Frequently neither of these two situations exist, and
both the change in movement and the height center
change occur at a proportional rate. This is illustrated
in figure 2-2, view (C). From a sequence of charts 24
hours apart, it is shown that the low is filling at a
decreasing rate and also moving at a decreasing rate.
The height change value is 50 percent of the value 24
hours previously on the successive charts, and the rate
of movement is 75 percent. We then assume this
constant percentage rate to continue for the next 24
hours, so the low is forecast to move 225 nautical miles
and fill only 15 meters.
Accelerations may be handled in a similar manner
as the decelerations shown in figure 2-2. Also, a
sequence of 12-hour charts could be used in lieu of
24-hour charts to determine past trends.
CRITICAL ECCENTRICITY.— When a
migratory system is unusually intense, the system may
extend vertically beyond the 300-hPa level. Advection
considerations, contour-isotherm relationships,
convergence and divergence considerations, and the
location of the jet max will yield the movement vector.
These principles are applied in the same manner as when
the movement of long waves are determined. The
eccentricity formula may be applied to derive a
movement vector, but only when a nearly straight
eastward or westward movement is apparent.
Migratory lows also follow the steering principle and
the mean climatological tracks. The climatological
tracks must be used cautiously for the obvious reasons.
The rise and fall centers of the time differential charts
are of great aid in determining an extrapolated
movement vector, and extrapolation is the primary
method by which the movement of a closed low is
determined.
Certain cutoff lows and migratory dynamic cold
lows lend themselves to movement calculation by the
eccentricity formula. The conditions under which this
formula may be applied are:
. The low must have one or more closed contours
(nearly circular in shape).
. The strongest winds must be directly north or
south of the center. The location of the max winds
determines the direction of movement. When the
strongest winds are the easterlies north of the low, the
low moves westward; when the strongest winds are the
westerlies south of the low, the low will move eastward.
The low will also move toward the weakest diverging
cyclonic gradient and parallel to the strongest current.
Systems moving eastward must have a greater speed in
order to overcome convergence upstream-there is
normally convergence east of a low system.
The eccentricity formula is written:
E c = V - V ´ - 2 C
or
2 C = V - V ´ - E C
where
Ec 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 determine
the latitude of the center of the low and the spread (in
degrees latitude) between the strongest winds in the low
and the center of the low. Apply these values to table
2-1 to determine the tabular value. Apply the tabular
value to the critical eccentricity formula to obtain 2C,
thus C. In determining the critical eccentricity of a
system, it is necessary to interpolate both for latitude and
the spread. A negative value for C indicates westward
movement; a positive value indicates eastward
movement.
LOCATION OF THE JET STREAM.— As long
as a jet maximum is situated, or moves to the western
side of a low, this low will not move. When the jet center
has rounded the southern periphery of the low, and is not
followed by another center upstream, the low will move
rapidly and fill.
Table 2-1.-Critical Eccentricity Value
Latitude
Spread (degrees latitude)
(degrees)
1°
3°
5°
10°
20°
80
.1
.9
2.5
--
--
70
.2
1.8
4.9
19.5
80.0
60
.3
2.6
7.1
27.0
115.0
50
.4
3.3
9.1
37.0
150.0
40
.4
4.0
10.9
43.5
175.0
30
.5
4.5
12.3
50.0
200.0
20
.5
4.9
13.3
53.0
--
10
.6
5.2
14.0
56.0
--
2-7
