  The prevailing conceptual model for the production of severe local storm
(SLS) environments over North America asserts that upstream elevated terrain
and the Gulf of Mexico are both essential to their formation. This work tests
this hypothesis using two prescribed-ocean climate model experiments with North
American topography removed or the Gulf of Mexico converted to land and
analyzes how SLS environments and associated synoptic-scale drivers (southerly
Great Plains low-level jets, drylines, elevated mixed layers, and extratropical
cyclones) change relative to a control historical run. Overall, SLS
environments depend strongly on upstream elevated terrain but weakly on the
Gulf of Mexico. Removing elevated terrain substantially reduces SLS
environments especially over the continental interior due to broad reductions
in both thermodynamic and kinematic parameters, leaving a more zonally-uniform
residual distribution that is maximized near the Gulf coast and decays toward
the continental interior. This response is associated with a strong reduction
in synoptic-scale drivers and a cooler and drier mean-state atmosphere.
Replacing the Gulf of Mexico with land modestly reduces SLS environments
thermodynamically over the Great Plains and increases them kinematically over
the eastern U.S, shifting the primary local maximum eastward into Illinois; it
also eliminates the secondary, smaller local maximum over southern Texas. This
response is associated with modest changes in synoptic-scale drivers and a
warmer and drier lower-tropospheric mean state. These experiments provide
insight into the role of elevated terrain and the Gulf of Mexico in modifying
the spatial distribution and seasonality of SLS environments.
