SHORT-RANGE WEATHER FORECASTING 
can be obtained by the geostrophie wind method, which 
usually yields good results with cold fronts but must 
be used with caution with warm fronts. Petterssen 
states that warm fronts will move with 60-80 per cent 
of the geostrophie wind, while Byers in the North Paci- 
fic observed warm fronts moving with 50 per cent or 
less of the normal component of the gradient wind. 
Further determmation of the movement of fronts 
and pressure centers should be obtained by the Petters- 
sen extrapolation formulas [42, Chap. IX]. These for- 
mulas give excellent results over oceanic areas, good 
results over homogeneous land areas, but rather poor 
results over mountainous sections. Since the movement 
of fronts and pressure systems is almost always chang- 
ing with time, the formulas rarely hold good longer 
than 24 hr. After that time, the forecaster must rely on 
the normal evolution of pressure systems and the large- 
scale factors indicative of acceleration or deceleration. 
The direction and rate of movement of pressure systems 
as indicated by the steering shown by the upper-air 
charts—current and prognostic—should be determined. 
In computing the displacement of pressure systems, the 
forecaster should be careful that the estimated dis- 
placement of each imdividual system agrees logically 
with the simultaneous displacement of adjacent 
systems. In the United States, errors occur most fre- 
quently when (1) rapidly moving occlusions, in a period 
of high mdex, cross the Pacific coastline with at least 
moderate intensity but become damped out by the 
time they should have reached central North America, 
and (2) a storm with a marked allobaric minimum 
reaches the North Pacific Coast during a period of 
moderate or high index simultaneously with the devel- 
opment of a rather strong but poorly organized depres- 
sion in the Great Plains and the Mississippi Valley. In 
the latter case, marked anticyclogenesis sets in 
immediately over the eastern Rockies and the Great 
Plains and the low frequently moves off more rapidly 
than expected. 
A forecast rule in general use states that wave cy- 
clones will move parallel to the warm-sector isobars 
when the latter are parallel to the isotherms and this 
movement is not inconsistent with the pressure tend- 
encies. As a rule, in a family of lows, each low will 
develop a course farther south than its predecessor but, 
in strong northwest steering, successive lows usually 
trend toward a more northerly course. 
Thus, a number of approximations of prognostic 
positions of fronts and pressure systems can be ob- 
tained. The results should be compared and a final 
approximation derived which, largely on the basis of 
experience, appears most logical from the physical proc- 
esses evidenced by the most recent observations. 
2. Forecast of changes in intensity of pressure sys- 
tems. The forecaster will, of course, keep in mind the 
normal deepening and subsequent filling of the typical 
cyclone as it passes through its life cycle. Cyclones 
developing in middle latitudes will be attended by 
maximum deepening when the northward component 
in direction of movement is greatest. The relationship 
between direction of movement, intensity of meridional 
730 
flow, and availability of moisture is obvious. According 
to Petterssen, the rate of deepening remains constant 
as long as a warm sector remains on the ground and 
for 6-12 hr after occlusion sets in. Changes in the three- 
and twelve-hourly pressure tendencies should be 
watched carefully since they provide a basis for esti- 
mating the rate of deepening and filling. 
Intensification or weakening of pressure systems can 
be detected by mspection of central pressures in the 
systems on the last several regular and interim synoptic 
charts, and by the position of the maximum isallobar 
relative to the center, correcting for the diurnal varia- 
tion in pressure. 
Some significant rules for determining fillmg or deep- 
ening have been summarized by Y. J. and M. B. Oliver 
[40] as follows: 
a. A wave will deepen or a front become more pro- 
nounced if the 10,000-ft wind field possesses cyclonic 
vorticity and the wave has a temperature contrast 
through it. 
b. A wave will weaken and a front will undergo 
frontolysis if the 10,000-ft wind field possesses anticy- 
clonic vorticity. 
c. If there are several waves along a front, the one 
with the most intense cyclonic vorticity aloft will de- 
velop at the expense of the others. This is usually the 
one nearest the axis of the trough aloft (at 10,000 ft). 
d. Waves at the surface will deepen if the 700-mb 
contours diverge ahead of them 
e. Waves at the surface will weaken if the 700-mb 
contours converge ahead of the wave. 
Austin [1] found no definite relationship between the 
lapse rate of temperature above the center of cyclones 
and their future change in intensity. In the same study 
he tested changes in cyclone intensity with the spacing 
of isotherms at 10,000 ft and for the layer between 700 
and 500 mb. That cyclones are observed in regions of 
strong temperature contrast was confirmed; otherwise 
no definite correlation was established. In this study, 
cyclones apparently were not classified according to 
the stage of development. 
3. Determination of cyclogenesis, anticyclogenesis, 
frontogenesis, and frontolysis. Cyclogenesis, or the for- 
mation of wave cyclones on stationary or slow-moving 
cold fronts, is one of the more difficult problems facing 
the forecaster. There are certain preferred areas for 
wave development, such as the southern portion 
of mountain ranges where deformation of the cold front 
is induced. Under certain conditions, flat waves emerge 
from this region every 24 hr or so and move rapidly in 
a general easterly or northeasterly direction. These 
cannot be forecast in detail more than 12 hr in advance, 
and for longer periods precipitation should be forecast 
without attempting to define times of beginning and 
ending. 
Waves form most frequently on stationary fronts or 
slowly moving cold fronts. The formation of a wave 
may be indicated by a new surge of pressure rises in 
the cold air, from the general pressure field, deforma- 
tion of the front as it passes over mountain ranges, and 
cyclonic circulation such as may frequently be observed 
