paragraphs. (See figure 4-22, views (A) and (B), for
typical warm and cold air advection patterns at
850-hPa.)
The following rules for the movement of the
24-hour, 850-hPa level temperature change areas have
been devised
l Maximum cooling takes place between the
850-hPa contour trough and the 850-hPa isotherm ridge
east of the trough.
. Maximum warming takes place between the
850-hPa contour ridge and the 850-hPa isotherm trough
east of the contour ridge.
. Changes are slight with ill-defined
isotherm/contour patterns.
. Usually, little change occurs when isotherms and
contours are in phase at the 850-hPa level.
l The temperature falls at the 850-hPa level tend
to replace height falls at the 700-hPa level in an average
of 24 hours.
Conversely, temperature rises replace
height rises.
Figure 4-22.-Typical cold and warm air advection patterns at
850-hPa. (A) Cold; (B) Warm.
. With filling troughs or northeastward moving
lows, despite northwest flow behind the trough, 850-hPa
level isotherms are seldom displaced southward, but
follow the trough toward the east or northeast
l Always predict temperature falls immediately
following a trough passage.
. Do not forecast temperature rises of more than 10
to 2°F in areas of light or sparse precipitation in the
forward areas of the trough. If the area of precipitation
is widespread and moderate or heavy, forecast no
temperature rise.
. With eastward moving systems under normal
winter conditions (trough at the 700-hPa level moving
east about 10° per day), a distance of 400 nautical miles
to the west is a good point to locate the temperature to
be expcted at the forecasting point 24 hours later. A
good 850-hPa temperature advection speed seems to be
about 75 percent of the 700-hPa trough displacement.
The following is step-by-step procedures for
moving the 850-hPa 0°C isotherm:
1. Extrapolate the movement of the thermal ridge
and troughs for 12 and 24 hours. If poorly defined, this
step may be omitted. The amplitude of the thermal wave
may be increased or decreased subjectively if, during
the past 12 hours, there has been a corresponding
increase or decrease in the height of the contours at
500-hPa.
2. The thermal wave patterns will maintain their
approximate relative position with the 850-hPa level
contour troughs and ridges. Therefore, the 12- and
24-hour prognostic positions of the contour troughs and
ridges should be made, and the extrapolated positions of
the thermal points checked against this contour
prognosis. Adjustments of these thermal points should
be made.
3. Select points on the 0°C isotherm that lie
between the thermal ridge and trough as follows: one
or two points in the apparent warm advection area, and
one or two points in the apparent cold advection area.
Apply the following rules to these selected points.
. Warm advection area. If the points lie in a near
saturated or precipitation area, they will remain
practically stationary with respect to the contour trough.
If the points lie in a nonsaturated area, but one that is
expected to become saturated or to lie in precipitation
area, then it will remain stationary or move upwind
slightly to approximate y the prognostic position of the
0°C wet-bulb. If the point does not fall in the above two
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