|WHAT IS A "NEGATIVELY TILTED TROUGH"?|
METEOROLOGIST JEFF HABY
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.
The image below is that of a negatively tilted trough located across the Southeast U.S. Notice the trough tilts from the NW to the SE and wind speeds are highest within the negative tilt.
METEOROLOGIST JEFF HABY
The best chart to use when examining the trough / ridge pattern is the 500-millibar chart. As you know, troughs are synonymous with cooler than normal temperatures while ridges are synonymous with warmer than normal temperatures. Troughs can be tilted in one of three ways which include: positive tilt, neutral tilt, and negative tilt. In the Northern Hemisphere, a positively tilted trough tilts from the northeast toward the southwest, a neutrally tilted trough tilts from north to south, and a negatively tilted trough tilts from northwest toward southeast. As a mid-latitude cyclone develops, it tends to begin as a positive tilted trough and end as a neutrally or negatively tilted trough.
The atmosphere is most unstable when a large trough in association with a strong mid-latitude cyclone becomes NEGATIVELY TILTED. Why? Because on the right side of the trough, the negative tilt causes cold air advection in the upper levels of the atmosphere while the PBL is warm and humid (especially if this situation occurs east of the Rocky Mountains in the fall or spring). Cold air above warm air creates thermodynamic instability and convective instability. A strong jet streak can cause a trough to become negatively tilted and contributes to dynamic lifting. It is the jet stream and jet streaks that are responsible for causing troughs to become more amplified or less amplified. The jet streaks also contribute to the tilt of a trough. Look at the 500-mb chart each day and see if the troughs over the U.S. are highly or weakly amplified and positive, neutral, or negatively tilted.