A large trough of cooler low-pressure air will slide over the western U.S. this week and linger into Halloween, bringing slightly below-average temperatures and a growing window for rain in Southern California.
The Sacramento Bee
Temperatures were spot-on average across California this past weekend, but cooler air will arrive this week and create a window for rain as Halloween nears.
On Monday morning, a few showers will fall along the northern Coast range near Eureka and into the Klamath mountains in far Northern California. These showers won’t make their way to Sacramento, but a few passing clouds over the region are expected late morning and early afternoon as the decaying pulse of earlier rain passes. Otherwise, it will be a nice day with high temperatures in the low 70s across the Sacramento Valley, and slightly warmer further south in the San Joaquin Valley.
Mountain temps will remain cool in the 40s and 50s, with more freezing expected overnight.
The Sacramento Bee 
The Sacramento Bee
On Tuesday and Wednesday morning, low temperatures will drop well into the 40s, and they will collide with the dew point and produce patchy fog, especially in areas where it rained more heavily last week and away from urban cores. Sacramento International Airport, and areas towards the foothills such as Roseville or Folsom, will have the highest chances for fog overnight this week.
Temperatures will continue cooling into the weekend as a trough of cooler low pressure moves, with high temperatures falling into the 60s across the valley on Thursday underneath more cloudy skies.
Halloween weekend rain towards southern half of the state
The cooler low is poised to point rain off the ocean towards the southern half of the state this weekend, potentially meaning a wet round of trick-or-treating for those unlucky to get caught underneath a passing shower.
While Sacramento will escape any serious rainfall on Halloween, the same cannot be said for San Luis Obispo, Bakersfield, Los Angeles and the Sierra. Far Northern California could also see a return of coastal showers.
Forecast weekend rainfall across California, with greater amounts falling in the southern third of the state. The Sacramento Bee
These showers will be powered by the mass of cooler air centered over the Sacramento region, leading to below-average high temperatures as we swing into November. Expect highs in the low 60s or upper 50s and passing clouds. As cool air mostly stays put into early November, the overall pattern looks more unsettled beyond Nov. 5.
The Sacramento Bee
Sean Macaday covers the weather for McClatchy Media in California. Originally from Chicago, Sean worked 6 years covering the weather on local television in Minnesota. He holds a bachelor’s degree in meteorology and marine science from the University of Miami.