Longwave Trough
A trough in the prevailing westerly flow aloft which is characterized by large length and (usually) long duration.
Negative-tilt Trough
An upper level system which is tilted to the west with increasing latitude (i.e., with an axis from southeast to northwest). A negative-tilt trough often is a sign of a developing or intensifying system.
Positive-tilt Trough
An upper level system which is tilted to the east with increasing latitude (i.e., from southwest to northeast). A positive-tilt trough often is a sign of a weakening weather system, and generally is less likely to result in severe weather than a negative-tilt trough if all other factors are equal.
Pre-Frontal Trough
An elongated area of relatively low pressure preceding a cold front that is usually associated with a shift in wind direction.
Shortwave Trough
Also called Shortwave; A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave trough.
Trough
An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge.
Wave Trough
The lowest part of the wave.

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