754 
seen that when the isobars and isotherms are in phase 
but the isotherms possess the larger amplitude, the 
wave will move with moderate velocity. In the same 
case but with isobars of an amplitude larger than the 
isotherms, the perturbations will move slowly or even 
retrograde. When the isobars and isotherms are 180° 
out of phase, the wave will move very rapidly. When 
cold air moyes into the system, the wave will tend to 
deepen; when warm air moves into the system, the 
wave will usually weaken. 
5. Minor waves tend to intensify as they move into 
a major trough although maximum intensity is usually 
found as they reach the midway point between the 
major trough and the major ridge downstream. Sim- 
ilarly, a pressure fall intensifies as it moves into a major 
trough and a pressure rise intensifies as it reaches a 
major ridge. However, care should be exercised if the 
pattern is changing. 
6. Use of pressure tendencies at upper levels. Miller 
and Thompson [34] devised a method for the calcula- 
tion of 3-hr pressure changes from pilot-balloon obser- 
vations. It was hoped that these pressure tendencies 
might permit the use of Petterssen’s extrapolation for- 
mula, but this has not become standard practice largely 
because of the work involved and because only advec- 
tion is considered, while convergence and vertical 
motion (and any nongradient winds) are omitted from 
consideration. 
7. Rossby’s trajectory method. Rossby has devel- 
oped a trajectory technique which is essentially the 
determination of the future path of a system of particles 
based on the change in vorticity with change of latitude 
while the absolute vorticity is conserved. Fultz [24] has 
described the technique and the attempts which have 
been made to apply it to forecasting the 10,000-{t chart. 
Tests have shown that the 10,000-ft trajectories can 
be computed by this method for winds in major flow 
patterns with a significant degree of accuracy up to 72 
hr. Convergence and divergence are neglected in the 
equation and subjectivity is required in the selection 
or rejection of the computed winds. In certain situations 
wave computations are difficult and the constant ab- 
solute vorticity trajectory methods give better results. 
Use of this technique in extended-range forecasting is 
described by Namias [37]. 
8. Supergradient wind velocities. It has been noted 
that supergradient wind velocities in the eastern Pacific 
have often been attended or followed by an increase in 
pressure over the far western United States. The super- 
gradient condition results in a net unbalanced force on 
each unbalanced particle directed toward the right and 
thus in a piling up of air on that side. This has been 
observed in other regions and has become known as 
“anticyclogenesis upstream.” 
9. “Cutoffs.” Occasionally a rise of pressure will 
appear behind a trough, move eastward in the maxi- 
mum westerlies and cut off the southern portion of the 
trough resulting in a closed circulation in that area. 
This tends to happen in preferred locations, for example, 
in the far southwestern United States. The closed low 
will either move southwestward off the coast of southern 
WEATHER FORECASTING 
California and usually dissipate or remain stationary 
for around 48 hr awaiting the arrival of a new fall in 
pressure from the north. Somewhat similarly, warm 
highs may be cut off. Closed lows may develop in almost 
any section, usually building upward from the surface, 
and their prognostication is very important because of 
the extensive bad weather associated with them. This 
development may be detected 12-24 hr in advance 
when a slowly moving isallobaric minimum is located 
south of a weak westerly gradient. 
For the most part, the 700-mb prognostic chart 
should be prepared independently of the surface 
prognostic chart since one of its principal uses is as a 
check on the latter. However, there may be a few ele- 
ments of the surface prognostic chart which will be 
subject to little or no question and the 700-mb chart 
should be consistent with respect to these elements. 
The usual order of operations followed by the analyst 
making the 700-mb prognosis in the Weather Bureau- 
Air Force-Navy Analysis Center! is as follows: 
The prognosticator makes a 700-mb analysis. and then 
draws a thickness chart (1000-700 mb) which also includes 
the 850-mb flow. While the thickness chart is being made, an 
assistant makes the 24-hr height change chart. A tentative 
prognosis is then made by application of the various tech- 
niques discussed in the following paragraphs. Then, in consul- 
tation with the surface prognostic analyst, a series of checks 
and adjustments of the two prognoses is made to make them 
mutually consistent, with special attention given to surface 
patterns and to trends determined from closer examination of 
old data or from a check of new three-hourly data. This in- 
formal discussion of the general situation is followed by a 
discussion of the points of difference or of inconsistency be- 
tween the 700-mb and the surface charts and is participated 
in by the two prognostic analysts and the supervising analyst. 
This leads to the final version of the prognosis. 
No one procedure or set of procedures is used regularly in 
making the 700-mb prognosis. Of the wide variety available, 
the choice depends upon the analyst and the occasion. Good 
judgment is of paramount importance in making a prognosis. 
The analyst relies on it in determining rather quickly to what 
extent pure extrapolation and persistence can be used with 
better results than a time-consuming physical forecast. At- 
tempts to explain the physical basis for the processes going 
on in the atmosphere lead to involved discussions and argu- 
ments so that such attempts are limited to those features 
which are worth the valuable time required for them. Those 
features which, from the analysis, provide indications that 
extrapolation is a good prognosis, plus those features which 
show definite indications of persistence, frequently make up 
a large portion of the chart. 
The semipermanent and characteristic points and lines 
serve as a starting point for the prognosis. Persistence is usu- 
ally the best forecast in regard to the semipermanent sys- 
tems: the Aleutian low, the oceanic highs, ete. However there 
is always the question of whether to move them or to fill or 
deepen them. A deep cold low, dominating a large area of the 
map, and the accompanying peripheral flow determine not 
only the features of the area of the low but also the course 
of any perturbations coming into the belt dominated by the 
1. The author is greatly indebted to Mr. Charles M. Len- 
nahan and Mr. J. R. Fulks of the Analysis Center for the out- 
line and description of the procedures used there for the prep- 
aration of the 700-mb prognostic charts. 
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