Los Angeles Weather In December What To Expect
December in Los Angeles marks a transition from the autumnal calm to the subtle heraldry of winter. It is a month where the city’s Mediterranean climate begins to show subtle shifts, trading intense summer heat for milder, more temperate conditions. This article provides a detailed forecast of what residents and visitors can anticipate regarding temperature, rainfall, and ocean conditions during the final month of the year.
The average high temperature in Los Angeles during December sits around 68 degrees Fahrenheit, a noticeable yet comfortable drop from the peak summer months. Overnight lows typically average around 48 degrees, creating a significant diurnal range that necessitates a light jacket or sweater for evening activities. While the city rarely experiences extreme cold, the humidity levels often make the cooler air feel more penetrating than the numerical temperature suggests.
Precipitation patterns in December are the most significant variable of the month, as this is the beginning of the region’s rainy season. Unlike the sporadic showers of spring, December introduces a more structured storm pattern influenced by the Pacific jet stream. Understanding this variability is crucial for planning outdoor events or simple commutes across the sprawling metropolis.
**The Mechanics of LA's December Skies**
The primary driver of December weather in Southern California is the shifting position of the Pacific storm track. During the summer, high-pressure systems dominate, creating the stable, dry conditions synonymous with beach weather. As the Earth tilts away from the sun, this high-pressure ridge migrates northward, allowing low-pressure systems to intrude into the region.
These atmospheric rivers and winter storms tap into the vast moisture reserves of the Pacific Ocean. When they make landfall, they deliver the majority of the region’s annual rainfall. However, the geography of Los Angeles creates a rain shadow effect. The coastal mountains often force the moist air upward, wringing out precipitation on the western slopes, while the eastern valleys and basins may remain surprisingly dry.
Meteorologists track these systems using tools like the Pacific-North American (PNA) pattern. A negative PNA pattern typically correlates with a more southerly storm track, sending rain directly toward California. Conversely, a positive pattern can divert storms northward, leaving Los Angeles basking in unseasonably dry and mild conditions. This variability means that December weather is never guaranteed, adding a layer of uncertainty to long-range planning.
**Temperature Trends and Microclimates**
While the citywide average provides a baseline, the reality of Los Angeles weather is highly localized. The urban heat island effect keeps downtown and densely built-up areas slightly warmer than the surrounding suburbs. Furthermore, the dramatic elevation changes within the county create distinct climatic zones that residents must navigate.
In the beach communities of Santa Monica and Venice, the ocean acts as a thermal regulator. The water retains summer heat, keeping coastal temperatures milder at night and slightly cooler during the day compared to the valleys. Just a few miles inland, in neighborhoods like Beverly Hills or the Westside, temperatures can be marginally warmer, particularly at night, due to reduced maritime influence and increased concrete density.
The valleys and basin areas, such as the San Fernando Valley, experience the most significant temperature swings. It is not uncommon for a morning in December to feel crisp and cool, with temperatures hovering in the 40s, only to rise to a pleasant 70 degrees under clear afternoon sun. This necessitates a flexible dressing strategy, often termed "layering," to accommodate the changing conditions throughout the day.
* **Coastal Zones:** Expect consistent onshore flows, leading to higher humidity, cloudier skies, and temperatures rarely dipping below 50 degrees at night.
* **Valley Floors:** These areas are prone to temperature inversions, where cool, dense air gets trapped beneath a layer of warmer air, leading to foggy mornings that may not clear until midday.
* **Mountain Foothills:** Communities in areas like the Santa Monica Mountains or the Verdugo Mountains will encounter significantly cooler temperatures, especially at night, and a higher likelihood of encountering frost or even snowfall at the highest elevations.
**Rainfall Expectations and Water Impact**
December is historically the second wettest month of the year in Los Angeles, trailing only January. The arrival of the first significant storm is often a relief, replenishing the local reservoirs and groundwater that have been depleted during the long, arid summer. However, the intensity of these storms can vary dramatically.
The Los Angeles Department of Water and Power monitors these patterns closely, as the rainfall directly impacts the city's water security. A series of atmospheric rivers can fill the aqueducts and restore levels in the San Gabriel Reservoir, easing concerns about water rationing. Conversely, a December with minimal precipitation can cast a long shadow over the water supply heading into the critical summer months.
When rain does arrive, it is often intense and concentrated. A single storm system can drop multiple inches of rain in a matter of hours. This contrasts sharply with the gentle, all-day drizzle sometimes associated with English winters. The downpours can lead to localized flooding, particularly in areas with poor drainage or near concrete channels designed to expedite water flow to the ocean. Drivers are frequently reminded of the dangers of driving through flooded roadways, a common hazard during the season.
**Ocean and Outdoor Activity**
The state of the Pacific Ocean in December dictates much of the outdoor lifestyle, even when the air temperature is mild. The ocean temperature along the LA coast averages around 58 to 60 degrees Fahrenheit. While technically swimmable for the hardy, most residents consider this too cold for prolonged aquatic recreation.
This temperature drop significantly alters the beach dynamic. Summer volleyball crowds dissipate, and the shoreline becomes a domain for dedicated surfers and storm watchers. The winter swells generated by North Pacific storms create legendary surf conditions. Spots like Mavericks in Half Moon Bay, though not technically Los Angeles city, embody the winter surfing ethos, attracting elite athletes to the towering waves generated by these distant storms. For the casual observer, the beaches transform into dramatic landscapes where the power of the ocean is on full display, crashing against the seawall with a force unseen during the calm summer months.
Ultimately, December in Los Angeles is a month of negotiation. It is the time when the city sheds its summer skin, revealing a more complex and dynamic personality. Residents trade sunglasses for layers, dust off rain boots, and adjust their routines to accommodate the whims of the winter sky. For the prepared observer, it offers a unique window into the region’s true climate character—a blend of startling sunshine, moody rain, and the invigorating chill that precedes the promise of spring.
