You need not place red and blue dots for the entire
chart-only for the area of interest.
Now, to use this advection chart, it should be
compared against the chart from the preceding 12 hours.
From comparison of the red and blue dots, you can
determine if there has been an increase or decrease in
the amount of warm or cold air advection in a particular
area, as well as any change in the intensity of advection.
Then, the advection type and amount, as well as
change, can be applied to determine the possibility of
new pressure system development.
FORECASTING THE MOVEMENT OF
SURFACE PRESSURE SYSTEMS
LEARNING OBJECTIVES Forecast the
movement of surface low- and high-pressure
systems by extrapolation, isallobaric
indications, relation to warm sector isobars,
relation to frontal movement, thickness lines,
relation to the jetstream, and statistical
techniques.
Whether you move the high- or low-pressure areas
first is a matter of choice for the forecaster, Most
forecasters prefer to move the low-pressure areas first,
and then the high-pressure areas.
MOVEMENT OF LOW-PRESSURE
SYSTEMS
Lows determine, to a large extent, the frontal
positions. The y also determine a portion of the isobaric
configuration in highs because gradients readjust
between the two. As a result of knowing the interplay
of energy between the systems, meteorologists have
evolved rules and methods for progging the movement,
formation, intensification, and dissipation of lows.
The general procedure for the extrapolation of
low-pressure areas is outlined below. Although only
movement is covered, the central pressures with
anticipated trends could be added to obtain an intensity
forecast.
1. Trace in at least four consecutive past positions
of the centers.
2. Place an encircled X over each one of these
positions, and connect them with a dashed line, (See
fig. 3-3.) If you know the speed and direction of
movement, as obtained from past charts, the forecasted
position can be calculated. One word of caution,
straight linear extrapolation is seldom valid beyond 12
hours. Beyond this 12-hour extrapolated position,
deepening/filling, acceleration/deceleration, and
changes in the path must be taken into consideration. It
is extremely important that valid history be followed
from chart to chart, Systems do not normally appear
out of nowhere, nor do they just disappear
3. An adjustment based on a comparison between
the present chart and the preceding chart must be made,
For example, the prolonged path of a cyclone center
must not run into a stationary or quasi-stationary
anticyclone, notably the stationary anticyclones, over
continents in winter. When the projected path points
toward such anticyclones, it will usually be found that
the speed of the cyclone center decreases and the path
curves northward. This path will continue northward
until it becomes parallel to the isobars around the
quasi-stationary high. The speed of the center will be
least where the curvature of the path is greatest. When
the center resumes a more or less straight path, the speed
again increases.
Extrapolation
First and foremost in forecasting the movement
of lows should be their past history. This is a
record of the pressure centers, attendant fronts, their
direction and speed of movement, and their
intensification/weakening. From this past history, you
can draw many valid conclusions as to the future
behavior of the systems and their future motion. This
technique is valid for both highs and lows for short
periods of time.
Figure 3-3.-Example of extrapolation procedure. X is the
extrapolated position.
3-3
