Frequently neither of these two situations exist, andboth the change in movement and the height centerchange occur at a proportional rate. This is illustratedin figure 2-2, view (C). From a sequence of charts 24hours apart, it is shown that the low is filling at adecreasing rate and also moving at a decreasing rate.The height change value is 50 percent of the value 24hours previously on the successive charts, and the rateof movement is 75 percent. We then assume thisconstant percentage rate to continue for the next 24hours, so the low is forecast to move 225 nautical milesand fill only 15 meters.Accelerations may be handled in a similar manneras the decelerations shown in figure 2-2. Also, asequence of 12-hour charts could be used in lieu of24-hour charts to determine past trends.CRITICAL ECCENTRICITY.— When amigratory system is unusually intense, the system mayextend vertically beyond the 300-hPa level. Advectionconsiderations, contour-isotherm relationships,convergence and divergence considerations, and thelocation of the jet max will yield the movement vector.These principles are applied in the same manner as whenthe movement of long waves are determined. Theeccentricity formula may be applied to derive amovement vector, but only when a nearly straighteastward or westward movement is apparent.Migratory lows also follow the steering principle andthe mean climatological tracks. The climatologicaltracks must be used cautiously for the obvious reasons.The rise and fall centers of the time differential chartsare of great aid in determining an extrapolatedmovement vector, and extrapolation is the primarymethod by which the movement of a closed low isdetermined.Certain cutoff lows and migratory dynamic coldlows lend themselves to movement calculation by theeccentricity formula. The conditions under which thisformula may be applied are:. The low must have one or more closed contours(nearly circular in shape).. The strongest winds must be directly north orsouth of the center. The location of the max windsdetermines the direction of movement. When thestrongest winds are the easterlies north of the low, thelow moves westward; when the strongest winds are thewesterlies south of the low, the low will move eastward.The low will also move toward the weakest divergingcyclonic gradient and parallel to the strongest current.Systems moving eastward must have a greater speed inorder to overcome convergence upstream-there isnormally convergence east of a low system.The eccentricity formula is written:E c = V - V ´ - 2 Cor2 C = V - V ´ - E CwhereEc is the critical eccentricity y value.V is the wind speed south of the closed low.V´ is the wind speed north of the closed low.C is the speed of the closed low (in knots).To obtain the value of C, it is necessary to determinethe latitude of the center of the low and the spread (indegrees latitude) between the strongest winds in the lowand the center of the low. Apply these values to table2-1 to determine the tabular value. Apply the tabularvalue to the critical eccentricity formula to obtain 2C,thus C. In determining the critical eccentricity of asystem, it is necessary to interpolate both for latitude andthe spread. A negative value for C indicates westwardmovement; a positive value indicates eastwardmovement.LOCATION OF THE JET STREAM.— As longas a jet maximum is situated, or moves to the westernside of a low, this low will not move. When the jet centerhas rounded the southern periphery of the low, and is notfollowed by another center upstream, the low will moverapidly and fill.Table 2-1.-Critical Eccentricity ValueLatitudeSpread (degrees latitude)(degrees)1°3°5°10°20°80.1.92.5----70.21.84.919.580.060.32.67.127.0115.050.43.39.137.0150.040.44.010.943.5175.030.54.512.350.0200.020.54.913.353.0--10.65.214.056.0--2-7