MOVEMENT OF HGH-PRESSURE
SYSTEMS
In general, the methods for extrapolation of
low-pressure areas are applicable to the movement of
high-pressure areas.
The following are general considerations in
forecasting the movement of high-pressure systems:
. A surface high, or that portion situated under a
blocking high aloft, remains very nearly stationary.
. A high situated under or very near a jetstream is
steered by the current aloft.
l Cold, shallow highs are steered more easily than
the larger ones. The Canadian and Siberian highs move
little when there is no jet max in their vicinity or above
them, and they move rapidly when the jet max is present.
. Progressive warm highs move with a speed
consistent with that of the major ridges aloft.
. With straight westerly currents aloft, surface
highs are displaced equatorward.
. Highs tend to move in the direction of, and with
the speed of, the isallobaric centers; however, this rule
is not very reliable because the isallobaric rises often
follow the low rather than lead the high.
Steering is not used for high-pressure systems as
widely as for lows because high-pressure cells do not
have as great a vertical extent as low-pressure systems.
However, steering seems to work about 75 percent of
the time for cold highs.
FORECASTING THE INTENSITY OF
SURFACE PRESSURE SYSTEMS
LEARNING OBJECTIVES: Forecast the
intensity of surface low- and high-pressure
centers by using extrapolation, isallobaric
indications, relation to frontal movement, aloft
indications, weather type, and in relation to
normal storm tracks.
The changes in intensity of pressure systems at the
surface are determined, to a large extent, by events
occurring above the system.
EXTRAPOLATION
The 3-hour pressure tendencies reported in a
synoptic plot indicate the sum of the pressure change
due to movement of the system, plus that due to
deepening and filling. If the exact amount of pressure
change due to movement could be determined, it could
be assumed that the system would continue to deepen
or fill at that rate. However, it is not normally prudent
to assume that the current rate of change will continue,
nor just how much of the pressure change is due to
movement.
ISALLOBARIC INDICATIONS
Isallobaric analyses at the surface show the
following relationships between the isallobars and the
change in intensity of pressure systems:
. When the 3-hour pressure falls extend to the rear
of the low, the low is deepening.
l When the 3-hour pressure rises extend ahead of
the low, the low tends to fill.
. When the 3-hour pressure rises extend to the rear
of the high or ridge, the high or ridge tends to fill.
l Since low-pressure systems usually move in a
direction parallel to the isobars in the warm sector, and
since the air mass in the warm sector is homogeneous,
it is possible to assume that the pressure tendencies in
the warm sector are an indication of the deepening or
filling of the system. The effects of frontal passages
must be removed. Therefore, if a low moves parallel to
warm sector isobars, the 3-hour pressure tendency in the
warm sector is equal to the deepening or falling of the
system.
Remember that when you use the present 3-hour
pressure tendency values for any of the above rules, they
arc merely an indication of what has been happening,
and not necessarily what will be taking place in the
future. Consequently, if you use the tendencies for
indication of deepening or filling, you will need to study
the past trend of the tendencies.
RELATIVE TO FRONTAL MOVEMENT
Wave cyclones form most readily on stationary or
slow moving fronts. A preferred position is along a
decelerating cold front in the region of greatest
deceleration. Normally, the 700-hPa winds are parallel
to the front along this area.
Under conditions characteristic of the eastern
Pacific, a secondary wave cyclone may rapidly develop
(fig. 3-7, step 1). As the secondary wave forms on the
3-8
