lows move with the mean speed of the warm air above
them, they will be rapidly moving systems.
l If the highest mean temperatures occur under the
700-hPa ridge (isotherms and contours in phase), the
ridge itself is warm while the low is cold; therefore, the
low will move slowly.
. Lows move with a speed of approximately 50
percent of the thermal wind for the 1,000- to 500-hPa
stratum, and approximately 75 percent of the thermal
wind of the 1,000- to 700-hPa stratum.
Movement of Lows in Relation to the Jetstream
Some of the rules for moving lows in relation to the
jetstream position were mentioned previously. One
basic rule however, states that “highs and lows situated
under or very near the jetstream behave regularly and
follow the steering indications.” Minimum deviations
occur when the upper flow does not change with time.
Forecasting the Movement of Lows by
Statistical Techniques
Since it requires many years of experience and a
photographic memory to develop a mental catalog of
weather patterns, a weather type or normal path
classification is a boon to the inexperienced forecaster
in identifying situations from the past for application to
the present. There are many normal and average
conditions to regulate behavior patterns of future
movement and development. However, there are also
many deviations from the norm. The season of the year
and topographical influences are factors to be
considered. If we could catalog weather types or
average types, and the systems would obey these rules,
it would greatly simplify the art of forecasting.
However, as a rule, this does not occur. Use these
statistical techniques, but do not rely too heavily on
them.
Normal Tracks
In 1914, Bowie and Weightman published
climatological tables of the average, by month, of the
24-hour speed and direction of cyclonic centers in the
United States. The storms were classified with respect
to the point of origin and the current location of the
centers. Although these tables appear to be antiquated,
some of them resemble relatively recent classifications;
therefore, they are of some value to the present-day
forecaster.
The Marine Climatic Atlas also contains average
storm tracks for each month of the year for areas over
the oceans of the world. Other publications are
available that give average or normal tracks for other
areas of the world.
Prediction of Maritime Cyclones
This method is an empirically derived method for
objectively predicting the 24-hour movement and
change in intensity of maritime cyclones. The technique
requires only measurement of the 500-hPa height and
temperature gradients above the current surface center,
and determination of the type of 500-hPa flow within
which the surface system is embedded. Full details of
this method are described in The Prediction of Maritime
Cyclones, NAVAIR 50-1P-545.
The deepening prediction should be made first, as
this will often give a good indication of movement.
The explosive intensification of maritime cyclones
is a fairly common phenomenon, but is presently among
the most difficult problems to forecast. Conversely,
there are many situations in which it is important to
predict the rapid filling of cyclones. This technique
gives an objective method for predicting the 24-hour
central change in pressure of those maritime cyclones
whose initial positions lie north of 30 degrees north
latitude. Further, the technique applies to the winter
months only (November through March), although it
may be used with some degree of confidence in other
months.
The following factors are considered the most
important:
. The location of the surface cyclone center with
respect to the 500-hPa pattern.
. The strength of the 500-hPa flow above the
cyclone center.
l The 500-hPa temperature gradient to the
northwest of the surface center.
Of the lows that deepened, the deepening was in
general greater, the stronger the 500-hPa contour and
isotherm gradient. The study also indicated that the
preferred location for filling cyclones is inside the
closed 500-hPa contours, and that deepening cyclones
favor the region under open contours in advance of the
500-hPa trough. The remaining portions of the pattern
indicate areas of relatively little change, except lows
located under a 500-hPa ridge line fill.
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