Another thing... squall lines tend to be positively tilted storms whereas discrete tend to be negatively titled storms.

Negatively tilted storms allow more updraft that is perpendicular to the surface (creating shear) whereas positively tilted storms have a parallel wind field (straight line winds)

FYI... here are their meteorological society explanations.

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.

This post was edited on 3/1/12 at 4:19 pm