A shortwave trough is a smaller disturbance embedded in the larger upper-level flow, marked by a local dip in geopotential heights and a pocket of enhanced vorticity that moves downstream faster than the longwave pattern. On NOAA's upper-air charts, shortwaves are also called vort maxes, pockets of energy, or upper-level disturbances.
How it fits into the flow
The mid-latitude westerlies are organized into a few longwave troughs and ridges at any given time, typically three to seven of them ringing the hemisphere. Shortwaves are perturbations riding inside those longwaves. NOAA JetStream defines them as features shorter than about 3,700 miles, moving at roughly 23 to 35 mph depending on season, while the longwave pattern moves much more slowly.
As a shortwave rotates through the base of a longwave trough, it deepens and shifts the longwave temporarily. When a shortwave, sometimes called the kicker, approaches a stationary longwave trough, it can nudge that larger feature back into motion.
Why forecasters care
Ascent in the free atmosphere is driven in part by differential positive vorticity advection through a layer. A shortwave carries a compact vorticity maximum, so the region downstream of the vort max sits under a broad zone of forced lift. If that lift arrives over a warm, moist boundary layer, it removes the cap and sets off convection.
Timing the shortwave is often the whole question on a marginal severe day. A shortwave that arrives at peak heating over a moist warm sector can produce a robust event. The same shortwave arriving at midnight, after inhibition rebuilds, may pass with only elevated storms or nothing at all.
Where it can mislead
Shortwaves are noisy on model output and often shift 100 miles or more between runs. Two models that both show a strong day may place the vort max in very different spots, and that placement determines who sees storms.
A shortwave alone does not create severe weather. It supplies lift and enhances shear, but if moisture return is shallow or instability is limited, the passage can produce nothing more than a line of cumulus and a wind shift.