SHORT-RANGE WEATHER FORECASTING 
study of wave lengths with reference to the speed of 
movement of contour systems is especially important 
at the 500-mb level since this level is nearest the level 
of nondivergence. Also, when vorticity-trajectory com- 
putations are not made, some subjective study of the 
500-mb chart with respect to constant vorticity tra- 
jectories is possible and helpful in forecasting move- 
ments and other developments. 
7. Inspection of the latest pilot-balloon charts for 
trends and changes during the preceding six hours and 
for computation of the 6-hr trough and ridge move- 
ments. The forecaster should be familiar with the diur- 
nal variation of winds under 8000 ft. 
8. Checking the analysis of the last six- and three- 
hourly surface charts with the latest hourly sequences, 
noting recent frontal passages and movement of pre- 
cipitation areas. 
9. Reading the latest district forecasts from all avail- 
able forecast centers, noting areas where precipitation 
and any unusual weather are forecast as well as any 
weather that does not seem warranted by the prognostic 
charts, and attempting to rationalize the basis for the 
differing forecasts. 
10. Reading the synopses and forecasts from the 
various airway forecast centers, noting rate of movement 
given for fronts and pressure centers in the area where 
the forecast is made and also noting forecasts of un- 
usual weather. Forecast centers in mountainous and 
distant areas are usually in a better position to deter- 
mine these facts accurately than are remote centrals or 
forecast centers. 
11. Inspecting the surface prognostic chart from the 
weather central and noting deviations from it by the 
preceding district forecaster and probable deviations 
from it by district forecasters in other forecast districts 
as indicated by the district forecasts. This check should 
be repeated when the new six-hourly synoptic chart is 
completed and analyzed. 
Intermediate Steps in the Preparation of the Fore- 
cast. These may include: 
1. Analysis of the new six-hourly synoptic chart, 
using weather-central analysis, modifying it in accord- 
ance with more detailed observational data which the 
forecast center may have in and near its own district. 
2. Analysis of the pressure-change chart, forecasting 
future positions of each isallobaric center. 
3. Using the 24-hr prognostic chart from the weather 
central as a basis, checking the positions and intensities 
of fronts and pressure centers, making such revisions 
as are indicated by the latest surface and upper-air 
charts, using techniques described earlier (pp. 750- 
758). 
4. Final check based on subjective evaluation of 
broad-scale influences of centers of action, zonal indices 
and blocking highs. 
5. Final reconciliation of all conflicting indications, 
objective, subjective, dynamical, and empirical. 
Weather processes are logical, and conflicting indica- 
tions are evidence of an error in computation, in inter- 
pretation of the data and charts, or in reasoning. The 
original error may have been made by the observer, 
759 
the computer, the analyst, or the forecaster but, in 
most calculations and chartwork, the error should be 
fairly obvious from the large amount of reasonably 
accurate information available. The forecaster should 
make the strongest possible effort to harmonize the 
prognostic information derived from a fairly large num- 
ber of sources rather than to reject entirely a portion 
of it. Recomputation and re-evaluation may be neces- 
sary but, usually, the time required is well worth 
while. Most often a forecast error arises in the sub- 
jective weighting of the several factors from which the 
forecast is derived. 
Steps Recommended by Petterssen. The following 
steps have been listed by Petterssen [42, Chap. XI]: 
1. Displacement of pressure systems, fronts, etc. 
2. Forecasting deepening and filling. 
3 Determination of whether new systems will Honma 
4. Readjustment of displacements. 
5. Determination of the position and properties of 
air masses—detailed analyses of clouds, hydrometeors, 
adiabatic charts, moisture patterns aloft. Constant- 
pressure charts will reveal physical and kinematic con- 
ditions of the air masses in question. 
6. Determination of changes in air masses as they 
reach the forecast district. Possibilities of cooling, heat- 
ing, depletion or addition of moisture, subsidence, etc. 
7. Modification of local influences, mountain ranges, 
valleys, lakes, land and sea breezes, and other coastal 
effects. 
8. Last re-examination: What can upset the forecast? 
—deegree of certainty. 
Influence of Main Centers of Action. By following 
the steps outlined above, the forecaster can, with the 
primitive tools at his disposal, complete the pressure 
prognosis for the first 24 hr of the forecast period. 
Because of acceleration and deceleration factors which 
are constantly operating, extrapolation becomes a pro- 
gressively poorer tool with time. For periods in excess 
of 24 hr, the forecaster must expand his horizon in 
space and time. Data describing the instantaneous 
state of the atmosphere are inadequate to define, in any 
detail, the evolution of developments which may sig- 
nificantly affect the weather beyond 24 hr. Large-scale 
circulation changes cannot be ignored and important 
developments in one portion of the hemisphere may be 
compensated for in another far away. The forecaster 
must not become preoccupied with the latest hourly 
sequences or even the last six-hourly weather map. 
The influence of centers of action was pointed out as 
early as 1881 by Teisserene de Bort. The forecaster 
must keep close watch on deviations from the normal 
positions of the semipermanent areas of high and 
low pressure and the resultant position of the mean 
troughs and ridges and carefully consider their prob- 
able effects on migratory depressions and the preferred 
areas of cyclogenesis and anticyclogenesis under the 
prevailing type. 
The centers of action are a conservative factor in the 
prevailing weather, that is, there is a tendency for a 
prevailing weather type to continue. An example of the 
