The brown marmorated stink bug, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae), is an invasive pest that has been resident in California since 2006. To better understand its seasonal phenology, we used baited traps to estimate nymph and adult population densities in midtown Sacramento, the focal area of the Northern California invasion. Adult H. halys populations were found soon after trapping began in February (2015–2016) or March (2014); the first egg masses for 2014, 2015, and 2016 were found on 5 May, 17 April, and 12 April, respectively, and the first nymphs were found 3 June, 19 May, and 9 May, respectively. There were two generations per year, with one peak in June and another in September. Summer temperatures above 36°C in July and August were associated with reduced catches in traps of both nymphs and adults. This extreme heat may have helped to form two clear nymph peaks and suppressed egg deposition. In 2016, two trap types and four lures were also compared. Trap type influenced season-long nymph captures, with fewer nymphs in double cone traps than pyramid traps. Lure type influenced season-long trap catch, with more nymphs and adults trapped with the Rescue lure than the AgBio Combo lure, Alpha Scents, or Trécé Pherocon Combo lures, although this difference was only associated with the capture of nymphs and we did not compare for longevity or seasonal variation. These data are discussed with respect to H. halys' phenology from the mid-Atlantic region.