Three-dimensionally localized features are tested as initial conditions (ICs) in linear calculations and then applied to study a problem in nonlinear extratropical cyclogenesis. Localized means the structure is nonzero only in a small region of a larger domain. These features remain nearly coherent and stable in a linear calculation by constructing them from neutral eigenmodes having similar phase speed. The model is the QG potential vorticity tendency equation. The linear form of the model employs a zonal mean flow that can have both vertical and meridional shear but no variation along the mean flow. The construction of a localized feature needs fewer modes when meridional shear is present in the zonal mean flow

The nonlinear simulations are limited to considering whether nonlinear advection favors nonmodal growth (NG) or normal mode baroclinic instability. Calculations test several mean flows and initial condition amplitudes and structures that approximate observed traveling, but not developing, localized troughs. Linearly unstable normal modes are excluded from the ICs and that allows tracking of how quickly growing structures are created by nonlinear advection. We emphasize ICs with mid or upper tropospheric isolated troughs and sufficient amplitude to have either an “open wave” or a “closed contour” in the total streamfunction. Results for selected ICs and basic flows find little NG. Adjacent to the original trough, structures appear soon into the integration whose properties are more consistent with normal mode growth than with NG. Projecting the solution onto eigenmodes finds strong initial amplification of unstable normal modes by the nonlinear terms. In nonlinear integrations eddies evolve towards horizontal sizes that are greater than the linearly most unstable normal mode. Larger initial amplitude leads to faster breakdown of the localization. A leading upper high and trailing lower high form and the IC trough develops upstream-tilt; properties similar to observed cyclogenesis.