Windstorm
Description
Section titled “Description”High winds can occur year-round in Jefferson County. In the spring and summer, high winds often accompany severe thunderstorms. Damaging winds are typically those which exceed 60 mph. There are seven types of damaging winds:
Straight-line winds—Any thunderstorm wind that is not associated with rotation; this term is used mainly to differentiate from tornado winds. Most thunderstorms produce some straight-line winds as a result of outflow generated by the thunderstorm downdraft.
Downdrafts—A small-scale column of air that rapidly sinks toward the ground.
Downbursts—A strong downdraft with horizontal dimensions larger than 2.5 miles resulting in an outward burst or damaging winds on or near the ground. Downburst winds may begin as a microburst and spread out over a wider area, sometimes producing damage similar to a strong tornado. Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder.
Microbursts—A small, concentrated downburst that produces an outward burst of damaging winds at the surface. Microbursts are generally less than 2.5 miles across and short-lived, lasting only 5 to 10 minutes, with maximum wind speeds up to 168 mph. There are two kinds of microbursts: wet and dry. A wet microburst is accompanied by heavy precipitation at the surface. Dry microbursts, common in places like the high plains and the intermountain west, occur with little or no precipitation reaching the ground.
Gust front—A gust front is the leading edge of rain-cooled air that clashes with warmer thunderstorm inflow. Gust fronts are characterized by a wind shift, temperature drop, and gusty winds out ahead of a thunderstorm. Sometimes the winds push up air above them, forming a shelf cloud or detached roll cloud.
Derecho—A derecho is a widespread thunderstorm wind caused when new thunderstorms form along the leading edge of an outflow boundary (the boundary formed by horizontal spreading of thunderstorm-cooled air). The word “derecho” is of Spanish origin and means “straight ahead.” Thunderstorms feed on the boundary and continue to reproduce. Derechos typically occur in summer when complexes of thunderstorms form over plains, producing heavy rain and severe wind. The damaging winds can last a long time and cover a large area.
Bow Echo—A bow echo is a linear wind front bent outward in a bow shape. Damaging straight-line winds often occur near the center of a bow echo. Bow echoes can be 200 miles long, last for several hours, and produce extensive wind damage at the ground.
Straight-line winds may exacerbate existing weather conditions, such as blizzards, by increasing the effect on temperature and decreasing visibility due to the movement of particulate matters through the air, as in dust and snowstorms. High winds may also exacerbate fire conditions by drying out the ground cover, propelling fuel, such as tumbleweeds, around the region, and increasing the ferocity of existing fires. These winds may damage crops, push automobiles off roads, damage roofs and structures, and cause secondary damage due to flying debris. Shorter duration winds, such as wind gusts, can cause substantial damage to power lines. Winds with an intermediate duration, which sharply increase and last for a minute, are called squalls. Long-duration wind speeds have various names associated with their average strength, such as breeze, gale, storm, hurricane, and typhoon.
Downslope winds in Colorado are referred to as Chinook winds, after the Native American tribe of the Pacific Northwest. As shown in these downslope winds can occur with violent intensity in areas where mountains stand in the path of strong air currents. These warm and dry winds occur when the winds from the west blow across the Continental Divide and descend from the foothills and out onto the plains.
Figure 4-53 Chinook Wind Pattern
Section titled “Figure 4-53 Chinook Wind Pattern”Source: University of Colorado at Boulder ATOC Weather Lab
Wind can be very dangerous. Areas of wind shear, caused by various weather phenomena, can make treacherous situations for airplanes and other flying aircraft. When winds become too strong on the ground, boats can capsize, trees can be stripped of their branches or uprooted, and man-made structures become vulnerable to damage or destruction. The NWS can issue High Wind Watch, High Wind Warning, and Wind Advisory to the public. The following are the definitions of these issuances:
High Wind Watch—This is issued when there is the potential of high wind speeds developing that may pose a hazard or are life-threatening.
High Wind Warning—The 1-minute surface winds of 35 knots (40 mph) or greater lasting for one hour or longer, or winds gusting to 50 knots (58 mph) or greater, regardless of duration, that are either expected or observed over land.
High Wind Advisory—This is issued when high wind speeds may pose a hazard. Sustained winds 25 to 39 mph and/or gusts to 57 mph.
Jefferson County wind patterns range from light and breezy to severe gale force winds. There is usually some level of a constant breeze due to Jefferson County’s mountainous, Front Range, and plains topography.
Geographic Extent
Section titled “Geographic Extent”The entire planning area is susceptible to wind, windstorms, and wind associated with other storm systems that can have negative impacts on a community. Depending on the origination of the atmospheric system, its direction of travel, and its duration, a part of the planning area can be affected or the entire County. depicts wind zones for the United States. The map shows that the majority of the County falls into Zone II which is characterized by high winds of 160 mph. Typically, however, the hazard is predicted to affect between 50% and 75% of the planning area. Based on this information, the geographic extent rating for windstorms is significant.
Figure 4-54 Wind Zones in the United States
Section titled “Figure 4-54 Wind Zones in the United States”Previous Occurrences
High winds associated with other severe weather and stand-alone windstorms are common occurrences in Jefferson County. The mountainous terrain and foothills topography lends itself to regular conflicts between systems of high and low pressure. Most of Colorado’s most costly storms are hail-related and occurred in the Denver-metro area. Hail is usually accompanied by high winds; however the damages are not broken out to distinguish hail from wind damage. below shows recorded high wind events in Jefferson County between 1955 and 2019.
Figure 4-55 Jefferson County High Wind Events 1955-2019
Section titled “Figure 4-55 Jefferson County High Wind Events 1955-2019”The NCEI database recorded 184 separate High Wind events between January of 2000 and December of 2020 with wind speeds over 50 knots (approximately 57 mph). The most significant of those events are recorded below.
June 14, 1976 – 78 mph winds recorded at the Jefferson County Airport near Broomfield, 66 mph at Littleton.
June 6, 1983 – Report of a thunderstorm with associated winds measured at 61 knots (70 mph). August 15. 1982 – Report of a thunderstorm with associated winds measured at 61 knots (70 mph). August 13, 1983 – Report of a thunderstorm with associated winds measured at 84 knots (97 mph).
June 9, 1987 – Report of a thunderstorm with associated winds measured at 63 knots (73 mph). One death reported.
April 19, 1989 – Report of a thunderstorm with associated winds measured at 68 knots (78mph). May 16, 1990 – Report of a thunderstorm with associated winds measured at 60 knots (69 mph). May 26, 1993 – Report of a thunderstorm with associated winds measured at 70 knots (81mph).
October 26, 1995 – Report of a thunderstorm associated winds measured at 61 knots in Coal Creek Canyon (70 mph).
June 22, 1997 – Report of a dry microburst which produced 69 mph winds at the Jefferson County Airport.
June 10. 2000 – Report of a dry microburst which produced 67 mph winds at the Jefferson County Airport.
July 30, 2004 – Report of a thunderstorm associated winds measured at 62 knots (71 mph) in Evergreen.
July 20, 2009 – Golf ball-sized hail and strong winds battered roofs, uprooted trees and pounded vehicles in Wheat Ridge, Lakewood, and Arvada, and portions of neighboring Arapahoe County. The insured losses are totaled at more than $767.6 million in damage for Colorado’s 2009 severe weather season as of August 2009.
November 12, 2011 – Hurricane force winds up to 115 mph downed trees and power lines across the Front Range Mountains and foothills. The event resulted in 4 reported injuries and several thousand residential power outages.
April 17, 2018 – A powerful damaging wind event with gusts ranging from 60 to 90 mph. Approximately 64,000 Xcel Energy customers experienced some type of outage. One woman died as a result of the event after being struck in the head by a falling tree branch.
February 10, 2020 – Strong winds blew down power lines and power poles in the Town of Morrison. No property damage was reported, but downed power lines caused several power outages that forced some school and business closures.
Probability of Future Occurrences
Section titled “Probability of Future Occurrences”According to the NCEI, there have been 184 separate events with NCEI-recorded high winds above 57 mph (50 knots) in Jefferson County from January 2000 to December 2020. The methodology for calculating the probability of future occurrences is described in Section This formula evaluates that the probability of a Windstorm occurring in any given year is 100%.
This corresponds to a probability of future occurrences rating of highly likely. Magnitude and Severity
Damage from windstorms can be difficult to quantify. Wind, by itself, has not historically caused high insured dollar losses. For the insurance industry to track a weather event, it must be a large enough storm that insurance companies may declare it a catastrophe, and then damage estimates for auto and homeowner claims are collected and published. This generally equates to damages in excess of $25
million, though significant events impacting small communities are also tracked occasionally. demonstrates how destructive wind can be.
Figure 4-56 July 20, 2009 Damage in Wheat Ridge
Section titled “Figure 4-56 July 20, 2009 Damage in Wheat Ridge”Source: Fox News Online Photo Gallery
shows The Beaufort Wind Scale. The replication of the scale only reflects land-based effects.
Table 4-76 The Beaufort Wind Scale
Section titled “Table 4-76 The Beaufort Wind Scale”| Beaufort Number | Description | Windspeed (Knots) | Land Conditions |
|---|---|---|---|
| 0 | Calm | <1 | Calm. Smoke rises vertically. |
| 1 | Light air | 1 – 3 | Wind motion visible in smoke. |
| 2 | Light breeze | 4 – 6 | Wind felt on exposed skin. Leaves rustle. |
| 3 | Gentle breeze | 7 – 10 | Leaves and smaller twigs in constant motion. |
| 4 | Moderate breeze | 11 – 16 | Dust and loose paper raised. Small branches begin to move. |
| 5 | Fresh breeze | 17 – 21 | Branches of a moderate size move. Small trees begin to sway. |
| 6 | Strong breeze | 22 – 27 | Large branches in motion. Whistling heard in overhead wires. Umbrella use becomes difficult. Empty plastic garbage cans tip over. |
| 7 | Near Gale | 28 – 33 | Whole trees in motion. Effort needed to walk against the wind. |
| 8 | Gale | 34 – 40 | Some twigs broken from trees. Cars veer on road. Progress on foot is seriously impeded. |
| 9 | Strong gale | 41 – 47 | Slight structural damage occurs; slate blows off roofs |
| 10 | Storm | 48 – 55 | Seldom experienced on land; trees uprooted or broken; considerable structural damage |
| 11 | Violent storm | 56-63 | |
| 12 | Hurricane | 64+ |
Source: National Oceanographic and Atmospheric Association
and show typical levels of damage that can be expected based on windspeed.
Table 4-77 Damage to Institutional Buildings from High Wind
Section titled “Table 4-77 Damage to Institutional Buildings from High Wind”| Damage Description | Wind Speed Range (Expected Speed) |
|---|---|
| Threshold of visible damage | 59-88 MPH (72 MPH) |
| Loss of roof covering (<20%) | 72-109 MPH (86 MPH) |
| Damage Description | Wind Speed Range (Expected Speed) |
|---|---|
| Damage to penthouse roof & walls, loss of rooftop HVAC equipment | 75-111 MPH (92 MPH) |
| Broken glass in windows or doors | 78-115 MPH (95 MPH) |
| Uplift of lightweight roof deck & insulation, significant loss of roofing material (>20%) | 95-136 MPH (114 MPH) |
| Façade components torn from structure | 97-140 MPH (118 MPH) |
| Damage to curtain walls or other wall cladding | 110-152 MPH (131 MPH) |
| Uplift of pre-cast concrete roof slabs | 119-163 MPH (142 MPH) |
| Uplift of metal deck with concrete fill slab | 118-170 MPH (146 MPH) |
| Collapse of some top building envelope | 127-172 MPH (148 MPH) |
| Significant damage to building envelope | 178-268 MPH (210 MPH) |
Source: National Oceanographic and Atmospheric Association
Table 4-78 Damage to Electric Transmission Lines from High Wind
Section titled “Table 4-78 Damage to Electric Transmission Lines from High Wind”| Damage Description | Wind Speed Range (Expected Speed) |
|---|---|
| Threshold of visible damage | 70-98 MPH (83 MPH) |
| Broken wood cross member | 80-114 MPH (99 MPH) |
| Wood poles leaning | 85-130 MPH (108 MPH) |
| Broken wood poles | 98-142 MPH (118 MPH) |
Source: National Oceanographic and Atmospheric Association
Information from the event of record is used to calculate a magnitude and severity rating for comparison with other hazards, and to assist in assessing the overall impact of the hazard on the planning area. In some cases, the event of record represents an anticipated worst-case scenario, and in others, it is a reflection of common occurrence. The significant wind and windstorm events of record for Jefferson County are identified in the Previous Occurrences section of the windstorm hazard profile. Wind damage is usually identified by the number of insurance claims made as a result of a severe weather event. Wind is not broken out from a hailstorm, rainstorm, or a tornado. The damages inflicted on critical facilities and services (critical infrastructure) for Jefferson County are not specific to windstorm activity alone.
Based on these factors, the magnitude severity ratings for windstorm in Jefferson County would be negligible; however, if the windstorm is considered a component of the larger weather system its magnitude and severity rating would be upgraded to limited.
Climate Change Considerations
Section titled “Climate Change Considerations”According to the best data available at the time of this plan update, the future impacts of climate induced severe wind events are unclear.
Vulnerability Assessment
Section titled “Vulnerability Assessment”It can be assumed that the entire planning area is exposed to some extent to high wind events. Certain areas are more exposed due to geographic location and local weather patterns. Populations living at higher elevations with large stands of trees or power lines may be more susceptible to wind damage and black out. It is not uncommon for residents living in more remote areas of the county to be isolated after such events.
General Property
Section titled “General Property”All property is vulnerable during high wind events, but properties in poor condition or in particularly vulnerable locations may risk the most damage. Generally, damage is minimal and goes unreported. Property located at higher elevations and on ridges may be more prone to wind damage. Property located under or near overhead lines or near large trees may be damaged in the event of a collapse. Wind
pressure can create a direct and frontal assault on a structure, pushing walls, doors, and windows inward. Conversely, passing currents can create lift and suction forces that act to pull building components and surfaces outward. The effects of winds are magnified in the upper levels of multi-story structures. As positive and negative forces impact the building’s protective envelope (doors, windows, and walls), the result can be roof or building component failures and considerable structural damage.
People
Section titled “People”Windstorms can cause injury and death in Jefferson County. The highest risk demographic is to first responders who are dealing with emergency situations resulting from the windstorm. Those working or recreating outdoors will be susceptible to injury from wind borne debris. Winds can also be hazardous to hikers in areas of beetle or fire killed trees, which occurred when a hiker was killed by a falling tree in Rocky Mountain National Park in 2007.
Vulnerable populations also include the elderly, low income or linguistically isolated populations, people with life-threatening illnesses, and residents living in areas that are isolated from major roads. Power outages can be life threatening to those dependent on electricity for life support. In Jefferson County, 11% of Medicare Beneficiaries rely on electricity to live independently in their homes. Isolation of these populations is a significant concern. These populations face isolation and exposure during wind events and could suffer more secondary effects of the hazard. Hikers and climbers in the area may also be more vulnerable to severe wind events.
Critical Facilities and Infrastructure
Section titled “Critical Facilities and Infrastructure”High winds can cause significant damage to trees and power lines, blocking roads with debris, incapacitating transportation, isolating population, and disrupting ingress and egress. Of particular concern are roads providing access to isolated areas and to the elderly. Severe windstorms and downed trees can create serious impacts on power and above-ground communication lines. Loss of electricity and phone connection would leave certain populations isolated because residents would be unable to call for assistance.
Economy
Section titled “Economy”Economic impacts of severe wind are typically short term. These events can disrupt travel into and out of all areas of the county and create perilous conditions for residents, tourists, and nature alike.
The effect of high winds on power delivery is a relevant factor when assessing current development exposure. Xcel Energy provided data from one high wind event in 2009 when 2 days of high winds interrupted power for 67,128 customers. Xcel estimated it cost $167,820 to repair the outage equating to a cost of roughly $25,000 for every 10,000 customers impacted by high winds. FEMA Standard Values for Loss of Service for Utilities, located in Appendix C of the FEMA BCA Reference Guide, estimates that a power supply interruption costs the average person $126 per day of service outage. By this estimate, this event caused $16,916,256 in economic impacts or $8,458,128 per day of service interruption due to high winds.
Historical, Cultural, and Natural Resources
Section titled “Historical, Cultural, and Natural Resources”The environment is highly exposed to high winds. Environmental impacts include the downing of trees and localized flattening of plants by high wind. Natural habitats such as streams and trees risk major damage and destruction.
Future Development
Section titled “Future Development”Construction sites are particularly vulnerable to windstorms. Wind-borne construction materials can become hazards to life and property. New construction designed in accordance with the Jefferson County wind load map should be able to withstand or at least resist wind damage if properly constructed. Backup power systems in critical facilities could help mitigate impacts from power outages associated with windstorms.
The ongoing development along State Highway 93 is in a region of the County that is very vulnerable to high winds. Construction sites, both residential and transportation related (the Jefferson Parkway, a multi-
lane arterial planned to connect Highway 93 to Highway 36 through Arvada) could be at risk of wind borne construction materials.
Overall Hazard Significance
Section titled “Overall Hazard Significance”Windstorms in Jefferson County can have a particular impact on the planning area. Alone they can rip roofs from houses, collapse fences, tear off siding, project flying debris through windows, and uproot large trees. When accompanying other severe weather, like hail, damages are compounded. The geographic extent of the hazard is considered significant. The probability of future occurrences is considered highly likely and the magnitude/severity for the event of record is limited. The HMPC considers the hazard to have an overall impact rating of medium on Jefferson County. Overall, the data indicates that the overall hazard significance rating is medium.