surface and the 850-mb charts. The area of genesis will
show progressively colder temperatures at the surface
and aloft; however, the drop in the 850-mb temperatures
does not occur at the same rate as at the surface. This is
an indication that a very strong inversion is in the
process of forming. The air in the source region must
be relatively stagnant.
. High-level anticyclogenesis is indicated when
low-level warm air advection is accompanied by
stratospheric cold air advection. This situation has
primary application to the formation of blocks, as
high-level anticyclogenesis is primarily associated with
the formation of blocks and the intensification of the
ridges of the subtropical highs.
. Blocks should normally be forecast to form only
over the eastern portion of the oceans in the middle and
high latitudes. Warm air is normally present to the north
and northwest.
Formation Forecasting Principles (Lows)
There are certain conditions required in the
atmosphere, as well as certain atmospheric indicators,
for cyclogenesis to occur. The greater the number of
these indicators/conditions in agreement, the greater the
success in forecasting cyclogenesis. Some of them are
listed below:
l An area of divergence exists aloft.
. A jet maximum on the west side of a low
indicates deepening and southward movement.
. Cold air advection in the lower troposphere and
warming in the lower stratosphere is associated with the
formation of or deepening of lows.
Formation Forecasting Principles
(Cutoff Lows)
Another task in forecasting is that of the formation
of cutoff lows. Some of the indicators are as follows:
l They generally form only off the southwestern
coast of the United States and the northwestern coast of
Africa.
. The upstream ridge intensifies greatly. This
intensifying upstream ridge contains an increasing,
strong, southwesterly flow.
. Strong northerlies on the west side of the trough.
. Height falls move south or southeastward.
. Strong cold air advection occurs on the west side
of the upper trough.
Constructing Upper Level Prognostic Charts
The constant pressure prognostic chart is about to
take form. The forecasted position of the long wave
troughs and ridges have been determined and depicted
on the tentative prognostic chart. The position of the
highs, lows, and cutoff centers were then determined
and depicted on the tentative prognostic chart. Short
waves were treated in a similar fashion. Contours are
then depicted. The height values of the contours are
determined by actual changes in intensity of the
systems. The pattern of the contours is largely
determined by the position of the long waves, short
waves, and closed pressure systems. Contours are
drawn in accordance with the following eight steps:
1. Outline the areas of warm and cold advection in
the stratum between 500 and 200 hPa, and move the
thickness lines at approximately 50 percent of the
indicated thickness gradient in the direction of the
thermal wind.
2. Tentatively note, at several points on the chart,
the areas of height changes on the constant pressure
surface above the existing height values.
3. Move the areas of 24-hour height rises and falls
at the speed of the short waves, and note at several key
points the amount and direction of the height change
from the current chart.
4. Adjust the advected height changes, and, in turn,
adjust these for positions of the long waves, pressure
systems, and short waves.
5. Extrapolate heights for selected points at 500
hPa on the basis of the 24-hour time differential
indications and advection considerations, provided that
they are justified by the indications of high-level
convergence and divergence. When the contributions
from advection and time differentials are not in
agreement with convergence and divergence, adjust the
contribution of each and use accordingly.
6. Adjust the height values to the forecasted
intensities of the systems. These adjustments can lead
to the following:
. All factors point toward intensification
(deepening of lows and filling of highs).
. One factor washes away the contribution
made by another, and the system remains at or near its
present state of intensity.
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