![]() The left mover typically moves faster than the original storm, the right mover, slower. Splitting Storm A thunderstorm which splits into two storms which follow diverging paths (a left mover and a right mover). Spin-up - A small-scale vortex initiation, such as what may be seen when a gustnado, landspout, or suction vortex forms. ![]() ![]() Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere. Speed Shear The component of wind shear which is due to a change in wind speed with height, e.g., southwesterly winds of 20 mph at 10,000 feet (32 km/h at 3,000 meters) increasing to 50 mph at 20,000 feet (80 km/h at 6,100 meters). The SPC is responsible for providing short-term forecast guidance for severe convection, excessive rainfall (flash flooding) and severe winter weather over the contiguous United States. SPC - Storm Prediction Center A national forecast center in Norman, Oklahoma, which is part of NCEP. Soundings are used extensively in severe weather forecasting, e.g., to determine instability, locate temperature inversions, measure the strength of the cap, obtain the convective temperature, etc. Sounding A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location. See High Risk, Moderate Risk, Enhanced Risk, Marginal Risk, General Thunderstorms, convective outlook. Wind gusts 75 mph (120 km/h) or greater.15% probability for severe hail (≥1" / ≥2.5 cm) or severe wind (≥58 mph / ≥93 km/h) probability WITH OR WITHOUT 10% or greater probability of hail 2" (4.8 cm) or greater in diameter OR.5% probability or greater tornado probability OR.Slight Risk (of severe thunderstorms) An area of organized severe storms, which is not widespread in coverage with varying levels of intensity. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave. Shortwave (or Shortwave Trough) A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn. Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Shelf Cloud A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances. Shear Variation in wind speed ( speed shear) and/or direction ( directional shear) over a short distance. Structural wind damage may imply the occurrence of a severe thunderstorm. Severe Thunderstorm A thunderstorm which produces tornadoes, hail 1" (2.5 cm) or more in diameter, or winds of 50 knots (58 mph / 93 km/h) or more. Such clouds generally are associated with cool moist air, such as thunderstorm outflow. S's Scud (or Fractus) Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts.
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