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Indicators of Possible Recurvature |
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Click here to Order your Radar Equipment Online Indicators of Possible Recurvature Some of the indicators of possible recurvature are as follows:. If the base of the polar westerlies lowers to 15,000 to 20,000 feet west of the storms latitude, and remains in this position, recurvature may then be expected to occur.. However, if there is the building of a dynamic high or an eastward movement of this high to the rear of the advancing trough, and the westerlies dissipate in the low latitudes, the storm will move past the trough to its south and continue its westward path. . The above rule also holds true in cases where the polar trough moves from the west against a blocking high. The higher latitude portion of the trough continues to move eastward while the southern segment of the trough is retarded and is no longer connected with the upper portion of the trough.. Recurvature may be expected when an anchor trough is about 500 miles west of the storm and when the forward edge of the westerlies is from 500 to 700 miles to the west. Correlated with this parameter, R. J. Shafer found that a spot value of the thickness between 850 to 500 hPa 7.5 degrees of latitude to the northwest of the storm was one of the most valuable of all parameters on recurvature or as an indication of future movement of Atlantic hurricanes. This thickness reflects the relative strength of cold troughs to the west essential for recurvature. Low values of thickness, 14,000 feet (approximately 4,270 meters) or less, almost always indicate recurvature; and high values, 14,200 feet (4,330 meters) or more, generally indicate continued westward motion.. The major trough west of the storm (in the westerlies) is slowly progressive.. Long waves are stationary or slowly progressive. . There is a rapid succession of minor troughs aloft. . The climatological mean track indicates recurvature (use with caution). . When the neutral point at the southern extremity of the trough in the westerlies at the 500-hPa level lies at or equatorward of the latitude of the cyclone, recurvature into the trough will usually occur. In this situation, the cyclone would normally be under the influence of southerly winds from the upper limits to a level well below the 500-hPa level while approaching the trough. . When the subtropical ridge at the 500-hPa level is broad and consists of large anticyclones, recurvature usually occurs. This case represents a low index situation in which the cyclone remains under the influence of a single, large, slow-moving anticyclone for a relatively long time. . Weak troughs exist between two separate subtropical high cells. Sometimes tropical storms move northward through these very weak breaks in subtropical highs. Nonrecurvature The following flow patterns are associated with nonrecurvature:. Strong subtropical anticyclone or ridge to the north of the storm with the mean trough in the westerlies located far to the west of the longitude of the storm. If this pattern develops strongly over the western oceans or continents, a storm will generally be driven inland and dissipate before it recurves into the western end of the ridge.. Flat (small-amplitude waves) in the westerlies at latitudes near or north of the normal position. A narrow subtropical ridge separates the westerlies from the tropical trough. In many cases the mean trough in the westerlies may be located at the same longitude of the storm.This information is now available on CD in Adobe PDF Printable Format |
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