Atmospheric River (AR) has become so popularized in the media, it is now a household term that is sometimes misused and misunderstood. This story tracks the historic atmospheric river events of early February 2024 as they developed and then swept through California.
We reviewed the science behind ARs in previous stories on this website, particularly during the storms and floods of last winter, 2023. You will also find numerous stories with discussions and analyses of these events in my forthcoming California Sky Watcher, which will be published by Heyday in a few months. And here we are in 2024, again covering the powerful storms that are feeding off ARs. But this is an El Niño year, which can change everything. (You will also find stories on our website and in my forthcoming California Sky Watcher publication that summarize the science behind El Niño events.) As middle latitude cyclones sweep off the Pacific, winds that blow counterclockwise into these massive storms may first pass over exceptionally warm ocean currents. Such warm water evaporates, loading tropical and subtropical air masses with moisture and latent heat energy, which can be dragged up and into our middle latitudes and then into California, sometimes directed by dipping troughs within winter’s jet stream patterns. Such was the case during the record 1982-83 and 1997-98 El Niño storms and floods. I’ve been lucky to have experienced and monitored these and other El Niño storm years and to appreciate how each develops its own personality and leaves a different legacy. The sum of El Niño + migrating low pressure troughs + ARs + climate change conspires to make this season’s storms more volatile and difficult to predict.
When atmospheric rivers aim at California landscapes, they encounter tall mountains that are aligned perpendicular to the moisture streams. This forces the moist air to rise, expand, and cool to its dew point. The results are much higher precipitation totals (compared to what we might expect from the storms alone) on the windward sides of mountain fronts (facing the Pacific). This diagram comes from the National Weather Service Office in Oxnard.
The start of our 2023-2024 West Coast rainy season developed with its own idiosyncrasies, as Pacific storms and streams of moisture were first steered over blocking high pressure ridges and up into the Pacific Northwest, mostly missing southern California. As January turned toward February, circulation patterns changed. Shielding high pressure spasmodically and then abruptly shifted away, allowing powerful Pacific cyclones and their weather fronts to penetrate farther south. There were early hints that exceptional moisture and energy was juicing up incoming storms. Monster waves pummeled the coast at the end of December, 2023. (Check out our previous website story on waves.) By late January, weather fronts finally started holding together after drenching Northern California, fueling very localized and historic downpours into Southern California. Classic El Niño scenes began to splash into national news stories. Cars were shown tumbling helplessly through cascading floodwaters during the wettest January day (2.73 inches on January 22) in San Diego history. More vehicles were submerged and surrounding freeways were closed by floodwaters when 2 1/2 inches of rain poured down in less than one day on neighborhoods around Long Beach as January flipped to February. These were just dress rehearsals for the big show that would appear a few days later.
By early February, the National Weather Service was tracking one of the most potentially powerful and dangerous atmospheric rivers in recent history as it gathered and aimed its meandering firehoses at Central and Southern California. There was no hedging in this forecast that first used the dreaded “unprecedented” a few days ahead and then advertised at least two days of “High Risk for Life Threatening and Damaging Flooding.” Forecasters gave us plenty of warning as the big show migrated down the coast and burst into Southern California on Sunday, February 4, 2024. More than 5 inches of rain fell across the LA County coastal plain within the first 24 hours of the storm. Totals were much higher along surrounding mountain slopes. As even more rain poured down during the second day, accumulating water triggered widespread flooding and hundreds of mudslides and debris flows, destroying hillside homes and infrastructures. Within three days, rainfall totals of more than 7-8 inches had accumulated across the LA Basin (from the coast to the inland valleys, down to Long Beach), while up to a foot of rain deluged surrounding mountain slopes. These three-day totals represented more than half the average rainfall for an entire year at numerous weather stations. Orange and San Diego Counties joined the soaking party a little later, but still had impressive totals. The following is a chronological play-by-play image essay that sets the stage and then follows this historic AR event. I wrote much of this while under a prolonged flood warning; enjoy the wild ride.
Whether you are a weather regular or new to all of this … to create the best effect, skim down through the images in order as if you are viewing a film loop.
The 2023-2024 rainy season began relatively dry, especially in Southern California. A strong high pressure ridge blocked incoming storms for a few days into late January, 2024. Southern California coastal temperatures soared over 80 degrees F while warm 70s spread into northern California’s coast and valleys beneath the dense, sinking air columns. Here, you can see the last remnants of valley (tule) fog evaporating in the Central Valley and snow capping the Sierra Nevada Mountains. Source: NOAA/National Weather Service. This water vapor image shows incoming moisture off the Pacific as it is blocked and directed north of California. Dry, offshore flow dominates across most of California. But the resilient high pressure is being nudged to the east and that will open the storm door during the final days of January. Source: NOAA/National Weather Service. This 500mb map shows pressure trends roughly halfway (about 18,000 feet or 5,600 meters) up through the density of our atmosphere, where winds often steer our storms. Upper-level high pressure that has produced days of warm, dry weather is shifting east of California on January 29, 2024. Notice the upper-level trough of low pressure (drifting from over the Pacific Ocean), which will bring stormy weather to the state. Source: National Center for Environmental Prediction, Weather Prediction Center.
At the surface, dominating high pressure that produced dry offshore breezes is drifting east of California. A wet weather front is approaching from the northwest on January 29, 2024. Source: National Centers for Environmental Prediction, Weather Prediction Center. Two days later (January 31), the cyclone is approaching the West Coast. Its cold front will make landfall in Northern California and then spread south. Source: NOAA/National Weather Service.The center of the middle latitude cyclone spins counterclockwise just off the Washington coast. But it is also circulating moisture and a frontal system down the California coast, which will hold together and even locally strengthen as it sweeps into Southern California. Source: NOAA/National Weather Service.High, middle, and low clouds condense in an increasingly moist air mass over the Southern California coast on January 31, 2024. Note how the multiple layers of clouds are streaming from southeast to northwest (lower left to upper right), toward the cyclone (low pressure system) that was already soaking Northern California.This view is from Santa Monica during sunset on January 31, 2024, looking toward the northwest and the approaching storm. Rain from this initial storm and atmospheric river is spreading down the coast. Clouds streaming and banking against the mountains in the distance (our backs are to the wind) will soon drop downpours as the first atmospheric river and cold front race through Southern California. But it is only a dress rehearsal for the historic atmospheric river that will follow in a couple of days.An impressive frontal boundary sweeps into Southern California. The storm taps into an atmospheric river, dragging moisture off the Pacific, dumping heavy bursts of rain that cause local flooding. But a more powerful storm and AR will follow behind it. Source: NOAA/National Weather Service.This 500 mb map (February 1, 2024) displays the deep upper-level trough that has formed, digging south along the Pacific Coast. These upper-level pressure and wind patterns are supporting the first storm and AR in this sequence. Source: National Centers for Environmental Prediction, Weather Prediction Center.Though the wet frontal system has passed well to our east, scattered unstable popcorn cumulus clouds are racing off the Pacific and into the state as if to chase the storm away. They bring passing showers, snow to the mountains, and even some isolated thunderstorms, mostly west of our major mountain ranges. But note that big cold mother low spinning off the Pacific Northwest coast and the solid mass of clouds entering the image from the west, as if pointing directly toward the California coast. The main event is not far behind. Source: NOAA/National Weather Service.By February 3, a massive moisture plume is organizing off the California coast as the large Pacific mother low circulated closer to Northern California. Notice the dryer air under a high-pressure ridge that has shifted far to the south. Source: NOAA/National Weather Service.On February 3, the storm stage was set. A powerful atmospheric river had organized and was being dragged toward the coast just to the south of the cyclone that swirls counterclockwise up near the Oregon border. Source: NOAA/National Weather Service.This shield of high cirrostratus clouds, on February 3, 2024, will gradually lower and thicken to altostratus and then nimbostratus as a low-pressure trough and atmospheric river drives off the Pacific and toward Southern California. (Note the faint and nebulous sun dog forming to the right of the sun.) These conditions are what we would expect to experience when a typical warm front approaches, announcing the arrival of one of our beneficial winter storms. But satellite images and NWS computer models indicate that this incoming storm and its atmospheric river will be anything but typical. The thick overcast will persist for two days and nights to produce historic rains. Southern California’s iconic palm trees lean away from the dry, desiccating Santa Ana winds that have deformed them and brought periods of warm, dry chamber of commerce weather during autumn and winter. But such iconic sunny days fade into a distant memory on February 3, 2024. The blanket of dull, gray stratiform clouds in the background is lowering and thickening ahead of what will prove to be one of the wettest atmospheric river episodes in LA history.These are some of the warnings issued just as the storm moved in. Though weather forecasters’ ominous predictions were difficult to believe, we now know that the National Weather Service nailed it. Thankfully, they were able to warn us far ahead of time. Source: NOAA/National Weather Service Weather Prediction Center.By February 4, the potent atmospheric river was being driven directly into Central and Southern California around the deep low-pressure system just off the north coast. Source: NOAA/National Weather Service.This water vapor image shows how the counterclockwise-spinning storm over Northern California is dragging a plume of moisture up from the subtropics and even the tropical Pacific. As it is driven from southwest to northeast, the moisture must be lifted up and over our mountain ranges, resulting in copious amounts of orographic precipitation. Source: NOAA/National Weather Service.This surface weather map (February 4, 2024) covers the entire Pacific Ocean. A powerful storm, packing gale-force winds and torrential rains, is skimming along the California coast (far right). The historic atmospheric river is flowing parallel to and just ahead of the trailing cold front, connecting to latitudes and air masses far to the south. After dumping torrential rains within a few days, this California storm will eventually yield to that big high-pressure system out over the Pacific. Just behind the high-pressure ridge (farther west) is another Pacific cyclone with characteristic warm, cold, and occluded fronts circulating and sweeping clashing air masses around it. We must wonder what other weather surprises the rest of winter might bring during this volatile El Niño year. But first, Southern California will have to deal with this historic weather event. Source: NOAA/National Weather Service Ocean Prediction Center.This upper-level (500mb) map from February 5, 2024 shows the deep low pressure trough that has meandered far to the south just off the West Coast. California is situated below the eastern side of the upper trough, where accelerating cyclonic winds (flowing southwest to northeast over the state) encourage unstable air to rise, fortifying the stormy low pressure system at the surface. Source: NOAA/National Weather Service Ocean Prediction Center.By February 5, 2024, the AR had stalled over Southern California, flowing from southwest to northeast, driven by a deep low pressure trough wobbling off the coast. Nature’s firehose had already produced record 24-hour rainfall totals (5+ inches) across and around LA County and there was plenty more to come. Unstable scattered showery weather ruled in the cold air north of the front. But there had been no measurable rain in San Diego, on the southern edge of the front. To prove how narrow these AR precipitation bands can be, the first 24-hour precipitation totals ranged from more than five inches to zero along just 100 miles of Southern California coastline! By the next day, rain finally spread over San Diego County as the cold front finally began advancing toward the southeast again.This February 5 surface weather map shows the deep low-pressure system on the north coast that circulated and drove the historic AR across Southern California. Extreme pressure gradients powered widespread damaging and deadly winds (measured at over 80 mph), which toppled thousands of trees and cut power to hundreds of thousands of northern Californians. Source: National Centers for Environmental Prediction, Weather Prediction Center.By February 6, the upper trough was on the move, pushing the AR into Arizona. As the low pressure trough drifted east, unstable showers and thunderstorms rotated over Southern California, adding to historic precipitation totals. As expected, snow levels dropped to lower elevations in the cold air behind the front, leaving thick blankets of powder at mountain resorts. Source: NOAA/National Weather Service.This water vapor image from February 6 shows the trough finally moving east, triggering more showers over Southern California and pushing the AR into Arizona. Source: NOAA/National Weather Service.It’s still raining on February 6 (note the thick nimbostratus clouds in the background), but the AR has passed to the east and the Los Angeles River is finally receding. Look carefully for stranded debris on the submerged trees to estimate the height of yesterday’s peak flows.A closer look at the high-water debris draped on trees in the channeled LA River suggests that this flood of water came very close to becoming a much more catastrophic event. Luckily, it was not immediately followed by another massive storm.Here are the three-day rain totals (February 4-7) for this historic winter storm. Note how they closely resemble the forecast from days earlier. Source: NOAA/National Weather Service Weather Prediction Center.Mostly fair-weather cumulus clouds puff up and drift around on the day after the big storm and AR have passed to the east. You might look for these colorful signs that a winter storm has passed by your neighborhood. They frequently form in marginally unstable moist air behind cold fronts, as higher pressure begins to nudge in. You will often notice them appearing to chase the weather front, drifting from west to east or north to south with the wind. Watch the flat bottoms form when rising thermals expand and cool to their dew (saturation) points. The white cauliflower tops boil higher when latent heat of condensation is released within the cloud as vapor condenses to liquid drops or freezes to form ice crystals.Near the horizon, a distant train of puffy fair weather stratocumulus clouds are leftovers from the week’s historic storm. They are streaming from right to left, pushed by chilling northwesterly winds that swept in behind the front, which has moved far inland. Middle-level clouds have started drifting over them.I’ve never seen this much water flowing here before. Also on the day after the storm, streams and waterfalls appeared across trails in the most unlikely tiny drainages, such as here above Los Leones Canyon in Pacific Palisades. Nearly a foot of rain fell on these slopes during the three previous days, at least half of their total average annual precipitation.Big Pod Ceanothus (Ceanothus megacarpus) is one of the many species that benefit from our winter rains. Their small white blossoms (standing out here just a day after our historic storm) decorate Southern California’s coastal slopes, announcing that it is February and spring isn’t far away.But wait! As her northwesterly winds churn up the ocean, nature still has a trick up her sleeve. The lingering low-altitude post-storm stratocumulus clouds are suddenly being joined by middle-level clouds, filling the sky and warning that there is one more small disturbance approaching. A miniature fast-moving cold front was swinging around the backside of the exiting low-pressure trough and down the coast during the evening of February 7. This quick hitter would dump more rain and mountain snow, but it was the final performance here for the four-day dramatic show that had stormed across California and finally swept east, into the Rocky Mountains, and across the continent. High pressure promised to follow with several days of fair weather. This was the scene at 6,000 feet elevation in Fern Valley, Idyllwild on the morning of February 8, 2024, after the storm’s final snow showers covered these parts of the San Jacinto Mountains. As is often the case, cold winds circulating out of the northwest brought bursts of snow to lower elevations on the backside of the exiting low-pressure trough (refer to the two final satellite images in this story). Such a winter scene will likely be followed by days of fair weather as higher pressure moves in and the storm continues its journey farther east across the continent. Photo by Steve Chadwick.
If you are still hungry for disaster scenes from the historic storm and AR of early February, 2024, there are plenty of media sources. Here are a few:
