Bromus tectorum (downy brome) is an invasive winter annual grass species naturalized throughout western North America. Within the small grain production region of the Pacific Northwest (PNW) downy brome is a ubiquitous and competitive weed. Previous research has documented regional specificity of downy brome phenotypic development, while considerable variation in phenotypic development has been noted between regions. In 2011 and 2012 130 downy brome accessions were collected from across the PNW small grain production region. A genotype-by-sequencing approach was employed to call molecular markers, generate population genetic statistics, and classify 88 of the 130 downy brome accessions into genetically similar clusters. Individuals were assigned to one of six genetic clusters using 384 single-nucleotide polymorphisms and discriminant analysis of principal components clustering approach. Accessions were transplanted to three common garden field sites to document and model the timing of development. The timing of development stages was modeled against cumulative growing-degree-days (GDD) to develop herbicide application thresholds to aid in control of downy brome within small grain fields. The estimate for mature seed production varied from May 18th to June 20th depending upon the location of the common garden. Earlier production of mature seed was observed following more severe winters compared to mild winters, implying a role of vernalization regulating the timing of development. Greenhouse experiments were initiated to characterize the response of early, intermediate, and late-to-flower downy brome accessions to various vernalization treatments and quantify expression of Brachypodium distachyon gene vernalization 1 (BdVRN1). Downy brome flowering in response to vernalization treatments was linked to the expression of BdVRN1, implying the molecular controls of flowering in downy brome are similar to the controls of other temperate grass species. Downscaled climate modeling was paired with Global Climate Change models to project downy brome development thresholds under future climate scenarios. Downy brome development is anticipated to advance 16 to 34 days across the small grain production region of the PNW. The earlier development of downy brome will require earlier control inputs, which may conflict with earlier rainfall projected under future climate scenarios.