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. l l aloft. l Long  waves  are  stationary  or  slowly  progressive. There is a rapid succession of minor troughs The   climatological   mean   track   indicates recurvature  (use  with  caution). l 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. Motion  During  Recurvature The following rules apply during recurvature only: . When the radius of recurvature of a storm is greater than 300 miles, it will not decelerate and may even accelerate. The storm will slowdown if the radius of recurvature is less than 300 miles. In general, when the radius of recurvature is large, it is usually very uniform. A small radius will occur along a brief portion of the track. (See fig. 11-2.) . A large radius of recurvature is to be expected if the high northeast of the tropical cyclone has a vertical axis (fig. 11-3 (A)). This will occur when long waves are  stationary. . When the high slopes south to southeast with height (fig. 11-3 (B)), the cyclone is transferred rapidly from the influence of upper easterlies to that of the upper westerlies and the track has a short bend. This occurs when long waves are progressive. The following rules refer to short-term (24 hours or less)  forecasting: .  Tropical  cyclones  move  toward  the  area  of greatest surface pressure falls (12- to 24-hour pressure change). Figure 11-2.-(A) Recurvature at a constant speed; (B) First decelerating and then accelerating. 11-7