As wind blows across the many different levels of the atmosphere, there are changes in wind speed and direction. These changes in speed and direction over a relatively short distance in the atmosphere are known as wind shear. Wind shear is most prevalent in the high latitudes as well as close to the jet stream; however, this shear plays a crucial role in the tropics as well. One critical factor in determining tropical cyclone development is calculating the shear of the environment over which the tropical cyclone will be above.
Through research and observation, two of the primary reasons that ordinary disturbances in the tropics can acquire tropical characteristics are due to the presence of warm sea surface temperatures as well as low values of wind shear. As a tropical cyclone is developing, heavy thunderstorms will build up near the center. Given a favorable environment, the whole system from sea level up to around 50,000 feet in the atmosphere will eventually begin to turn counter-clockwise (or cyclonically). Without wind shear, the turning within the tropical system will be uniform, or vertically aligned, which helps to keep the storm intact and likely strengthening. If, however, strong wind shear is present, then a system's structure will be vertically tilted toward the direction that the wind shear is blowing. A vertically tilted system is inefficient at drawing in warm and moist air from the ocean necessary for intensification; therefore, the storm never fully develops tropical characteristics.
If a well-defined hurricane moves from a region of low wind shear into an area of high wind shear, the vertically aligned center of the storm will become tilted in the direction of the wind shear and likely cause weakening or "tearing apart" of the center of the storm.
It is essential for meteorologists to determine and monitor wind shear across any tropical basin in order to forecast a tropical system's intensity as accurately as possible.
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Earliest substantial snow -- 4 inches fell.
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