From: Haby Hints:
NEGATIVE TILTED TROUGH- A trough which tilts from the northwest toward the southeast. This situation creates instability since cold temperatures in the mid and upper levels advect over warmer air at the surface. A negative tilt is a sign a trough is mature.
POSITIVE TILTED TROUGH- A trough which tilts from the northeast toward the southwest. Often a trough is positive tilted in it's development stage.
From: National Weather Service:
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.
