a bow.Heat Bursts, extremely hot, dry bursts of air from a thunderstorm that warm by compressional heating.Floods and Flash FloodsFlash Floods, floods that rise rapidly with little or no advance warning. Often caused by stalled or slow-moving thunderstorms causing heavy rainfall over a small area.Training, when thunderstorms move quickly, but keep passing over the same area. Can also cause flooding.Distribution of ThunderstormsCombination of warmth and moisture make equatorial landmasses especially conducive to thunderstorm formation. Occur every 1/3 days.In regards to the U.S., Gulf Coast states with a maximum in Florida experience the most thunderstorms. A second maximum exists in the Rockies. Pacific Coastal areas and interior valleys receive the least.In most areas, thunderstorms form primarily in the summer during the warmest part of the day. Central and southern California is an exception, thunderstorms are most common in winter and spring. In the Central Plains, they are most common at
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