Terrain over western Washington state regulates how low-level moisture is advected across the state from west to east. While gaps in the Cascade Range allow some low-level moisture to be advected in from the Pacific, the bulk of the moisture advection is confined to levels above 6,000 to 6,500 ft (~800 mb).

Differences in the terrain representation over the Spokane's county warning area (CWA) impact how moisture is represented by the numerical prediction models. For a model with a lower terrain resolution like the AVN, an artificially lower (higher) terrain will result in higher (lower) column integrated values like total precipitable water (TPW). The effects of model resolution and terrain are important to keep in mind when making quantitative comparisons between model, satellite, and other in-situ data.

The AVN smoothes terrain features across Washington, resulting in lower mountains and in general lower elevations. This creates artificially high TPW values in the model output fields and also has implications on model initialization.