How Should I Use the Location of Troughs Information in Flight Planning?

An aviation meteorologist explains why troughs are important enough to depict on prog charts.

Wednesday, August 24, 2022 at 07:38 AM ET

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PROG charts on the aviationweather.gov site depict a dotted yellow line shown as "troughs." [Courtesy: aviationweather.gov]

Key Takeaways:

Surface pressure troughs are elongated areas of low pressure that indicate a wind shift and pressure change (fall then rise), similar to a front but without separating air masses or typically having distinct weather.
Most non-frontal surface troughs do not cause significant weather like clouds or precipitation, especially in dry or stable conditions, and often dissipate above the planetary boundary layer.
Despite generally being benign, troughs can act as a catalyst for convection and adverse weather if sufficient moisture and atmospheric instability are present.
Pilots should identify troughs on PROG charts as potential wind shift lines and differentiate them from other dashed lines like intermediate isobars or convective outflow boundaries (which are not depicted on PROG charts).

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Q: The PROG charts on the aviationweather.gov site depict a dotted yellow line shown as “troughs.” The location is seemingly random and sometimes between two closely positioned high pressure areas with no forecast precipitation and doubtful significant weather at those spots. Can you explain why this is important enough to depict on the PROG charts? How should I use the location of troughs information in my flight planning?

A: Before every flight, pilots should become familiar with the latest surface analysis and PROG charts to identify the major weather systems that are expected to affect the conterminous U.S. and southern Canada. These charts are made available on the aviationweather.gov and wpc.ncep.noaa.gov websites and depict an isobaric analysis—or forecast of pressure (in millibars) adjusted to mean sea level—that includes a few familiar features such as high and low pressure centers, as well as the location and type of surface frontal zones. They also may include a few unfamiliar features such as drylines, squall lines, convective outflow boundaries, intermediate isobars, and what are referred to as surface pressure troughs.

A front is an example of a surface pressure trough. [Courtesy: aviationweather.gov]

In general terms, a trough is simply an elongated area of low pressure with no distinct low-level pressure center. A trough is an axis (line) that defines both a wind shift and pressure change like a frontal zone.