l If the pressure gradient is forecast to increase,
decrease the component by the least percentage.
. If the pressure gradient is forecast to decrease,
decrease the component normal to the front by the
highest percentage value.
l If the pressure gradient is forecast to remain
static, decrease the component normal to the front by the
middle percentage value as listed above.
Upper Air Influences on the Movement of
Surface Fronts
A number of the rules relating the upper air contours
to the movement of fronts were discussed in the AG2
TRAMAN, volume 1. You saw that a slow moving cold
front has parallel contours behind the front, and in a fast
moving cold front, the contours were at an angle to the
front, and at times normal to the front.
Some additional rules are stated below:
. During periods of strong, continued westerly
flow aloft (high index) over North America, surface
fronts move rapidly eastward. A rule of thumb, the front
will move eastward at a speed that is 50 percent of the
500-hPa flow and 70 percent of the 700-hPa flow.
l Cold fronts associated with cP outbreaks are
closely dependant on the vertical extent of the northerly
winds. The following relationships are evident: For cP
air to push southward into the Great Basin from British
Columbia, strong northerlies must exist to at least 500
hPa over the area; for cP air to push southward into the
Gulf of Mexico, northerly and/or northwesterly winds
must extend, or be expected to extend, to at least 500
hPa as far south as Texas; for cP air to push southward
over Florida to Cuba, northerlies must extend to at least
500 hPa as far south as the Gulf States.
FORECASTING THE INTENSITY
OF FRONTS
The following text deals with the forecasting of the
intensity of fronts, as well as indicators of frontogenesis
and frontolysis.
Frontogenesis
Surface fronts generally intensify when one of the
following three conditions and/or combination occurs:
1. The mean isotherms (thickness lines) become
packed along the front.
2. The fronts approach deep upper troughs.
3. Either or both air masses move over a surface
that strengthens their original properties.
Frontogenesis occurs when two adjacent air masses
exhibit different temperatures and density, and
prevailing winds bring them together. This condition,
however, is the normal permanent condition along the
polar front zone; therefore, the polar front is
semipermanent.
Generation of a new front, or the intensifying of an
existing front, occurs during the winter months along
the eastern coasts of the American and Asian Continents.
During this time the underlying surface (ocean) is much
warmer than the overlying air mass.
Frontolysis
Weakening or dissipation of fronts occurs when:
The mean isotherms become more perpendicular
to the front or more widely spaced.
The surface front moves out ahead of the
associated pressure trough.
Either or both air masses modify.
The front(s) meet with orographic barriers.
FORECASTING ISOBARIC
CONFIGURATION
LEARNING OBJECTIVES: Evaluate isobaric
configuration in preparation of surface charts.
Isobars may be constructed on the surface prognosis
either by computing the central pressures for the high
and low centers and numerous other points on the
surface prognosis chart and drawing the isobars, or by
moving the isobars in accordance with the surface
pressure 3-hour tendencies and indications.
A thickness prognosis is used in constructing the
isobaric configuration on your forecast. The steps for
constructing forecasted isobaric configuration by using
the thickness chart are as follows:
1. At a selected point, determine the difference
between the present 500-hPa height and the forecasted
500-hPa height. A forecasted rise at the 500-hPa level
is positive; a forecasted fall is negative.
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