ISOTHERM-CONTOUR RELATIONSHIP.—
Little movement will occur if the isotherms and contours
are symmetrical (no advection). Lows will intensify
and retrogress if cold air advection occurs to the west
and fill and progress eastward if warm air advection
occurs to the west.
FORECASTING THE INTENSITY OF
UPPER LEVEL AND ASSOCIATED
SURFACE FEATURES
Many of the same considerations used in the
movement of closed centers aloft may also apply to
forecasting their intensity. Extrapolation and the use of
time differentials aid in forecasting the change and
magnitude of increases and decreases. Again, rise and
fall indications must be used in conjunction with
advection considerations, divergence indications, and
other previously discussed factors.
Intensity Forecasting Principles (Highs)
The following text discusses how atmospheric
conditions affect the forecasted intensity of high
pressure systems.
l Highs undergo little or no change in intensity
when isotherms and contours are symmetrical
. Highs intensify when warm air advection occurs
on the west side of the high.
. Highs weaken when cold air advection occurs on
the west side of the high.
. Blocking highs usually intensify during
westward movement and weaken during eastward
movement.
. Convergence and height rises occur in the
downstream trough when high-speed winds with a
strong gradient approach low-speed winds with an
anticyclonic weak gradient. This is often the case
in ridges where the west side contains the high-speed
winds; the ridge intensifies due to this accumulation
of mass.
This situation has also been termed
overshooting. This situation can be detected at the
500-hPa level, but the 300-hPa level is better suited
because it is the addition or removal of mass at higher
levels that determines the height of the 500-hPa
contours.
. Rise and fall centers on the time differential chart
indicate the changes in intensity, both sign (increasing,
decreasing) and magnitude of change, if any, in
decimeters. The magnitude of the height rises or falls
can be adjusted if other indications reveal that a slowing
down or a speeding up of the processes is occurring, and
expected to continue.
Intensity Forecasting Principles (Lows)
The following text discusses how atmospheric
conditions affect the forecasted intensity of
low-pressure systems.
l Lows and cutoff lows deepen when cold air
advection occurs on the west side of the trough.
. Lows fill when warm air advection occurs on the
west side of the low.
. Lows fill when a jet maximum rounds the
southern periphery of the low.
. Lows fill when the jet maximum is on the east
side of the low, if another jet max does not follow.
. Lows deepen when the jet max remains on the
west side of the low, provided the jet max to the west of
the low is not preceded by another on the southern
periphery or eastern periphery of the low, for this
indicates no change in intensity.
l The 24-hour rise and fall centers aid in
extrapolating both the change and the magnitude of falls
in moving lows. Again, these rise and fall indications
must be considered along with advection factors,
divergence indications, and the indications of the
contour-isotherm relationships.
FORECASTING THE FORMATION OF
UPPER LEVEL AND ASSOCIATED
SURFACE FEATURES
The following text deals with the formation of upper
level and associated surface features, and how
atmospheric features affect them.
Formation Forecasting Principles (Highs)
The following are atmospheric condition indicators
that are relevant to the formation of highs.
l Cold air masses of polar and Arctic origin
generally give no indication of the formation of highs at
the 500-hPa level or higher, as these airmasses normally
do not extend to this level.
. The shallow anticyclones of polar or Arctic
origin give indications of their genesis primarily on the
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