The trough axis connects the center of a trough or the center of lowest closed height contour (beginning of rotation axis) through the region of maximum cyclonic curvature. A trough axis (in association with a deep low pressure/heights) can be analyzed at each level of the troposphere (surface pressure trough, 850 mb, 700 mb, 500 mb and 300 mb). If the trough axis occurs in the same general region on each of the charts, then the low pressure is referred to as VERTICALLY STACKED. However, if the upper level trough axis (500 mb and 300 mb) is located at a significantly different location than the low level trough axis (850 mb and 700 mb), then the trough is referred to as TILTED WITH HEIGHT.
Strong developing low pressures tend to be tilted with height. Why? Heights is the troposphere are determined primarily by the average temperature of the troposphere below a pressure level of interest. Example: The 500 millibar height is influenced by the average temperature from the surface to the 500 millibar level. The vertical depth of cold air tends to be shallow behind the cold front boundary but is much deeper a few 100 miles behind the cold front boundary. Cold temperatures through a deep layer of the troposphere will produce low heights. The cold air in association with a developing surface mid-latitude cyclone is generally located to the northwest of the surface low. Therefore, heights (the trough axis) will tilt with height from the surface to the upper level (tilts toward the northwest / tilts toward the cold air).
Once lows become occluded, cold air wraps completely around the cyclone. When this occlusion occurs, the low will become vertically stacked since the temperature are about the same in every direction from the low center. Occlusion = vertically stacked = the weakening of a cyclone. Deep developing low pressure = tilted to the NW with height = strengthening cyclone.