Los Angeles Part 2

Since Los Angeles is a city in close proximity to the coast, a Mediterranean climate (Mediterranean climate and Köppen Climate Classification System: http://www.eoearth.org/view/article/162272/), and surrounded by mountains, Santa Monica and San Gabriel, the two air masses that affect the region are maritime tropical in the summer and maritime polar in the winter. The picture below shows the path the air masses take but as one can discern the only two that actually travel towards southern California are the maritime air masses. The Continental Tropical air masses may travel into southern California by means of the Santa Ana winds during the colder seasons.

Source: http://www.geog.ucsb.edu/~joel/g110_w08/lecture_notes/midlat_surface/agburt09_01.jpg

One interesting note is that as the mP air mass travels over the San Gabriel mountains it loses its moisture and that is one reason why the eastern side of southern California generally varies greatly in temperature than the western side. (http://classroom.synonym.com/air-mass-influences-weather-pacific-coast-other-6144.html) Los Angeles is generally affected by cold fronts in the winter which would be responsible for the, somewhat, cold and wet climate around that time. Taking a look at a accuweather.com surface map (http://www.accuweather.com/en/us/national/weather-surface-maps) showing frontal system for the US for the next few days one notices that Southern California is due for another cold front by Friday which would probably bring some cold and wet weather. 

Source: http://www.intellicast.com/National/Surface/Mixed.aspx

This map shows surface analysis and clouds over the continental United States. Looking at Los Angeles, the red star, one would notice minimal cloud coverage as well as a neighboring low pressure system in Arizona as well as an approaching cold front approaching due to a mid-latitude pacific cyclone (Raphael, Marilyn, and Gerald Mills. "The Role Of Mid-Latitude Pacific Cyclones In The Winter Precipitation Of California.")

The San Gabriel mountain range is also responsible for the Santa Ana winds, strong winter winds that bring plenty of heat. As high pressure builds over the deserts they bring strong and cool winds over the mountains that heat adiabatically as they cross the mountains which can sometimes drive temperatures in Los Angeles to 90F in December or January. These winds generally only happen during the colder months because the LA basin in warmer than the deserts during fall and winter and are responsible for most of the wild fire season in Southern California. (http://people.atmos.ucla.edu/fovell/ASother/mm5/SantaAna/santa_ana_faq.html)

Source: http://cnap.ucsd.edu/santa_ana.html