Cyclones are important generators of precipitation in
the Tropics as well as in midlatitudes.
Factors to be considered in arriving at an accurate
forecast are listed below; these factors are not listed in
any order of importance:
The source region of the parent air mass.
Nature of the underlying surface.
The type and slope of the front(s).
Wind and contour patterns aloft.
Past speed and direction of movement of the
low or front(s).
Familiarization with the normal weather
patterns.
As pointed out earlier, a thorough understanding of
the physical processes by which precipitation develops
and spreads is essential to an accurate forecast.
FRONTAL AND OROGRAPHIC
CLOUDINESS AND PRECIPITATION
There are unique cloud and precipitation features
and characteristics associated with the cold and warm
fronts, as well as orographic barriers. The following text
discusses these features and characteristics.
Cold Front
You will find it helpful to use constant pressure
charts in conjunction with the surface synoptic
situation in forecasting cold frontal cloudiness and
precipitation. When the contours at the 700-hPa level
are perpendicular to the surface cold front, the band of
weather associated with the front is narrow. This
situation occurs with a fast-moving front. If the front is
slow moving, the weather and precipitation will extend
as far to the rear of the front as the winds at the 700-
hPa level are parallel to the front. In both of the above
cases, the flow at 700 hPa also indicates the slope of the
front. Since the front at the 700-hPa level lies near the
trough line, it is apparent that when the flow at 700
hPa is perpendicular to the surface front, the 700-hPa
trough is very nearly above the surface trough; hence,
the slope of the front is very steep. When the 700-hPa
flow is parallel to the surface front, the 700-hPa trough
lies to the rear of the surface front and beyond the
region in which the flow continues parallel to the front.
Consequently, the frontal slope is more gradual, and
lifting is continuing between the surface and the 700-
hPa level at some distance to the rear of the surface
front.
Another factor that contributes to the distribution of
cloudiness and precipitation is the curvature of the flow
aloft above the front. Cyclonic flow is associated with
horizontal convergence, and anticyclonic flow is
associated with horizontal divergence.
Very little weather is associated with a cold front if
the mean isotherms are perpendicular to the front.
When the mean isotherms are parallel to the front,
weather will occur with the front. This principle is
associated with the contrast of the two air masses;
hence, with the effectiveness of lifting.
Satellite imagery provides a representative picture
of the cloud structure of frontal systems. Active cold
fronts appear as continuous, well-developed cloud
bands composed of low, middle, and high clouds. This is
caused by the upper wind flow, which is parallel, or
nearly parallel, to the frontal zone (fig. 4-5).
Figure 4-5.—An active cold front.
4-4
