In this study, a month-long simulation of gravity waves over North America and North Atlantic Ocean is performed using the mesoscale model MM5 for January 2003, verified with Advanced Microwave Sounding Unit-A (AMSU-A) radiance observations in the upper troposphere and lower stratosphere. According to the monthly mean statistics, four regions of strong gravity wave activities are found both in the simulation and the AMSU-A observations: northwestern Atlantic, Appalachian Mountains, Rocky Mountains and Greenland, respectively. Those over the northwestern Atlantic Ocean are strongly associated with the midlatitude baroclinic jet-front systems, while the other three regions are apparently collocated with high topography. Imbalance diagnosis and numerical sensitivity experiments of a strong gravity wave event during January 18-22 show that the gravity waves are strongly linked to the unbalanced flow in the baroclinic jet-front system. The gravity waves are usually radiated from the upper tropospheric jet exit region with maximum nonlinear balance equation residual ( Δ NBE; key indicator of flow imbalance), distinctly different from other surface sources. Flow imbalance related strongly to tropopause folding and frontogenesis of the large-scale background flow. Similar wave characteristics are simulated in experiments with different microphysics and grid resolutions. The Δ NBE is again shown to be a good predictor for jet-front related gravity waves, suggesting its potential application to gravity wave parameterizations for global and climate models.