In forecast discussions you will come across the terms positive, neutrally and negatively tilted trough. When a trough is positive or neutrally tilted it is usually not referenced at such. However, when it becomes negatively tilted it will be referenced as such. The tilt of a trough is the angle the trough axis makes with lines of longitude. A negatively tilted trough tilts horizontally (parallel to surface) from the northwest to the southeast. What is the big deal about having a negative tilt? (1) indicates a low pressure has reached maturity, (2) indicates strong differential advection (middle and upper level cool air advecting over low level warm air advection). This increases thermodynamic instability. (3) Indicates good vertical wind shear. A deep low pressure, a negatively tilted trough, and a warm and moist warm sector combination east of the Rockies often produces a severe weather outbreak. What causes the negative tilt? (1) Strong middle and upper level winds wrapping around the base of the trough, (2) A strong jet streak near the base of a trough (3) A ridge to the east of the trough (like a sideways inverted trough) (4) Occlusion of low pressure. Below are a positive and negative tilted trough.   |