| February 06 2024 |
Significant Western U.S. Atmospheric River Event: (2/3 - 2/6)
By: Scott Kleebauer, WPC Meteorologist
Meteorological Overview:
A significant Atmospheric River event impacted portions of the Western U.S with heavy snow, high winds, and torrential rainfall that would break multiple records across portions of Southern California. On February 3rd, a digging upper trough amplified off the Pacific Northwest with a surface low beginning to slowly intensify off the coast of Oregon. The surface low began to inject a prolonged plume of moisture across the northern Sierra, as well as the Oregon/California coastal plain with some modest rainfall and snowfall totals accompanying the prevailing moisture flux. Conditions across the west, especially into California deteriorated rapidly moving into the 4th and 5th as the surface cyclone off the Pacific coast intensified very quickly with the surface pressures dropping from approximately 1004 millibars (mb) to as low as 978 mb off the coast of northern California. This was a textbook case of rapid cyclogenesis with the pressure falls dropping close to 1 millibar per hour, the defining characteristic for a “bombing cyclone”, an evolution typically reserved for potent east coast low pressure systems.
The surface low meandered off the Pacific coast through the 5th, bringing a persistent onshore flow component into the central California coast along with favored dynamics focused within a diffluent area of ascent along the leading edge of a 140 kt. jet streak off the Pacific. A surface cold front extended from the north-central California coast down across southern California, a presentation that would prove to be pivotal in the evolution of the heavy rain and associated flood pattern across Southern California. The cold frontal orientation from south to north within the confines of the Transverse and Peninsular Ranges across Southern California provided a meteorological “highway” for deep layer moisture to advance poleward out of the Pacific with roots traced back to the tropical Pacific as noted by integrated-vapor transport (IVT) indices exceeding 800 kg/ms, a classification of a strong atmospheric river signal.
The orthogonal flow against the terrain enhanced upslope ascent within the coastal mountain ranges to the north of Santa Barbara and Los Angeles, leading to prolific rainfall over the span of 24-48 hours beginning the morning of the 4th through the 5th. Farther north into the central and southern Sierra range, the same entrenched moisture transport would progress farther inland into the terrain with a strong upslope enhancement thanks to the 8-10k+ ft. terrain provided by the regional mountain chain. Cold air was plentiful given the time of year and lower heights brought in by the mean west coast trough allowing for the higher elevations over the interior to experience a significant snowfall with widespread blizzard conditions. Strong jet dynamics and favorable mixing from the upper atmospheric column allowed for winds to soar well above high wind thresholds with instances of 100+ mph gusts reported at multiple high elevation locales, including some major ski resorts from as far north as Truckee, CA southward to Mammoth Mountain and surrounds. This only added to the prolific snowfall totals that eventually occurred over the multi-day period, compounding the impacts from the storm that would take several days to alleviate.
By the 6th, the storm finally started to weaken and progress inland with the heavy rain and snowfall footprint shifting into the interior Southwest U.S, but not before one last enhancement across the southernmost part of California near San Diego and the adjacent terrain.Despite rising heights associated with the passage of the trough, the ascent pattern remained favorable within the tail end of the upper jet streak that continued to maintain intensities around 130 kts. as it shifted farther inland. Waves of convective precipitation would enhance local rainfall rates with localized flooding persisting into parts of southern Los Angeles, Orange, and San Diego counties with flooding even extending into the desert areas of southeast California and southern Nevada. The storm would finally exit by the 7th with some lingering showers and elevated thunderstorms over Southern California on its way out, but the significant impacts would finally subside and the recovery process could begin.
Impacts:
Substantial rain and snowfall impacted much of the state of California, along with some extreme high wind reports littered across the coastal and interior mountain areas. Rainfall was widespread within the lower elevations with the most impressive totals coming out of Southern California within the Transverse and Peninsular Ranges. The highest mark for the storm came out of Topanga, California with rainfall totals just breaking the 14” mark, according to the National Weather Service (NWS) in Oxnard, the local Los Angeles forecast office. Topanga is located on the eastern flank of the Transverse Range and was subject to several waves of heavy precipitation from the morning of February 4th through part of the 6th leading to astronomical totals for the area. Other totals included; 11.64” at the Bel Air Hotel northwest of Los Angeles, as well as Sepulveda Canyon to the north of Santa Monica near Beverly Hills. Downtown Los Angeles near the University of Southern California campus eclipsed the 9” mark by the morning of the 7th allowing the region to see about 65% of its seasonal rainfall within a span of 48-60 hours. Widespread 5+” reports encompassed much of the region extending from Point Conception down to Long Beach, California with some major cities like Santa Barbara (5.42”), Thousand Oaks (7.05”), Santa Clarita (7.25”), Pasadena (8.45”), and Malibu Hills (9.59”) all receiving significant totals. Farther north across the Bay area, totals were not as prolific compared to their southern neighbors, but there were still several reports exceeding 3”, including some of the stations located within the coastal King Range south of Eureka, and the coastal stretch extending west and north of San Francisco and Santa Rosa.
High wind and copious amounts of snowfall were also a major concern with the event, and a combination of the two occurred in the Sierra Nevada’s causing extensive, prolonged life-threatening blizzard conditions to elevations above 7000 ft. Several major ski resorts had to shut down for multiple days due to the dangerous wind and snowfall causing travel to become fairly impossible due to the low visibility and significant accumulation on the roads. Cars and trucks along Interstate 80 near Truckee and Donner Pass became stranded for several hours during the height of the event on the 5th, with state Highway Patrol and various local plow services having to help free some of the vehicles stuck within the extreme conditions. Snowfall totals were very difficult to measure due to the combination of very high winds and drifting snow. Estimates for the storm were anywhere from 3-6 ft. within the higher elevations, but reanalysis of the precipitation totals and expected snow ratios signals places above 8500 ft. may have received slightly higher amounts. Mammoth Lakes, California was able to measure nearly 3 ft. by the end of the storm, but much of the area was plagued by very high drifts exceeding 8 ft. that would make cleanup and travel in and out of the area a virtue of futility. Winds over the Sierra’s were potentially a bigger story than the snowfall amounts, with wind gusts over 70 mph a common occurrence during the period from the morning of February 4th all the way through first half of the 5th before subsiding slowly toward the end of the storm's lifecycle. Some recorded gusts included a 128 mph reading from Mammoth Lakes, California, one of the highest recorded wind speeds the area has seen since its data collecting inception. Other wind gusts included some 80+ mph readings in multiple areas of the San Gabriel’s, and some near 80 mph gusts up off of Donner Pass to the west of Reno, Nevada. San Francisco also saw some of its strongest recorded winds in history with some coastal areas near the Bay eclipsing 70 mph with a 77 mph gust recorded in San Francisco proper with San Francisco International managing to reach 58 mph, a defined severe gust based on NWS warning criteria. Places along the coast near the Bay area also saw some incredible wind speeds with the two highest readings outside Mammoth Lakes breaking 100 mph in Lagunitas- Forest Knolls and Pablo Point both reaching 102 mph during the peak of the wind surge on the 4th.
The unfortunate component of a storm of this magnitude is the damage impacts these types of systems can produce. Reports of mudslides and landslides became all too common within the terrain of Southern California with the extreme rainfall that was produced. According to an article by the Los Angeles Times, L.A authorities responded to over 550 mudslide calls with several of the mudslides inundating or taking out homes within the hill side to the north and west of Los Angeles. Evacuation orders and warnings were issued for certain municipalities within Los Angeles, Orange, Ventura, and San Bernardino counties due to the fear of mud and landslides along with significant flooding. High winds over the Bay area knocked out power to hundreds of thousands as they toppled trees and impacted the local power infrastructure to the region. Multiple fatalities were reported throughout the event with the most common occurrence from trees being uprooted and falling on homes with residents inside. A State of Emergency was issued by Governor Newsom of California and President Biden extended his hand to the state to help with any necessary aid during the storm's height and post-storm clean up. This event was one of the more impactful Atmospheric River events in recent memory and one that many will never forget across parts of the Golden State.
