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.

. 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.

. 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.

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.

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.

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.

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