36-hour prognostic charts for these levels. In this way,
changes both in space and time can be considered. For
a direct application for a short period of time, transfer
the position of the low center to the concurrent 700-mb
chart. For direction, move the center in the direction of
the contours downstream and slightly inclined to the left
for low-pressure areas.
Experience with moving
systems of this type will soon tell you how much
deviation should be made. For speed of the surface
cyclone, average the basic current downstream over
which the cyclone will pass (take into consideration
changes indirection and speed of flow over the forecast
period). Take 70 percent of this value for the mean
speed for 24 hours. Move the low center along the
contours, as described above, for this speed for 24 hours.
This should be your position at that time.
For the 500-mb chart, follow the same procedure,
except use 50 percent of the wind speed for movement.
If these two are not in agreement, take a mean
of the two. There may be cases where the 500-mb
chart is the only one used. In this case, you will not
be able to check the movement against the 700-mb
FORECASTING PRINCIPLES. T he
following empirical relationships and rules should be
taken into account when you use the steering technique:
. Warm, unoccluded lows are steered by the
current at the level to which the closed low does not
extend. When so steered, lows tend to move slightly to
the left of the steering current.
l Warm lows (unoccluded) are steered with the
upper flow if a well-defined jet is over the surface center
or if there is no appreciable fanning of the contours aloft.
Low-pressure systems, especially when large, tend to
move slightly to the left of the steering current.
. Rises and falls follow downstream along the
500-hPa contours; the speed is roughly half of the
The 3-hour pressure rises and
pressure falls seem to move in the direction of the
700-hPa flow; while 24-hour pressure rises and pressure
falls move with the 500-hPa flow.
. Cold lows, with newly vertical axes, are steered
with the upper low (in the direction of upper height
falls), parallel to the strongest winds in the upper low,
and toward the weakest contour gradient.
. Occluded lows, the axes of which are not vertical,
are steered partly in the direction of the warm air
. A surface low that is becoming associated with a
cyclone aloft will slow down, become more regular, and
follow a strongly cyclonic trajectory.
. Surface lows are steered by jet maximums above
them, and deviate to the left as they are so steered. They
move at a slower rate than the jet maximum, and are
soon left behind as the jet progresses.
. During periods of northwesterly flow at 700 hPa
from Western Canada to the Eastern United States,
surface lows move rapidly from the northwest to the
southeast, bringing cold air outbreaks east of the
. If the upper height fall center (24 hour) is found
in the direction in which the surface cyclone will move,
the cyclone will move into the region or just west of it
in 24 hours.
Direction of Mean Isotherms (Thickness Lines)
A number of rules have been compiled regarding the
movement of low-pressure systems in relation to the
mean isotherms or thickness lines. These rules are
outlined as follows:
. Unoccluded lows tend to move along the edge of
the cold air mass associated with the frontal system that
precedes the low; that is, it tends to move along the path
of the concentrated thickness lines. When using this
method, you should remember that the thickness lines
will change position during the forecast period. If there
is no concentration of thickness lines, this method
cannot be used.
l When the thickness gradient (thermal wind) and
the mean windflow are equal, the low moves in a
direction midway between the two. This rule is more
reliable when both the thermal wind and the mean
windflow are strong.
l When the menu windflow gradient is stronger
than the thickness gradient, the low will move more in
the direction of the mean windflow.
l When the thickness gradient is stronger than the
mean windflow gradient, the low will move more in the
direction of the thickness lines.
l With warm lows, the mean isotherms show the
highest temperature directly over the surface low, which
is about halfway between the 700-hPa trough and ridge
line. This indicates the mean isotherm and 700-hPa
isoheights are 90 degrees out of phase. Since warm