Abstract This study develops an objective method of identifying terrain-trapped airflows (TTAs) along the coast of Northern California and documenting their impact on orographic rainfall. TTAs are defined as relatively narrow air masses that consistently flow in close proximity and approximately parallel to an orographic barrier. A 13-winter-seasons dataset is employed, including observations from a 915-MHz wind profiling radar along the coast at Bodega Bay (BBY, 15 m MSL) and surface meteorology stations at BBY and in the coastal mountains at Cazadero (CZD, 478 m MSL). A subset of rainy hours exhibits a profile with enhanced vertical shear and an easterly wind maximum in the lowest 500 m MSL, roughly the same depth as the nearby coastal terrain. Both flow features have a connection to TTAs along the coast of Northern California. Based on the average orientation (320°–140°) and altitude of nearby topography, mean wind direction in the lowest 500 m MSL ( ) between 0°–140° is used as the initial criterion to identify TTA conditions. Application of this threshold yields a CZD/BBY rainfall ratio of 1.4 (3.2) for TTA (NO TTA) conditions. More detailed analysis of the relationship between and orographic rainfall reveals that an upper threshold of 150° more precisely divides the TTA and NO-TTA regimes. A sensitivity analysis and comparison with a TTA documented in a previous case study show that the best TTA identification criteria correspond to with a duration of at least 2 h. This objective identification method is applied to seven case studies in Part II of the present study.