Eaton Canyon is situated within the south-southwest facing coastal slopes of Southern California, offering it a unique climate year-round. The San Gabriel Valley of Los Angeles County is considered to have a Mediterranean climate characterized by hot, rainless, long summers and cool, damp, short winters. Much of the vegetation, however, is more representative of a hot semi-arid desert due to the infrequent but heavy nature of winter storms.
Weather conditions in Eaton Canyon are typically friendly year-round for almost all outdoor recreation. However, conditions can get dangerous when heavy rain falls or temperatures get too high. Always check the forecast and take basic safety precautions before visiting the canyon.
Winter
Winter in Eaton Canyon is predominantly mild, damp, and breezy. During this season, cold rain storms from the North Pacific arrive periodically and dump heavy rain on the steep mountain slopes. Between the months of December and April, Eaton Canyon sees approximately 85% of its average rainfall for the entire year. It is critical that rainstorms reach Southern California during this time, as the other times of year are naturally very dry.
California winters can be exceptionally variable from year to year in terms of both temperature and rainfall. While some winters are cool and wet, and others can be mild and dry. In winter 2017 and winter 2019, Eaton Canyon experienced several feet of rainfall, cool temperatures, and even low elevation snow events on local mountain slopes. During winter 2020 and winter 2021, the weather was exceptionally mild all winter long, with occasional heat waves, sporadic rainfall, and relatively dry and windy conditions.
During these months, visitors to Eaton Canyon need to be mindful of rainstorms as the canyon has a history of deadly flash flooding. Visitors should note that Eaton Canyon and all access points are closed when the National Weather Service issues either a ‘flash flood watch’ or ‘flash flood warning’ for the San Gabriel Valley. If heavy rain begins to fall while visiting Eaton Canyon, it is highly advisable that visitors make their way away from the wash as quickly as possible. Flash flooding can occur even if it is not raining where you are, and rock slides are common within El Precipicio Gorge north of the Mount Wilson Toll Bridge during heavy rain.
Wintertime temperatures in Eaton Canyon are cool to warm, with the average daytime highs around 70 degrees in December, January, and February. Heavy rainstorms can keep daytime temperatures in the 50s, while traditional Santa Ana winds and warm spells can push temperatures as high as the low 90s. Overnight low temperatures are typically in the 40s throughout the winter, but can occasionally drop into the mid 30s during the coldest winter mornings. Occasional frost does occur, especially higher up in elevation.
Cold, northerly Santa Ana winds can bring strong wind events to Altadena and Pasadena during the overnight hours in winter, though they typically subside after sunrise. Thunderstorms are an infrequent occurrence. Snow can occasionally accumulate down to 3,000 ft. in the canyon during the coldest of storms. Sometimes, flakes themselves can fall in the canyon, though this only happens once or twice per decade.
Spring
The spring months in Eaton Canyon are variably warm, damp and sunny, and they last from March through early June. Many flowering plants begin to bloom during this time as the final Pacific rain storms of the year depart. Moss grows, tall grasses cover the hills, and beautiful Western Sycamores begin to leaf out. For these reasons, spring is the most attractive time of year to visit Eaton Canyon as warm, sunny conditions can coincide with a gushing river and waterfall.
In late spring, the infamous coastal California marine layer becomes more prominent and conditions can turn rather overcast in May and June. Some years are cloudier than others, but these gloomy conditions can really keep temperatures cool during the morning hours. Low clouds and fog are an occasional occurrence within the canyon.
Temperatures slowly warm through the spring months, with average highs warming from the low 70s in March to the low 80s in early June. Heatwaves can push temperatures well over 100°F as early as April, though excessive heat in spring is very uncommon. Rainstorms can keep temperatures much cooler well into May should they occur. Overnight low temperatures slowly begin to warm as well, and are typically in the low-50s by May.
Summer
Summer in Eaton Canyon begins suddenly in mid-June and is extremely hot, dry, and long. Summer-like conditions can last well into October in Southern California, a span of five months. It typically does not rain whatsoever during this period, and so vegetation begins to dry out.
The average high temperature in Eaton Canyon in July, August and September is well over 90°F. On average, Eaton Canyon sees 13 days per year with an afternoon temperature of 100°F or warmer, and 35 afternoons with a high temperature of 95°F or warmer. Overnight low temperatures remain on the cool side compared to the rest of the desert southwest, frequently falling below 70 degrees. However, heatwaves and offshore winds can keep overnight temperatures in the canyon in the 80s and even 90s.
The hottest air temperature ever recorded in Pasadena was 115°F on September 5, 2020. The hottest overnight low temperature on record in Eaton Canyon was a sweltering 96.3°F at the Pinecrest Drive weather station on the night of September 5-6, 2020. Temperatures remained over 100°F for all but 30 minutes that night.
It is highly advisable that visitors to Eaton Canyon in summer avoid the hottest daytime hours. Heat exhaustion is a common occurrence, and it can be deadly. Learn the signs before heading out.
Eaton Canyon Falls is the most accessible waterfall in all of Los Angeles due to the canyon’s proximity to the 210 Freeway and the Metro Gold Line. Due to the height of the local mountains, this waterfall essentially has an endless source of water and unlike many other waterfalls in Southern California, it does not dry up in summer, even during extreme drought. Mind the crowds during this time, bring plenty of water to drink, and avoid the peak heat hours of the afternoon.
Autumn
Autumn in Eaton Canyon remains very hot, dry, and smoky compared to summer. Temperatures begin to slowly cool, especially at night as summer comes to a close. Do not be fooled, however, as heat waves can remain very powerful.
After months baking in the hot Southern California sunshine, Eaton Canyon becomes exceptionally dry regardless if the previous winter’s rains were significant. Wildfire danger is very high in autumn as vegetation dries out, Santa Ana winds push new wildfires that do start, and fire fighting resources are stretched thin across the state.
September is the most likely month to contain the hottest temperature for the entire summer, with October holding that title in 10% of all years since records began in 1908. Temperatures cool significantly in November, though heat waves can still push temperatures towards 100°F.
Cold North Pacific rain storms return to Eaton Canyon sometime between October and early January. Exactly when depends on the atmospheric setup during the given year. The first rainfall of winter always breathes new life into the canyon, suppresses wildfire danger, brings our waterfall back to life, and releases an addictive aroma canyon-wide that brings naturalists back every year. Visitation in the canyon typically picks up after the first rainfall of the season.
Climate Change
Regionally, the western United States is being severely impacted by anthropomorphic climate change. Hotter temperatures, more intense droughts, longer wildfire seasons and heavier rainfall are symptoms of this process. Water shortages have become a major problem in recent years as groundwater is depleted, Lake Mead in Nevada is drained, and winter snowmelt becomes insufficient to refill reservoirs. Extremely wet years that force reservoirs to let go of water (Lake Oroville, 2017) are followed by extremely dry years that make us wish we could get that water back.
Eaton Canyon experiences climate change mostly in the form of drought and higher temperatures. April, September and October are the fastest warming months of the year with climate change, while December is the slowest warming month. Average temperatures in Pasadena have risen approximately 6.5°F since weather record keeping began in 1908. A significant portion of this warming has been from the urban heat island (UHI) of Los Angeles, where dark roofs and pavement help heat the city more than surrounding rural areas. Urban Heat Islands are a form of climate change, should be taken seriously, and have deadly consequences when they remain unaddressed.
Rainfall in California is not necessarily decreasing with climate change. Instead, it is becoming more erratic from year to year, with bone dry years being succeeded by exceptionally wet winters. California always had a variable climate related to winter precipitation, but climate change is making this variability even more extreme. In Eaton Canyon, this means more years like 2019 where the mountain slopes above Eaton Canyon received 40” of rain, and more years like 2021 where the same slopes barely record 7” of measurable rainfall for 18-straight months.
Locally, the net change in rainfall is a loss. While Eaton Canyon remains a Mediterranean climate, the semi-arid desert climate classification has expanded west of the Southern California mountains and now includes much of Los Angeles, San Bernardino and the southern San Gabriel Valley. Climate change threatens many species in Eaton Canyon that cannot survive the ever stronger heat waves. Temperatures reached all time record highs on July 6, 2018 across the Los Angeles basin, only for another heat wave on September 6, 2020 to shatter those new records.
