FORECASTING THE INTENSITY OF
TROUGHS AND RIDGES
Forecasting the intensity of long wave troughs and
ridges often yields nothing more than an indication of
the expected intensity; that is, greater than or less than
present intensity. For instance, if deepening or falling is
indicated, but the extent of deepening or tilling is not
definite, the forecaster is forced to rely on experience
and intuition in order to arrive at the amount of
deepening or tilling.
FNMOC upper level charts
forecast the intensity of upper waves with a great deal
of success.
If available, you should check your
intensity and movement predictions against these
prognoses.
Extrapolation
Patterns on upper level charts are more persistent
than those on the surface. Therefore, extrapolation
gives better results on the upper air charts than on
surface charts. When you use height changes aloft, the
procedure is to extrapolate height change and add or
subtract the change to the current height values.
Use of Time Differentials
The time differential chart is discussed in the AG2
TRAMAN, volume 1.
The time differential chart constructed for
the 500-hPa level shows the history of changes that
have taken place at the 500-hPa level at 24-hour
intervals.
In considering the information on the
time differential chart, those centers with a well
defined history of movement will be of greatest value.
Take into consideration not only the amount of
movement, but also the changes in intensity of the
centers. Centers with no history should be treated with
caution, especially with regard to their direction of
movement which is usually downstream from the
current position. Information derived from the time
differential chart should be used to supplement
information obtained from previous considerations, and
when in agreement, used as a guide for the amount of
change.
Normally, the 24-hour height rise areas can be
moved with the speed of the associated short wave
ridges, and the speed of the fall centers with the speed
of the associated short wave troughs. It must be
remembered that height change centers may be present
due to convergence or divergence factors and may not
have an associated short wave trough or ridge. Be
cautious not to move a height change center with the
contour flow if it is due primarily to convergence or
divergence. However, with short wave indications, a
change center will appear and move in the direction of
the contour flow.
Once you have progged the movement of the height
change centers and determined their magnitude, apply
the change indicated to the height on the current 500-mb
chart. You should use these points as guides in
constructing prognostic contours.
Isotherm-Contour Relationship
In long waves, deepening of troughs is associated
with cold air advection on the west side of the trough
and filling of troughs with warm air advection on the
west side of the trough. The converse is true for ridges.
Warm air advection on the western side of a ridge
indicates intensification, and cold air advection
indicates weakening.
This rule is least applicable
immediate yeast of the Continental Divide in the United
States, and probably east of any high mountain range
where westerly winds prevail aloft. In short waves,
deepening of troughs is associated with cold air
advection on the west side of the trough and falling of
troughs with warm air advection, particularly if a jet
maximum is in the northerly current of the trough and
tilling is indicated by warm air advection on the western
side.
In reference to the above paragraph, the advection
is not the cause of the intensity changes, but rather is a
“sign”
of what is occurring.
High level
convergence/divergence is the cause.
Effect of Super Gradient Winds
Figure 2-1, views (A) through (D), shows the effect
of the location of maximum winds on the intensity of
troughs and ridges.
Explanation of figure 2-1 is as follows:
l When the strongest winds aloft are the westerlies
on the western side of the trough, the trough deepens
[fig. 2-1, view (A)].
. When the strongest winds aloft are the westerlies
at the base of the trough, the trough moves rapidly
eastward and does not change in intensity [fig. 2-1, view
(B)].
. When the strongest winds are on the east side of
the trough, the trough fills [fig. 2-1, view (C)].
2-4
