It has high winter temperatures and high summer temperatures too.
The main reason for Sacramento's warmth compared to some other cities at the same latitude is, like in many cases, due to the city's surrounding topography, as can be seen in the topographic map below:
In regards to the climate of Sacramento, it is probably better to consider why it is seasonally warmer than cities on the same latitude.
Sacramento is located in a regionally low elevation Great Valley (also Central Valley) (1), in between two roughly parallel mountains ranges that converge in the north of the Valley in the Klamath Ranges. The Coastal Range on the west and the Sierra Nevada to the east (2).
The Köppen climate classification for Sacramento is
Csa, which is defined as a Mediterranean environment with hot summers (1). The Coastal Range largely shields the city (and the Valley) from the climate (especially much of the humidity) of the Maritime climate to the west, and the Continental climate to the east (2). Essentially, according to the WRCC, the climate of the Valley (including Sacramento) is intermediate between the climates either side of the bordering ranges (2).
The very cold winters that can occur in the Continental climate are blocked by the Sierra Nevada. Even if cold air does get over the mountain, the air warms as it compresses as descends into the Valley, and hence is milder when it reaches the Valley (2).
In regards to the Urban Heat Island, The US EPA (3) performed measurements and observations of Sacramento between 1998-2002, revealing that there is an urban heat island of the order of an average increase in temperature in the city of 0.5-1°C (0.9-1.8°F) compared to the surrounding non-urban areas.