Hailstorms

Description

Hailstorms are any storm events where hailstones fall. Hail forms when updrafts carry raindrops into extremely cold areas of the atmosphere where the drops freeze into ice. Hail falls when it becomes heavy enough to overcome the strength of the updraft and is pulled by gravity towards the earth. The process of falling, thawing, moving up into the updraft and refreezing before falling again may repeat many times, increasing the size of the hailstone. Hailstones are usually less than two-inches in diameter, but have been reported much larger and may fall at speeds of up to 120 mph. Hailstorms occur throughout the spring, summer, and fall in the region, but are more frequent in late spring and early summer. These events are often associated with thunderstorms that may also cause high winds and tornadoes. Hail causes nearly $1 billion in damage to crops and property each year in the United States. Hail is also one of the requirements which the National Weather Service uses to classify thunderstorms as severe. If hail more than ¾ of an inch is produced in a thunderstorm, it qualifies as severe.

The National Weather Service classifies hail by diameter size, and corresponding everyday objects to help relay scope and severity to the population. The table below indicates the hailstone measurements utilized by the National Weather Service.

Table 4-52 Hailstone Measurements

Average Diameter	Corresponding Household Object
.25 inch	Pea
.5 inch	Marble/Mothball
.75 inch	Dime/Penny
.875 inch	Nickel
1.0 inch	Quarter
1.5 inch	Ping-pong ball
1.75 inch	Golf-Ball
2.0 inch	Hen Egg
2.5 inch	Tennis Ball
2.75 inch	Baseball
3.00 inch	Teacup
4.00 inch	Grapefruit
4.5 inch	Softball

Source: National Weather Service

In Colorado, hail is one of the most damaging of natural hazards. In fact, the 1996 July hailstorm set a record for most damaging hailstorm on a national level at that time. According to the 2018 State Hazard Mitigation Plan, the damaging hail season in Colorado ranges from mid-April to mid-August. Colorado’s Front Range, including the entire planning area, is located in the heart of Hail Alley, which receives the highest frequency of large hail in North America and most of the world. According to an April 2020 report from the National Insurance Crime Bureau (NICB), Colorado had the second highest number of insurance claims involving hail from 2017-2019. The Rocky Mountain Insurance Information Association (RMIIA) also reports that hailstorms have caused upwards of $5 billion over the last 10 years.

Figure 4-38 Jefferson County Hail Events

Geographic Extent

Hailstorms occur during severe storms, which are regional in nature. However, just as the amount of precipitation in the form of snow or rain may vary significantly within a single storm, so may the amount, size, and duration of hail within a severe storm. In general, hail can fall anywhere in Colorado. The areas where hail is most frequently reported with damaging effects are in the eastern plains, where hail damages crops and livestock, and in the Denver metro area, where hailstorms damage buildings, cars and trees, and may cause driving conditions to deteriorate. The extent of impact ranges from limited, where a single community within the planning area is affected, to significant, where more than 50% of the County was impacted. There are no known incidents where a single hailstorm impacted more than 75% of the County; however, so while hail is possible anywhere in the planning area, it is not likely to affect the entire area simultaneously. below illustrates the location and magnitude of hail events within and adjacent to the planning area from 1955-2019.

Based on this information, the geographic extent rating for hailstorms is significant. Previous Occurrences

Since hailstorms are so prevalent in Colorado, the most useful previous occurrences to examine are those which caused a particularly high amount of damage or incurred some other unique cost or impact. The NCEI database records 422 hail events in the planning area between January 1, 1950 and December 31, 2020. Nineteen of those storms reported hailstones at least two inches in diameter; however, some of these individual storm records reflect the different size hailstones for the same storm event, so the data is somewhat skewed. Several selected incidents, including some not captured in the NCEI database, are profiled below. These selections illustrate the severity of the hail hazard for the jurisdiction and are representative of the range and risk but are not comprehensive.

June 13, 1984 – A mega rain/hailstorm occurred on June 13, 1984. Severe thunderstorms crossed northern Jefferson County and western Adams County dropping 2 to 4” rain and 1” to 3.5” diameter hail. There was serious flooding in Arvada, Westminster, Wheat Ridge and Lakewood. Damage was estimated at $350-$400 million ($723-$825 million in 2008 dollars) damage in Jefferson County.

July 11, 1990 – A storm with hailstones of up to 2.75” in diameter incurred 13 injuries in the planning area. A companion entry for the same date indicated the hail size was 1.75” but that 47 injuries were reported, which were mostly documented in Elitch Gardens (then located in Denver County). The RMIIA placed the total insured hail damages for the affected area at $625 million ($1.03 billion in 2009 dollars). The storm impacted Adams, Arapahoe, Boulder, Denver, Elbert, Jefferson and Larimer counties, with the heaviest damages reported in Jefferson County. Additional accounts indicate that this was the costliest hailstorm in U.S. history, as hail ranged along the entire Front Range. Jefferson County also suffered severe damages to aircraft at the Jefferson County Airport, power and utilities were disrupted to thousands of residents, and storm drains clogged with hail flooded roads three to six feet deep in Arvada.

June 1, 1991 – Intense thunderstorms formed in northern Jefferson County on June 1, 1991. These storms flooded streets and urban streams from Columbine County Club through Lakewood into Golden with 0.75” to 1.5” diameter hail and 1.5” to 3.5” rainfall in less than 1 hour. I didn’t have information on the estimated damage for this event.

October 1, 1994 – An afternoon hailstorm, lasting for nearly three hours as it crossed the Denver metro area, produced hail ranging from pea to golf ball sizes. Damages and incidents reported in the planning area include Arvada, Edgewater, and Wheat Ridge. Other impacted areas included Denver, Boulder, Last Chance, Bennett, Strasburg, Wiggins, Penrose, and the Buckley Air National Guard Base near Aurora.

Overall insured estimates, sourced by RMIIA, totaled at $225 million ($326 million in 2009 dollars).

May 22, 1996 – A severe thunderstorm producing large hail ranging in size from 3/4 to two inches in diameter rumbled across the northwest and northern portions of the Denver metropolitan area. The thunderstorm apparently developed from an outflow boundary generated from the supercell thunderstorm that moved across extreme northeastern Colorado earlier in the evening. The storm developed near the foothills and moved east northeast across northern portions of the metro area. The hardest hit areas were cities of Arvada and Westminster, northwest of Denver. The insurance industry estimated $60 million in damage to homes and personal property and $62 million in damage to automobiles for a total of $122

million in insured losses ($166.8 million in 2009 dollars). This estimate also included the cities of Golden, Thornton, and Wheat Ridge.

June 8, 2004 – A series of hailstorms stretching along the Front Range from Colorado Springs to Larimer County and out to the eastern border of the state dropped hailstones ranging from dime to golf ball sized. The hail in Jefferson County fell mostly between 7:00 and 8:00 pm across Evergreen and Golden. The next afternoon, Morrison, Conifer, and Lakewood were all impacted by large hailstorms as well.

Statewide, insurance damages were reported at $146.5 million ($166.4 million in 2009 dollars). This storm was classified as the eighth most costly hailstorm event in Colorado history as of July 2009.

May 24, 2007 – Several fast-moving storms dropped substantial amounts of hail in the foothills southwest of Denver. One hailstorm impacted U.S. Highway 285 near Aspen Park, where state patrol reported two inches of pea-sized hail fell on the highway, causing it to become snow packed and slick. Four associated accidents were reported shortly thereafter, including three roll-overs in a 10-minute period of time. No injuries were reported, and damages were estimated at $20,000 ($20,700 in 2009 dollars (most recent data available)).

July 20, 2009 – In an unusual overnight storm, rain, winds and golf-ball sized hail battered roofs, uprooted trees, damaged homes, and pounded vehicles in Wheat Ridge, Lakewood, Arvada and Englewood. Most of the damage in this storm are attributed to property losses, with 32,900 homeowner claims and 19,500 automobile claims filed as of July 27, 2009, which amounts to $350 million in insurance claims based on preliminary estimates. While the entire Denver metro area was impacted by the storm, the most significant damages were reported in Jefferson County. This storm is projected to be the second costliest natural disaster in Colorado, in terms of insured losses.

May 8, 2017 – A severe afternoon thunderstorm produced what would become the most expensive insured catastrophe in Colorado state history, and the second costliest hailstorm in US history. Hailstones recorded in the event ranged in size from 0.75 inches to 2.75 inches in diameter depending on the location and impacted a large highly populated area of Jefferson County including the cities of Lakewood, Arvada, and Wheat Ridge. According to NCEI, an estimated 150,000 auto insurance claims and 50,000 homeowner insurance claims were filed. The event severely damaged and forced a six month closure of the Colorado Mills Mall in Lakewood, resulting in an estimated monthly loss of $350,000 in lost sales tax revenue in addition to lost business revenue. The total damage cost of the event totaled around $2.3 billion.

Probability of Future Occurrences

The record of previous occurrences, as discussed earlier, is incomplete as well, but provides a useful reference for hailstorms which produced significant size stones and/or caused damage. Calculating that Jefferson County experiences six hail events per year is less useful than determining how frequently the planning area may experience a severe event. According to RMIIA, there have been eight severe hailstorms which caused more than $100 million in damages that impacted Jefferson County in some way since 1990. Since the last plan update, the NCEI records have been updated to include the 2017 event and an event in Columbine that caused over $350 million in damage to property. This data will be used to determine the probability of a severe hailstorm in Jefferson County.

There have been 87 severe incidents, defined as hailstones 1 inch or greater in diameter in the 2018 Colorado State Hazard Mitigation Plan, involving Jefferson County since 1990. The methodology for calculating the probability of future occurrences is described in Section This formula evaluates that the probability of a severe hailstorm occurring in any given year is 290%. If the same methodology is applied to all hailstorms (including those that cause minimal damage), then there have been 422 events since 1950, for a span of 70 years. This indicates that Jefferson County can expect an average of 6 hailstorms per year.

This corresponds to a probability of future occurrences rating of highly likely. Magnitude and Severity

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 event of record for Jefferson County occurred on May 8, 2017. According to the RMIAA, the event caused $2.3 billion in damages to property in the jurisdiction. This storm was the costliest in Colorado history and the second costliest storm in US history.

Also of note are the July 20, 2009 and July 11, 1990 hail events. The former of these events resulted in

$767.6 million in insured damages according to the RMIAA. The latter resulted in 60 direct injuries in the duration of the event, and damages inflicted on critical facilities and services (critical infrastructure) resulted in a loss or disruption of serves for a minimal amount of time. Documented injuries were considered critical, though the medical response of the jurisdiction was considered minimally impacted.

According to the RMIAA, eight of the top ten hazard events in Colorado by the amount of insured loss were either entirely hail-related or involved hail as a hazard. RMIAA also ranks Colorado 2nd in the U.S. for hail insurance claims.

Based on these factors, the magnitude severity rating for hailstorms is considered critical. Climate Change Considerations

According to the 2018 Colorado State Hazard Mitigation Plan, the future impacts of climate change are

expected to influence future hail events. Ongoing efforts to reduce Colorado’s greenhouse gas emissions and adapt to a changing climate, such as the Colorado Climate Plan and the Climate Change in Colorado Report, will help to reduce the impacts of climate induced hazard such as hail.

Vulnerability Assessment

All assets located in Jefferson County can be considered at risk from severe hail events. This includes 100% of the County’s population, and all buildings and infrastructure within the County.

General Property

Research into the damages inflicted by this hazard indicates the hazard has a high impact on the entire planning area, and perhaps the greatest economic impacts. Hail impacts anything exposed to the event, including structures, infrastructure, landscaping, personal property and vehicles, people, agriculture, and livestock. Jefferson County has the highest number of reported injuries due to hail in the state. Hail is also the costliest insured-losses natural disaster to impact the state of Colorado, with nine separate incidents falling within the top ten disasters list for the state. Existing development remains exposed to hail with minimal mitigation opportunities. Individuals can mitigate exposure by remaining indoors and away from windows during hailstorm events. Vehicles can be parked under shelters to help minimize damage costs incurred in that arena. However, in many cases it is impossible to move existing development away from the impact areas. For example, hail heavily impacts the economic contributors who house merchandize outdoors, such as car retailers, home improvement stores and gardening stores. Damage to landscape and agriculture is also almost impossible to prevent, as the plants cannot be transported indoors for the storm.

People

Exposure is the greatest danger to people from hail, for those caught outside in the open without shelter. Large hail has the potential to cause significant bruising, concussions, the potential for broken bones, and even death. The impacts of hail on vulnerable populations can be more severe. Low income families are more likely to live in poorly constructed homes that are more likely to be damaged, and are more likely to be uninsured or underinsured, making it more difficult for them to recover from hail events. Individuals with disabilities may need more assistance after a major event, especially if transportation or utility services are disrupted. Severe weather warnings must use methods that reach vision or hearing-impaired people and those with limited English proficiency.

Critical Facilities and Infrastructure

Hail can lead to the temporary incapacitation of roads when small hail stones build up so deep, they block roads. Hail has also been observed to block storm drains and prevent proper runoff, potentially resulting in flooding as a secondary hazard. Most structures, including the County’s critical facilities, should be able to provide adequate protection from hail but the structures could suffer broken windows and dented exteriors. Those facilities with back-up generators are better equipped to handle a severe weather situation should the power go out.

Economy

The economic impact from hail can be severe on impacted areas, and potentially long lasting. As mentioned throughout this section, hail is the costliest hazard experienced in the planning area. Direct damages have totaled $5 billion over the last 10 years (averaging to $500,000,000 per year), but severe indirect economic impacts can also be felt through businesses forced to close for repairs. For example, the 2017 event led to the city of Lakewood losing an estimated $350,000 in monthly sales tax revenue due to a several month closure of the Colorado Mills mall. Impacts such as these can result in lost revenue and employment, adding to the impact of direct damage costs. Insurance helps to offset some, but not all, of these losses.

Historical, Cultural, and Natural Resources

While hail is a natural environmental process, it can cause significant environmental damage, breaking tree limbs, damaging trees and other plants in bloom, and destroying crops. Some cultural and historic properties may also potentially be at risk of damage from hail.

Future Development

Consideration for future development may include the use of resilient landscaping or the construction of covered parking to minimize those losses. The increased availability of accurate, real-time weather forecasting and alerts the most some protection to both residents and visitors. In some cases, the costs of future mitigation efforts, even in new future development, may outweigh the potential insurance losses; for example, Jefferson County does not generally consider shelters a cost effective mitigation effort in built environments.

Overall Hazard Significance

Hailstorms in Jefferson County have a significant impact on the planning area. The costs of hailstorms are higher than any other natural disaster currently documented for the planning area. In addition, Jefferson County reports the highest number of hail-related injuries in the state at 60. 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 critical. The HMPC considers the hazard to have an overall impact rating of low on the County. The data indicates, however, that an overall impact rating of high is most appropriate.

While hailstorms are not as high profile as other natural disasters such as tornadoes, blizzards, or floods, the amount of damage they inflict on the planning area is hugely significant. The hazard is frequent enough in occurrence to pose a significant financial risk to the planning area, and though mitigation measures are limited, the hazard deserves due consideration in the overall profile effort.