SHORT-RANGE WEATHER FORECASTING 
peripheral flow. The speed of these perturbations may also 
be inferred so that the pattern of the region in the vicinity 
of their prognosticated locations may be deduced. In this way 
persistence not only controls its own area but extends its in- 
fluence and prognostic value to adjoining regions and, with 
diminishing influence, to more distant regions. The dominant 
warm highs have a similar role in the prognosis since they 
persist and tend to be blocking highs. Any cold trough which 
is strong enough to move a semipermanent warm high is 
usually so obviously strong that at least a slight movement 
of the high will be included in the prognosis. 
As in all forecasting, the first step in pure extrapolation is 
to find some element that is conservative enough on the 
particular occasion to be used as a starting point. This ele- 
ment may very well be different from the one used on the 
previous day and may even be different each day for a week 
or more. Thus the speed of the front, the speed of the low, 
the speed of the isallobaric trough, etc., may be different from 
the corresponding points and lines of a high or ridge, and one 
may be conservative when the others are not. 
With a sequence of good analyses as a basis, the prognos- 
ticator extrapolates the conservative characteristic points 
and lines, at uniform speed if they have been moving uni- 
formly—with positive or negative acceleration if they have 
shown signs of accelerating. Trends have to be checked closely 
so that the beginning of transition periods between high- 
and low-index conditions can be detected and the prognosis 
adjusted to conform to the increase or decrease in displace- 
ment. A blocking high, whether one of the warm oceanic 
highs or a cold high over the continent, has to be considered 
in its effect on the flow. 
The complex interrelations between sea-level isobars and 
700-mb contours make any feature on the sea-level chart 
important imsofar as it affects the 700-mb contours. Con- 
versely, since one of the forecaster’s main interests in the 
700-mb prognosis is its value as an aid in forecasting surface 
wind, weather, and temperature changes, any important fea- 
ture on either the surface or the 700-mb chart is usually an 
important feature on the other. It should not, however, be 
implied that the 700-mb prognosis is strictly dependent on 
the surface prognosis; often the reverse is true because some 
features aloft can be prognosticated with greater certainty, 
and in such cases the upper-air prognosis will influence the 
surface prognosis. 
Because of the close coordination required between the 
surface and the 700-mb prognoses, it is difficult to separate 
completely the factors used in making the one from those 
used in making the other. One of the best aids that the person 
responsible for the 700-mb prognosis can have is a competent 
surface prognosticator working with him. A good surface 
prognosis is the best way to point up the inconsistencies in 
the 700-mb prognosis. 
The 700-mb prognosticator must be familiar with the sea- 
level analysis and this familiarity influences him not only in 
the movement of systems at 700 mb but also in the deepening 
and filling of the systems. The 700-mb and sea-level analyses 
and prognoses require parallel and coordinated treatment. 
It is inherent in the whole prognostic procedure that the sur- 
face chart tends to be the dominant one, because the surface 
network of stations is much denser and provides more fre- 
quent reports than the upper-air network. Therefore, there is 
a tendency to start from the surface when making a 700-mb 
prognosis, and final revisions are usually made on the basis 
of the latest three-hourly map. Any system which is important 
enough to be included in the 30-hr surface prognosis is of 
sufficient magnitude to affect, even though slightly, the 700- 
mb contours. 
755 
The close connection between the sea-level pressure pat- 
term and the 700-mb surface makes many studies of sea-level 
systems applicable indirectly in the prognostication of 700-mb 
contours. Thus Bowie and Weightman normals, C.1.T. types, 
Palmer’s method, analogues, etc., all contribute to the 
700-mb prognosis. However, here again good judgment is 
the prime factor; there is always the question of how much 
such objective measures should be modified or ignored. They 
cannot be used blindly. Considerable qualitative use is made 
of the characteristic behavior of storms in a typical sequence. 
The path and speed of the storms, the isobar and contour 
patterns, all contribute to the final version of the prognosis. 
Any deepening of the surface systems will usually cause a 
corresponding deepening of the 700-mb systems. Therefore, 
any kinematic or dynamic effects which are known from ex- 
perience or from theory to favor deepening of surface systems 
will have to be considered in making a 700-mb prognosis. Con- 
sideration must be given to such kinematic processes as the 
converging of two katallobaric systems or of the movement 
of a low in such a direction as to take it under progressively 
lower contours at 200 mb, and to such dynamic processes as 
the converging of fresh cA and m7’ air masses. 
Sometimes an inactive front (the most pronounced cases 
are on the Texas and Louisiana coasts) will become very ac- 
tive and rapid cyclogenesis will take place on it because of 
the approach of a cold trough at 700 mb or higher and the 
subsequent coupling of the cold trough with the sea-level cold 
front. This coupling has the effect of steepening the slope of 
the cold front and of increasing its vertical extent. As a result, 
the total mass of air involved and the total temperature differ- 
ence are much greater than those involved in the original 
front. The deepening storm quickly affects the 700-mb con- 
tours and a very important feature of the chart is created. 
These storms usually move fast since they are generated in 
well-developed upper troughs. 
A frequently recurring feature is the movement of sea- 
level lows and highs into the region of the mean trough. This 
process usually causes a mutual deepening of the sea-level 
and of the 700-mb troughs; similarly it causes weakening of 
the sea-level high and a corresponding weakening of the 
700-mb ridge. 
Frequent use is made of the 200-mb contours in forecasting 
the deepening of surface lows. Often the high-level contours 
will be only slightly influenced by the surface system, and if 
the future position of a surface low is known, it is found from 
experience that when the low moves into a region of lower 
200-mb height, it will deepen by approximately 60 per cent 
of the amount which would result from projection onto the 
ground of the change in height of the 200-mb level above the 
surface low. This effect causes a corresponding change in the 
700-mb surface. 
Contrary to the general rule that a fast-moving low will 
not deepen, lows in well-established troughs with a strong 
gradient on the warm side will move rapidly and if the move- 
ment has a good component northward, the storm will deepen. 
This feature of movement or steering with the upper winds 
is generally known but it has many modifying factors. 
Cold air moving into the region to the rear of an upper- 
level trough will tend to cause the trough to retrograde. The 
timing of the action of the mechanism is of the utmost im- 
portance. If the initial trough is in a position where it will 
move (i.e., where topographic and kinematic reasons favor 
its continued movement), the cold air will tend only to make 
the flow more westerly and cause the trough to move faster. 
Ordinarily, however, the cold air will cause the trough to 
move more slowly or even to retrograde. 
Frequently supergradient winds are noticed, or strong winds 
