The winter synoptic evolution of the western North Atlantic and its influence on the atmospheric boundary layer is described by means of a regime classification based on Self-Organizing Maps applied to 12 years of data (2009–2020). The regimes are classified into categories according to daily 600-hPa geopotential height: dominant ridge, trough to ridge eastward transition (trough-ridge), dominant trough, and ridge to trough eastward transition (ridge–trough). A fifth synoptic regime resembles the winter climatological mean. Coherent changes in sea-level pressure and large-scale winds are in concert with the synoptic regimes: (a) the ridge regime is associated with a well-developed anticyclone; (b) the trough-ridge gives rise to a low-pressure center over the ocean, ascents, and northerly winds over the coastal zone; (c) trough is associated with the eastward displacement of a cyclone, coastal subsidence, and northerly winds, all representative characteristics of cold-air outbreaks; and (d) the ridge–trough regime features the development of an anticyclone and weak coastal winds. Low clouds are characteristic of the trough regime, with both trough and trough–ridge featuring synoptic maxima in cloud droplet number concentration (Nd). The Nd increase is primarily observed near the coast, concomitant with strong surface heat fluxes exceeding by more than 400 W m −2 compared to fluxes further east. Five consecutive days of aircraft observations collected during the ACTIVATE campaign corroborates the climatological characterization, confirming the occurrence of high Nd for days identified as trough. This study emphasizes the role of boundary-layer dynamics and aerosol activation and their roles in modulating cloud microphysics. Plain Language Summary The synoptic evolution of boundary layer clouds over the western North Atlantic is described by means of a regime classification based on Self-Organizing Maps. The analysis is able to capture events with low and high low-cloud coverage. High-cloud coverage days are associated with cold-air outbreaks (CAOs). The combination of cold and dry conditions gives rise to an enhancement of surface heat fluxes during CAO, consistent with an increase in cloud fraction. In addition, prevailing winds during CAO days explain the occurrence of a synoptic maximum in cloud droplet number concentration, linked to transport of continental aerosol over the ocean. Overall, the dynamics of midlatitude low clouds substantially differ from archetypal stratocumulus clouds regimes.