Section 10: Windstorm

Press Enter to show all options, press Tab go to next option

windstormWhy are Severe Windstorms a Threat to the Southern California?

Severe wind storms pose a significant risk to life and property in the region by creating conditions that disrupt essential systems such as public utilities, telecommunications, and transportation routes. High winds can and do occasionally cause tornado-like damage to local homes and businesses. Severe windstorms can present a very destabilizing effect on the dry brush that covers local hillsides and urban wildland interface areas. High winds can have destructive impacts, especially to trees, power lines, and utility services.

Windstorm Characteristics in Southern California

Santa Ana Winds and Tornado-Like Wind Activity
Based on local history, most incidents of high wind in the City of West Covina are the result of the Santa Ana wind conditions. While high impact wind incidents are not frequent in the area, significant Santa Ana Wind events and sporadic tornado activity have been known to negatively impact the local community.

What are Santa Ana Winds

"Santa Ana winds are generally defined as warm, dry winds that blow from the east or northeast (offshore). These winds occur below the passes and canyons of the coastal ranges of Southern California and in the Los Angeles basin. Santa Ana winds often blow with exceptional speed in the Santa Ana Canyon (the canyon from which it derives its name). Forecasters at the National Weather Service offices in Oxnard and San Diego usually place speed minimums on these winds and reserve the use of "Santa Ana" for winds greater than 25 knots." These winds accelerate to speeds of 35 knots as they move through canyons and passes, with gusts to 50 or even 60 knots.

"The complex topography of Southern California combined with various atmospheric conditions create numerous scenarios that may cause widespread or isolated Santa Ana events. Commonly, Santa Ana winds develop when a region of high pressure builds over the Great Basin (the high plateau east of the Sierra mountains and west of the Rocky mountains including most of Nevada and Utah). Clockwise circulation around the center of this high pressure area forces air downslope from the high plateau. The air warms as it descends toward the California coast at the rate of 5 degrees F per 1000 feet due to compressional heating. Thus, compressional heating provides the primary source of warming. The air is dry since it originated in the desert, and it dries out even more as it is heated."

These regional winds typically occur from October to March, and, according to most accounts are named either for the Santa Ana River Valley where they originate or for the Santa Ana Canyon, southeast of Los Angeles, where they pick up speed.

What are Tornados

Tornadoes are spawned when there is warm, moist air near the ground, cool air aloft, and winds that speed up and change direction. An obstruction, such as a house, in the path of the wind causes it to change direction. This change increases pressure on parts of the house, and the combination of increased pressures and fluctuating wind speeds creates stresses that frequently cause structural failures.

In order to measure the intensity and wind strength of a tornado, Dr. T. Theodore Fujita developed the Fujita Tornado Damage Scale. This scale compares the estimated wind velocity with the corresponding amount of suspected damage. The scale measures six classifications of tornadoes with increasing magnitude from an "F0" tornado to a "F6+" tornado.

The chart below depicts the Fujita Tornado Damage Scale.

Table 10-1 Fujita Tornado Damage Scale


Unlike tornados, microbursts, are strong, damaging winds which strike the ground and often give the impression a tornado has struck. They frequently occur during intense thunderstorms. The origin of a microburst is downward moving air from a thunderstorm's core. But unlike a tornado, they affect only a rather small area.

University of Chicago storm researcher Dr Ted Fujita first coined the term "downburst" to describe strong, downdraft winds flowing out of a thunderstorm cell that he believed were responsible for the crash of Eastern Airlines Flight 66 in June of 1975.

A downburst is a straight-direction surface wind in excess of 39 mph caused by a small-scale, strong downdraft from the base of convective thundershowers and thunderstorms. In later investigations into the phenomena he defined two sub-categories of downbursts: the larger macrobursts and small microbursts.

Macrobursts are downbursts with winds up to 117 mph which spread across a path greater than 2.5 miles wide at the surface and which last from 5 to 30 minutes. The microburst, on the other hand is confined to an even smaller area, less than 2.5 miles in diameter from the initial point of downdraft impact. An intense microburst can result in damaging winds near 270 km/hr (170 mph) and often last for less than five minutes.

"Downbursts of all sizes descend from the upper regions of severe thunderstorms when the air accelerates downward through either exceptionally strong evaporative cooling or by very heavy rain which drags dry air down with it. When the rapidly descending air strikes the ground, it spreads outward in all directions, like a fast-running faucet stream hitting the sink bottom.

When the microburst wind hits an object on the ground such as a house, garage or tree, it can flatten the buildings and strip limbs and branches from the tree. After striking the ground, the powerful outward running gust can wreak further havoc along its path. Damage associated with a microburst is often mistaken for the work of a tornado, particularly directly under the microburst. However, damage patterns away from the impact area are characteristic of straight-line winds rather than the twisted pattern of tornado damage."

Tornados, like those that occur every year in the Midwest and Southeast parts of the United States, are a rare phenomenon in most of California, with most tornado-like activity coming from micro-bursts.

Local History of Windstorm Events

While the effects of Santa Ana Winds are often overlooked, it should be noted that in 2003, two deaths in Southern California were directly related to the fierce condition. A falling tree struck one woman in San Diego. The second death occurred when a passenger in a vehicle was hit by a flying pickup truck cover launched by the Santa Ana Winds.

Table 10-2

The following Santa Ana wind events were featured in news resources during 2003:

The following is a glimpse of some major Santa Ana wind/windstorm events to hit the local area:

Table 10-3 Major Windstorms / Santa Ana Wind Events

Orange County Area from 1961- 2001

Date Location and Damage
November 5-6, 1961 Santa Ana winds. Fire in Topanga Canyon
February 10-11, 1973 Strong storm winds:. 57 mph at Riverside, 46 Newport Beach. Some 200 trees uprooted in Pacific Beach alone
October 26-27, 1993 Santa Ana winds. Fire in Laguna Hills
October 14, 1997 Santa Ana winds: gusts 87 mph in central Orange County. Large fire in Orange County
December 29, 1997 Gusts 60+ mph at Santa Ana
March 28-29, 1998 Strong storm winds in Orange County: sustained 30-40 mph. Gust 70 mph at Newport Beach, gust 60 Huntington Beach. Trees down, power out, and damage across Orange and San Diego Counties. 1 illegal immigrant dead in Jamul.
September 2, 1998 Strong winds from thunderstorms in Orange County with gusts to 40mph. Large fires in Orange County
December 6, 1998 Thunderstorm in Los Alamitos and Garden Grove: gust 50-60 mph called "almost a tornado"
December 21-22, 1999 Santa Ana winds: gust 68 mph at Campo, 53 Huntington Beach, 44 Orange. House and tree damage in Hemet.
March 5-6, 2000 Strong thunderstorm winds at the coast: gust 60 mph at Huntington Beach Property damage and trees downed along the coast
April 1, 2000 Santa Ana winds: gust 93 mph at Mission Viejo, 67 Anaheim Hills
December 25-26, 2000 Santa Ana winds: gust 87 mph at Fremont Canyon. Damage and injuries in Mira Loma, Orange and Riverside Counties
February 13, 2001 Thunderstorm gust to 89 mph in east Orange

The following chart is a glimpse of major tornado-like events to hit the Southern California region:

Table 10-4

Windstorm Hazard Assessment

Hazard Identification

A windstorm event in the region can range from short term microburst activity lasting only minutes to a long duration Santa Ana wind condition that can last for several days as in the case of the January 2003 Santa Ana wind event. Windstorms in the City of West Covina area can cause extensive damage including heavy tree stands, exposed coastal properties, road and highway infrastructure, and critical utility facilities.

The map shows clearly the direction of the Santa Ana winds as they travel from the stable, high-pressure weather system called the Great Basin High through the canyons and towards the low-pressure system off the Pacific. Clearly the area of the City of West Covina is in the direct path of the ocean-bound Santa Ana winds.

Vulnerability and Risk

With an analysis of the high wind and tornado events depicted in the "Local History" section, we can deduce the common windstorm impact areas including impacts on life, property, utilities, infrastructure and transportation. Additionally, if a windstorm disrupts power to local residential communities, the American Red Cross and City resources might be called upon for care and shelter duties. Displacing residents and utilizing City resources for shelter staffing and disaster cleanup can cause an economic hardship on the community.

The prevailing wind in West Covina is a result of the Southern California onshore breeze that usually occurs every afternoon. The force of this wind is influenced by the differential temperatures over the ocean and inland areas. The force of the wind can also be affected by any associated high and low pressure areas. This afternoon breeze is a southwest wind, meaning that it blows from the southwest to the northwest.

Although West Covina is not located in a hazardous wind zone or special wind region, the City has experienced wind damage in the past. Most wind damage in the City of West Covina has been the result of combination storms of wind and rain. West Covina does experience Santa Ana conditions and the resulting high winds during this dry period. Wind conditions may add to the destruction associated with other natural hazards; such as wind driven wildfires, or as mentioned before the combination of rain saturated ground and the additional forces caused by the wind.

Historically, storm winds have caused the greatest amount of damage in West Covina. With these types of winds there is no particular area of the City that is more susceptible than another. On September 2, 1998 a late summer storm blew through the City and surrounding areas. This storm caused numerous utility wires to drop, with the West Covina Fire Department responding out to twelve calls of wires down, two tree fires, and a structure fire possibly caused by the surge in power as lines went down. Along Azusa Avenue, multiple power poles had snapped. Post-storm recovery required $18,364.00 in crew time to handle 250 requests for removal of hazardous trees and limbs.

On November 30, 1982 storm winds as high as 75 mph along with heavy rains swept through the area. The Maintenance Department lists damage to over 2000 trees in the City, 600 of these city-owned trees. Over 1000 calls for service were taken by the Communications Department, the City's 911 center. Complicating communications the repeater for Fire and Police communications was blown out of alignment, and both Public Safety agencies religned on the Maintenance Department channel to communicate via radio.



Community Windstorm Issues

What is Susceptible to Windstorms

Life and Property
Based on the history of the region, windstorm events can be expected, perhaps annually, across widespread areas of the region which can be adversely impacted during a windstorm event. This can result in the involvement of City of West Covina emergency response personnel during a wide-ranging windstorm or microburst tornadic activity. Both residential and commercial structures with weak reinforcement are susceptible to damage. Wind pressure can create a direct and frontal assault on a structure, pushing walls, doors, and windows inward. Conversely, passing currents can create lift suction forces that pull building components and surfaces outward. With extreme wind forces, the roof or entire building can fail causing considerable damage.

Debris carried along by extreme winds can directly contribute to loss of life and indirectly to the failure of protective building envelopes, siding, or walls. When severe windstorms strike a community, downed trees, power lines, and damaged property can be major hindrances to emergency response and disaster recovery.

The Beaufort Scale on the next page, coined and developed by Sir Francis Beaufort in 1805, illustrates the effect that varying wind speed can have on sea swells and structures:



Historically, falling trees have been the major cause of power outages in the region. Windstorms such as strong microbursts and Santa Ana Wind conditions can cause flying debris and downed utility lines. For example, tree limbs breaking in winds of only 45 mph can be thrown over 75 feet. As such, overhead power lines can be damaged even in relatively minor windstorm events. Falling trees can bring electric power lines down to the pavement, creating the possibility of lethal electric shock. Rising population growth and new infrastructure in the region creates a higher probability for damage to occur from windstorms as more life and property are exposed to risk.


Windstorms can damage buildings, power lines, and other property and infrastructure due to falling trees and branches. During wet winters, saturated soils cause trees to become less stable and more vulnerable to uprooting from high winds.

Windstorms can result in collapsed or damaged buildings or blocked roads and bridges, damaged traffic signals, streetlights, and parks, among others. Roads blocked by fallen trees during a windstorm may have severe consequences to people who need access to emergency services. Emergency response operations can be complicated when roads are blocked or when power supplies are interrupted. Industry and commerce can suffer losses from interruptions in electric services and from extended road closures. They can also sustain direct losses to buildings, personnel, and other vital equipment. There are direct consequences to the local economy resulting from windstorms related to both physical damages and interrupted services.

Increased Fire Threat
Perhaps the greatest danger from windstorm activity in Southern California comes from the combination of the Santa Ana winds with the major fires that occur every few years in the urban/wildland interface. With the Santa Ana winds driving the flames, the speed and reach of the flames is even greater than in times of calm wind conditions. The higher fire hazard raised by a Santa Ana wind condition requires that even more care and attention be paid to proper brush clearances on property in the wildland/urban interface areas.

Windstorm activity can have an impact on local transportation in addition to the problems caused by downed trees and electrical wires blocking streets and highways. During periods of extremely strong Santa Ana winds, major highways can be temporarily closed to truck and recreational vehicle traffic. However, typically these disruptions are not long lasting, nor do they carry a severe long term economic impact on the region.

Existing Windstorm Mitigation Activities

As stated, one of the most common problems associated with windstorms is power outage. High winds commonly occur during winter storms, and can cause trees to bend, sag, or fail (tree limbs or entire trees), coming into contact with nearby distribution power lines. Fallen trees can cause short-circuiting and conductor overloading. Wind-induced damage to the power system causes power outages to customers, incurs cost to make repairs, and in some cases can lead to ignitions that start wild land fires.


One of the strongest and most widespread existing mitigation strategies pertains to tree clearance. Currently, California State Law requires utility companies to maintain specific clearances (depending on the type of voltage running through the line) between electric power lines and all vegetation. Enforcement of the following California Public Resource Code Sections provides guidance on tree pruning regulations:

4293: Power Line Clearance Required
4292: Power Line Hazard Reduction
4291: Reduction of Fire Hazards Around Buildings
4171: Public Nuisances

The following pertain to tree pruning regulations and are taken from the California Code of Regulations:

Title 14: Minimum Clearance Provisions
Sections 1250-1258
General Industry Safety Orders
Title 8: Group 3: Articles 12, 13, 36, 37, 38
California Penal Code Section 385

treeFinally, the following California Public Utilities Commission section has additional guidance:

California Public Utilities Commission
General Order 95: Rule 35

Homeowner Liability:
Failure to allow a utility company to comply with the law can result in liability to the homeowner for damages or injuries resulting from a vegetation hazard. Many insurance companies do not cover these types of damages if the policy owner has refused to allow the hazard to be eliminated.

The power companies, in compliance with the above regulations, collect data about tree failures and their impact on power lines. This mitigation strategy assists the power company in preventing future tree failure. From the collection of this data, the power company can advise residents as to the most appropriate vegetative planting and pruning procedures. The following chart depicts some of the tree failure data collected by Southern California Edison in this comprehensive mitigation strategy:

Windstorm Mitigation Action Items

The windstorm mitigation action items provide direction on specific activities that organizations and residents in City of West Covina can undertake to reduce risk and prevent loss from windstorm events. Each action item is followed by ideas for implementation, which can be used by the Hazard Mitigation Planning Committee and local decision makers in pursuing strategies for implementation.

City of West Covina Mitigation Strategy Recommendations

LT - Wind #1: Underground utilities on Azusa Avenue from Aroma Drive to Amar Road. Relocate overhead utilities to conduits placed underground along Amar Road from Azusa Avenue to Temple Avenue.
Ideas for Implementation:

  • All new utility projects designed for underground service.
  • Identify areas where existing service needs to be better protected.
  • Coordinate with Southern California Edison on existing and proposed projects.
Coordinating Organization: Public Works
Timeline: Ongoing
Plan Goals Addressed: Partnerships and Implementation, Protect Life and Property
Constraints: To be addressed as new projects are brought through the city planning process.

LT - Wind #2: Reduce tree damage from storms throughout the City. Ideas for Implementation:
  • Work with Maintenance Department to determine best ways to limit damage to trees.
  • Work with Maintenance Department to develop tree-trimming schedule for optimum effectiveness to maintain healthy trees.
  • Determine tree planting plan to include type of trees to plant, when to plant, where easement trees will be planted, and how and when they will be maintained.
Coordinating Organization: Public Works
Timeline: Ongoing
Plan Goals Addressed: Protect Life and Property
Constraints: Incorporation into the current process, data entry and programming to track inventory of trees.

LT - Wind #3: Public Awareness Campaign: To provide public education materials to City of West Covina residents and all School District staff, parents and age-appropriate students with mitigation materials pertaining to the protection of life and property before, during, and after a windstorm.
Ideas for Implementation:
  • Compile mitigation brochures from the following organizations: FEMA; California Public Utilities Commission; County of Public Works; Southern California Edison; Tree Line Connection
  • Distribute these materials to City of West Covina residents and school district members. Materials can be distributed at City Council Meetings, Commission Meetings, City Hall, Parks and Recreation Centers, Fire Departments, Police Departments, Chamber of Commerce Meetings, School Administration Offices and other appropriate venues.
  • Create community PowerPoint seminar to be given at CERT/RACES joint hazard training event. Utilize presentation at future City Council Meetings or other public events as appropriate.
Coordinating Organization: City of West Covina Emergency Services
Timeline: Ongoing
Plan Goals Addressed: Public Awareness, Protection of Life and Property
Constraints: Development and distribution of Public Education materials.

LT - Wind #4: Create local City and utility awareness of tree pruning and Fire Code Sections relevant to wind-resistant utility operations
Ideas for Implementation:
Provide information to City Planning Departments and local utility companies encouraging compliance with State and Local tree clearance and integrity guidelines by:
  • Compile comprehensive list of pertinent State and local regulations
  • Send letters of encouragement from Hazard Mitigation Planning Committee and local City and School officials encouraging utility compliance with guidelines
Coordinating Organization: Planning Dept, Public Works, Emergency Services Offices
Timeline: Ongoing
Plan Goals Addressed: Public awareness; Partnerships and implementation
Constraints: Staff time in developing program.

LT - Wind #5: Encourage Critical City Facilities to purchase and/or test backup power facilities for use during a power failure. Create a equipment/testing log to ensure backup power equipment is in working service.
Ideas for Implementation:
  • Gather all databases of backup power equipment for critical facilities.
  • Test all critical facility backup power generators.
  • Keep an accurate record of equipment specification and testing date information.

Coordinating Organization: Public Works, Emergency Management
Timeline: Ongoing
Plan Goals Addressed: Emergency Services; Protect Life and Property
Constraints: None

Windstorm Resource Directory

State Resources

California Division of Forestry & Fire Protection
1416 9th Street
PO Box 944246
Sacramento California 94244-2460

Federal Resources and Programs

National Weather Service
Los Angeles/Oxnard Weather Forecast Office
520 North Elevar Street
Oxnard, CA 93030
Forecast and weather info: 805-988-6610
Administrative issues: 805-988-6615

Additional Resources

International Society of Arboriculture.
P.O. Box 3129
Champaign, IL 61826-3129
Phone: 217.355.9411
Fax: 217.355.9516


WINDSTORMS: Protect Your Family and Property from the Hazards of Violent Windstorms

Preparing Your Home for Severe Windstorms is available from

Section 10 - Windstorm End Notes

ii. Ibid
iii. Keith C. Heidorn
iv. Ibid
v. Ibid
vi. Ibid
vii., January 8, 2003
View Full Site