The Monster Santa Ana Event of 1969
Updated: Jul 14, 2018
The Santa Ana (SA) of November 1969 was an extraordinary event; long-lasting, violent and extremely destructive. SA conditions persisted over southern California for more than fourteen days beginning on November 17 and ending on November 30, 1969. It may have been one of the longest and most intense (in the top 3%) SA events in the last fifty years. According to an examination of hourly meteorological data and numerous SA-related articles published in the Los Angeles (LA) Times, the event continued with little interruption through November 30 when the area received just one day of relief from SA conditions before another arrived on December 2 which lasted for another six days. Initiated by a Great Basin to California coastal pressure gradient along with the establishment of an east-west temperature gradient, the event appears to have been sustained by the emplacement of a Rex Block pattern which allowed the event to persist for such an extraordinary number of days.
Fall and winter is notorious for the hot, dry Santa Ana wind, a katabatic-like wind that sweeps out of the desert, over the surrounding mountains and down into the densely populated coastal areas. While individual SA events are often simply warm, windy days along the southern California coast, an unfortunate by-product of an event can be the increase in the frequency and intensity of wildfires. Even weak SAs can quickly turn into catastrophic fire events posing a significant threat to this heavily populated region. A typical LA Times headline from the event dated November 28 reads “Warm Winds Take Chill off Holiday, Fire Situation Termed Critical as Humidity Drops” accompanied by a dramatic photograph of a fire department helicopter sortie to drop fire retardant. While most of the world was more concerned about the weather at Apollo 12’s splashdown site in the Pacific, for the residents of southern California, this particular SA was having a significant impact.
As the event progressed, the Times reported a small plane being flipped, trees and fences downed, sailboats capsized, power lines toppled, Christmas decorations damaged and numerous fires fanned by the gusting winds. Table 1 is a sixteen-day timeline summarizing noteworthy events that occurred during the SA as well as pertinent meteorological observations for each day of the event plus one day before and after. This table does not include observations from the December 2 event.
Societal and Environmental Impacts of SA Events
Large wildfires are not unusual for the dry chaparral in southern California. However, the impact of a population that has doubled since 1950 as well as the resultant extensive wild land-urban interface that continues to expand has intensified the destructive coupling of Santa Ana winds and wildfires (Westerling et al., 2004). Gusting SA winds, low humidity and high temperatures exacerbate fires catastrophically in a region where ignition sources and highly flammable scrubland fuel are in abundance. “Red flag warnings” are almost always issued in conjunction with SA events especially in vulnerable areas such as mountainous areas, canyons, passes, and in national and state forests. During the 1969 event, five separate fires are reported as being exacerbated by the SA wind on November 20 alone.
One fireman was quoted by the LA Times, saying “We had fist-sized fireballs all over the place.” A 2,000 acre blaze in Torrey Canyon near Moorpark was still out of control by that evening and had spread into another canyon. Residents of Moorpark were told to expect evacuations as the flames threatened several outlying neighborhoods. A number of movie sets were destroyed and a fireman in the area suffered injuries when flames were blown over a fire truck. By nightfall, over 400 men from the Ventura County and State Division of Forestry were on the fire lines. By the end of the event, eight more large fires were reported as being spread and intensified by the high winds. Large fires continued to burn in Los Angeles, Riverside and San Diego counties even after the SA event had dissipated.
Along with intensifying and prolonging wildfires in the area, SA events also affect coastal sea surface temperature (SST) and marine biological activity. Hu and Liu (2003) and Castro et al. (2006) showed that during SA events, event-induced upwelling and intense vertical mixing caused both sea surface sensible and latent heat fluxes to increase three-fold causing a decrease in SST as well as a considerable increase in chlorophyll-a. During strong to moderate SA events, pollution is advected offshore only to be replaced with significant dust and other particulates including ash from previously burned areas. Lu and Liu (2003) determined that the effects of a SA wind event could be observed as far as 1000 km off shore and that strong wind jets transported dust causing an oceanic response up to 400 km off the coast of southern California. However, weak SA events may exacerbate pollution by stagnating air over coastal communities. This situation arises when weak SA winds are countered by onshore flow, trapping pollutants which under strong SA conditions would be advected off shore (Jacobson, 2002).
The increase in particulates can pose significant health issues with vulnerable populations. Ash warnings are now commonly issued with SA events that occur in the aftermath of large fires. Minimally, the extremely low humidity and increase in air borne dust may induce headaches and sinus discomfort. Corbett (2006) found that emergency department visits for asthma increased 3.12 vs. 2.16 visits per day during SA events compared with other weather conditions and asthmatics presenting during Santa Ana winds appeared to be more ill, as judged by higher admission rates (21.9 vs. 18.7%). In general, the odds of someone seeking emergency care are increased by approximately 50% during peak fire-conditions (French et al., 2012).
In addition to the dust and ash, Coccidioidomycosis, or “valley fever”, is a fungal disease transmitted through the inhalation of Coccidioides immitis spores that are carried in dust. Coccidioidomycosis occurs in arid to semi-arid regions that are prone to dust storms, hot summers and warm winters. The disease is endemic in the southwestern U.S., southern California and parts of Mexico and local health officials report that the overall rate of the disease has increased in LA County over the last 10 years. The LA County Public Health Department notes that SA wind events cause “ideal conditions for disseminating” the spores and suggests warning persons at risk to avoid travel or outdoor activities when conditions are most dangerous for exposure (Acute Communicable Disease Control, 2010 Annual Morbidity Report).
The gusting winds and low-level wind shear that accompany SA events are also a danger to general aviation, it presents a hazard on large portions of the regions extensive freeway system and can cause extensive property damage. A cursory examination of the NTSB (National Transportation Safety Board) accident database revealed nine incidents from 1993 to the present in southern California involving general aviation aircraft and wind shear resulting from downslope winds. An aviation wind warning was issued at the beginning of the 1969 case study event after three airplanes were torn from their moorings at a regional airport.
Wind velocities over 100 mph have been recorded with intense SA events and numerous southern California city disaster preparedness plans include procedures for responding to SA wind events. These types of high wind events pose a danger to high profile vehicles that are easily blown by the wind and also cause blowing dust that reduces visibility. These conditions often necessitate the temporary closure of state and federal highways, in particular Interstates 8, 15 and the 215 freeway. Forecasters frequently issue high wind advisories in the region before and during the onset of these events.
During the 1969 event, numerous reports of damage dominated the headlines. On November 18, two mobile homes were overturned, trees were uprooted, windows were broken and signs were damaged across the area. On November 28, a plane parked at Orange County airport was flipped over by gusting winds; winds in Burbank overturned 112 decorated Christmas trees; a block of fencing around the Pasadena courthouse was blown down (Figure 3); and five Burbank police patrol cars were trapped after a fallen tree blocked a county road tying up eight officers until landscaping crews could remove the tree. Overhead signs are particularly prone to damage during high wind events and on November 28 a large sign stretching across a major street in Burbank was torn down by the winds. During the case study event there were numerous reports of trash cans being overturned and debris being blown about. The wind from SA events can accelerate to such velocities that sandblasting of structures and vehicles can occur. The California Highway Patrol (CHP) reported sand damage to cars traveling near Riverside on November 19, including the sandblasting of one of their patrol cars.
Unusually dangerous surf conditions that normally accompany a Santa Ana event frequently cause forecasters to issue high surf advisories for the waters around the region. The large, powerful waves generated by SA winds have the potential to generate strong rip currents within the surf zone as well as dangerous sneaker waves. The unusually high temperatures that occur during a Santa Ana is an additional incentive for local residents to flock to the beach or go boating thereby exposing even more people to the risk of swimming and surfing-related danger. Small craft warnings were issued immediately at the onset of the case study event on November 17 from Point Conception to Oceanside. On November 29, three boaters had to be rescued after winds snapped the rudder and then upended the boat.
SA wind events are also a challenge to regional utility companies since high velocity winds can damage overhead power lines and interrupt service to thousands in the densely populated areas of southern California. This was especially so in 1969, since most electrical and phone lines were above ground at that time. Five separate reports of power outages were reported in the LA Times during the 1969 event and presumably more occurred but were not mentioned in the paper. One report from November 19 stated that there were “dozens of power failures in the Banning-Beaumont area, one of them lasting up to eight hours.
Figure 9a and 9b, composite maps of the 500 mb geopotential height anomaly over the United States on November 16 (one day before the onset of the event) and on November 17 (the first day of the event), shows the notable absence of high pressure over the Great Basin. Figure 10, the surface maps created by the Weather Bureau for both days, confirms the absence of high pressure. In both figures 8 and 9, the high pressure system can be seen advancing into the Pacific Northwest, which eventually becomes stationary starting November 19 and remains in place through the rest of the event. This synoptic-scale feature, at this point cannot be contributing to the initial development of the SA.
A 700 mb air temperature anomaly and the cold pool associated with the thermodynamic mechanism for SA development establishes over the inland desert areas in California and Nevada by November 17. This suggests that the thermodynamic mechanism is initiating the event in the case study with both the synoptic and thermodynamic mechanisms sustaining the event through the remaining eleven days. The temperature gradient remains in place through 12/6 which may explain the reinitiating and continuation of the event past the rain-induced hiatus on 12/1. The dramatic arrival of the event at 7 pm, November 17, at March Air Force Base near Riverside is marked bya 36% decrease in relative humidity over just one hour. There is a 10º C rise in temperature in two hours as the event begins.
As the Event Progressed: Synoptic-Scale Mechanism
Santa Ana events occur when cool, dry air masses flow downslope from high elevation basins in the western North American interior toward lower atmospheric pressures off the Pacific coast (Finley and Raphael, 2007; Miller and Schlegel, 2006; Raphael, 2003; Sommers, 1978; Ryan, 1969; Fosberg et al., 1966). Triggers often include a cold front passing through the western United States often followed by an intensifying short wave trough. The upper to mid-level trough, a feature not seen in the 1969 event until November 27, moves over southern California and establishes strong northerly winds (Ryan, 1969). The arrival of this trough may explain the re-strengthening of the event on November 27 and 28. The dryness of a SA flow, the static stability and inversions, and the associated upper air patterns were all present during the 1969 event and indicate the presence of properly oriented subsiding, mid-tropospheric flow associated with SA events (Sommers, 1978). Upstream soundings characteristically show a dry adiabatic lower and middle troposphere during Santa Ana events.
Since the 500 mb pattern for SA development consists of a ridge over or just off the northern coast of California and a trough pushing south or southwest into Baja, the duration of a SA event often depends upon the duration of the ridge that builds behind the trough. During the 1969 event, the arrival on November 19 and persistence of the high pressure system over the Great Basin sustained the event until November 30, when the event begins to weaken slightly and withdraw for one day, and suggests the presence of a Rex Block pattern which would explain the synoptic longevity of the event. The strongest winds usually occur during the initial period of an event and are dependent on a strong mid-tropospheric trough-ridge system extending from northern to southern California.
The strongest SAIs during the event was 9.53 which occurred at March Air Force Base (AFB) in Riverside county and 8.03 at San Bernardino on November 19. The next highest SAIs occurred on November 28 with San Bernardino at 7.41 and Naval Air Station (NAS) Miramar in San Diego at 7.25 (Rowland, 2011). Warm temperatures over the ocean along the southern California coast increase the desert-ocean temperature difference and the resulting pressure gradient winds. The advection of cold air into the desert will also intensify SA wind development (Abatzoglou et al., 2013; Hughes and Hall, 2010). Both of these scenarios can occur independently of the synoptically-forced SA mechanism, however, Hughes and Hall (2010) concluded that in combination, the thermodynamic and synoptically-forced mechanisms produce the strongest SA events.
In the 1969 SA, both mechanisms seem to be playing a part in the formation and progression of the event, although the synoptic mechanism seems to play no role in the initiation of the event but rather in the intensification and sustainment of it through the last eleven days.
Rapid increases in surface pressure across the area invariably signal the end of the SA winds as well as when the 500 mb flow becomes more zonal or when a new trough position becomes established west of the northern California coast (Sommers, 1978). Eastward migration of the synoptic features that support the SA tends to decrease the pressure gradient which in turn favors dissipation of the SA event (Fosberg, et al., 1966). The advection of warmer air into the desert or of cooler air over the coast signals the withdrawal of the thermodynamic mechanism.
In the withdrawal of the 1969 event, a weak, upper level disturbance off the coast of northern Baja caused the advection of moist air over the southern California desert and mountains. This resulted in an increase in humidity, slight lowering of temperatures along the coast and scattered showers. However, this small reprieve only lasted for one day before SA conditions resumed for another six days. The persistent high pressure area over the Great Basin remained in place and as the upper level disturbance moved inland, SA conditions returned on December 2.
What is a Santa Ana?
Typically, Santa Ana events occur in the counties of Monterey, San Luis Obispo, Santa Barbara, and Ventura along the central California coast and Los Angeles, San Bernardino, Riverside, Orange and San Diego along the southern coast. There are, of course, many other canyons and passes in southern California where SA winds tend to intensify due to compression of air through this type of constricted topography (gap flow). The mountains and immediate vicinity typically experience high visibility; dry, northerly flow; windward static stability and an inversion at or near mountaintop level. As the air subsides, it is compressed, warmed adiabatically and the relative humidity reduced.
Sommers (1978) provides a working definition of a Santa Ana wind event based on surface measurements: wind direction within 30° of perpendicular to the local mountain orientation, relative humidity less than 30% and wind speeds greater than 8 m s-1 for mild, greater than 15 m s-1 for moderate and greater than 25 m s-1 for strong SA events. The highest gust reported by the Weather Bureau (now the National Weather Service) during the 1969 event, was an 80 mph gust on November 20 at the San Gorgonio Pass with gusts between 50 and 78 mph reported on seven separate days over the period.
Several meteorological variables are used to define a Santa Ana event, including relative humidity, temperature increases and synoptic patterns. But perhaps the most universally acknowledged characteristic of a SA event is the wind direction — blowing out of the north-northeast which orients the wind within 30° of perpendicular to the local mountain axis. In southern California, this means that surface winds are typically from the north-northeast. However, as might be expected, wind direction may vary from northwesterly in Los Angeles County to easterly in San Diego due to differences in mountain orientation between the two locations (Sommers, 1978). From November 17 through November 30, 1969, three stations; Oxnard, Ontario and Santa Ana; recorded four days where each hourly wind direction observation (24 observations) were from the east-northeast quadrant. The rest of the days during the event had the majority of the wind direction observations at all stations from that quadrant.
Another important characteristic of SA events is the very low humidity levels brought about by the adiabatic warming of air as downslope winds exit the western/southwestern (depending upon orientation) side of surrounding mountain ranges. During SA events, dew points range from -0.5° to 8° C with a mean minimum value of 0.34° C. In terms of relative humidity, readings of 5% are not unusual during SA events (Raphael, 2003). Frequently, even the sea breeze that often temporarily interrupts the normal SA flow during the morning hours is characterized by low humidity and may be residual SA-dried air that has been over the ocean only a short time (Fosberg, et al., 1966). The lowest relative humidity recorded during the 1969 event was 2% on November 29 in Los Angeles. The lowest dew point was -9.4° C recorded at March Air Force Base on November 19, 20, and 28.
As with humidity change, temperature change is largely due to adiabatic warming as air is compressed while being forced over the mountains. Soundings taken in the vicinity of SA wind events show this phenomenon clearly as cold air descends over the mountains beneath a stable layer and reaches the anomalously high temperatures as it arrives at sea level on the lee side (coastal side) of the mountains (Blier, 1998; Finley and Raphael, 2007; Fosberg, et al., 1966). The figure below is a skew-t plot of radiosonde data from Montgomery Field in San Diego on November 17. An inversion can be seen at around 850mb, creating the stable layer necessary for the initiation of the SA. The persistent inversion can be seen continuing in subsequent soundings on November 29 (the next to the last day of the event) showing a weakening of the inversion as well as an increase in height.
Temperatures in southern California coastal areas reached above 32° C and maintained those temperatures during the entire life of the event. The highest temperature of the 1969 event was 28.9° C recorded on November 25 at Santa Ana with 21° C being the average temperature for the area at that time of the year. Additionally, there is little diurnal variation in temperature during SA events. As strong, warm winds gradually erode the marine boundary layer, very warm air from above is allowed to penetrate further into coastal areas and finally out to sea. This displacement of the cool air pool on the western/southwestern side of the mountain barrier means that persistent downslope winds keep air mixed and prevents night time cooling which accounts for the low diurnal variation.
Santa Ana winds have a strong seasonality occurring mostly in late fall, winter and early spring. The synoptic conditions most favorable for SA development occur predominantly during the winter months when the north Pacific high pressure system moves farther south. An anticyclone in the Great Basin simultaneously occurring with a surface low pressure system off the California coast partly explains the initiation of SA wind events. The Great Basin high (GBH) develops when a cold front associated with a trough aloft passes through California. As high pressure builds over the Great Basin, a strong northeast-to-southwest surface pressure gradient is established over southern California (Abatzoglou et al., 2013; Hughes and Hall, 2010; Finley and Raphael, 2007; Sommers, 1978).This synoptic pattern corresponds seasonally with the winter temperature regimes over southern California of cold desert and warm coastal conditions which enhances the temperature differences between the desert and coastal areas. These synoptic conditions and the temperature differences in the region are major components in the development of Santa Ana events.
Conversely, during the summer, the Pacific high moves northward and the monsoon flow over the southwestern United States promotes westerly flow at local and regional levels, preventing SA development (Raphael, 2003). Mild temperatures prevail in coastal southern California in the winter due to the proximity to the ocean and the inland desert temperatures cool as distance increases away from the coast and the climate becomes more continental. Temperature differences between the desert and the coast reverse as the deserts warm during the summer and late spring, inhibiting the establishment of the temperature gradient which also fuels SA development (Abatzoglou et al., 2013; Hughes and Hall, 2010).
The SA wind typically develops at night or in the early morning when the atmosphere is the most stable. Atmospheric stability is a key component in these events as is the diurnal pattern of land/sea breeze. During the night, the sea breeze dissipates and the land breeze along the southern California coast augments the north-northeasterly flow associated with SA events. During the day, the sea breeze blows in the opposing direction to the SA winds resulting in a net reduction in the speed of both winds and reduced inland penetration by the sea breeze in the late afternoon (Fosberg et al., 1966, Keeley and Zedler, 2009).
The highest frequency of SA events occurs during the months of December and January. The mean number of events per season was determined to be 17.8 with an average duration of 2.5 days (Rowland, 2011). SA events can last up to a week or longer and 14 SA events lasted 10 days or longer. The mean number of SA days per season is 44 or 18.1% of the days during the months of September through April with the peak number of days occurring during the month of December. SA event frequency is relatively low in September and April (.99 and 1.7 respectively) with a monthly mean of 2.2 events per month for the eight months of the season which are considered “SA season” (Rowland, 2011). The monthly means then decrease consistently and events are rarely found between June and August due to the reversal in the coastal/desert temperature gradient and the shift in the dominant Pacific high pressure patterns situated west of California that promote westerly flow and inhibit northeasterly winds in southern California (Rowland, 2011; Finley and Raphael, 2007).
One of the most important components in the development of SA events is the formation of a mid-tropospheric stable layer (Hughes and Hall, 2010; Blier 1998; Sommers, 1978; Klemp and Lilly, 1975). Mountain waves can only form in a stable atmosphere. This is one reason SA events occur during the winter months as the static stability in the lower troposphere is somewhat greater than normal at that time of year (Klemp and Lilly, 1975). The stability of the atmosphere over the mountaintop is essential since buoyancy oscillation is a key dynamic in the formation of the mountain wave. A higher stability means that the oscillation will be stronger as lifted air tries to regain equilibrium after passing over the barrier thereby creating waves.
Recent research by Hughes and Hall (2010) and confirmed by Abatzoglou et al. (2013), indicates that cold winter surface temperatures in the desert could play a significant role in the formation of SA winds. Since SA occurrences peak in December, two months before the climatological peak in synoptic disturbances, an additional mechanism may be needed to explain SA occurrences that are out of phase from the large-scale mechanisms. This may explain the initiation of the case study SA in the absence of the synoptic environment. Their research indicates that a mesoscale pressure gradient between the very cold desert surface temperature and the warmer coastal areas would create strong offshore surface flow. Cold air can pool in the desert against the coastal topography resulting in strong density currents pouring through the gaps at the surface when the desert-ocean pressure gradient gets large enough (a katabatic-type mechanism). They found a strong linear relationship between this temperature difference and SA events: as the desert surface gets much colder than the atmosphere over the ocean at the same altitude the likelihood of having strong surface winds increases dramatically.
Further, the strongest SA days have the largest temperature gradient and are enhanced by the previously described synoptic patterns because of cold air advection into the desert. During the 1969 event, the average maximum coastal temperature was 23.5° C and the average minimum temperature in the desert was 7.2° C. From examination of the data, it appears that the temperature gradient temporarily withdraws from November 22 through November 24. On November 23, the average minimum desert temperature rose to 9° C and continued to rise with Palm Springs reporting a minimum temperature of 14.4° C on November 27, suggesting that the thermodynamic mechanism may have played less of a role during the middle of the event and may explain the slight weakening of the SAI at LA area stations on those days. (Data source: NCDC Integrated Surface Hourly Observations)
Local influences accompany a SA regardless of the macro mechanisms involved in event initiation. Gap flow, winds that are intensified due to localized response to topography, contributes to the strength of SA winds (Westerling et al., 2004; Sharp and Mass, 2004). Modeling done by Klemp and Lilly (1975) suggests that asymmetrical mountains, with a steep leeward slope provide the best topography for generating large-amplitude mountain waves. As might be expected, air crosses the mountains preferentially through gaps and passes. Gap flow in a channel and the entrainment of air from above the channel further accelerates winds due to the along-gap pressure gradient and provides an upper limit to the strength of the gap winds at the exit of the channel. Observations of gap flow indicate that the strongest winds are generally located in the exit areas of gaps. Acceleration down the pressure gradient, which is often largest at the exit, plays the most important role in the intensification of winds along a canyon.
Additional wind maxima may also be present within a gap due to Venturi and other hydraulic effects. Within a downward sloping canyon, gap and downslope winds can occur simultaneously during a SA event producing even stronger winds (Sharp and Mass, 2004).
Many of the areas having the highest wind gusts from the 1969 event were in the area of canyons and passes. A gust of 80 mph, the highest recorded wind speed during the entire event, occurred at the San Gorgonio Pass. Fontana, located at the base of the mountains near San Bernardino, also suffered much wind damage including the overturning of mobile homes, downed trees, damaged signs and broken windows. Gusts there were reported as high as 72 mph on November 18 and 78 mph on November 19 where police patrol cars were damaged from blowing sand. Close by in San Bernardino, situated directly below Cajon Pass, classes were cancelled at CSU San Bernardino on November 19 because of high winds and three mobile homes were overturned. Airplanes were damaged when they were torn from their moorings at the nearby Banning airport. Hundreds of trees were uprooted and glass windows were broken in parts of San Bernardino County below the mountain passes.
A complete list of references can be supplied upon request.
The case study, from which this post was excerpted, was originally published in November 2015 as part of the requirements for passing the comprehensive exams for the Environmental Dynamics PhD program at the University of Arkansas.