section 4.3.12 - tornado, windstorm€¦ · tornadoes can occur at any time of the day, with most...

Sectom 4.3.12 – Risk Assessment – Tornado, Windstorm

Westmoreland County Hazard Mitigation Plan 4.3.12-1 June 2020

4.3.12 Tornado, Windstorm

This section provides a profile and vulnerability assessment of the tornado and windstorm hazard. The wind hazard includes various types of wind events, including windstorms and tornadoes, which are defined below.

A tornado appears as a rotating, funnel-shaped cloud that extends from a thunderstorm to the ground with whirling winds that can reach 250 miles per hour (mph). Damage paths can be greater than 1 mile wide and 50 miles long. Tornadoes typically develop from either a severe thunderstorm or hurricane as cool air rapidly overrides a layer of warm air. Tornadoes typically move at speeds between 30 and 125 mph and can generate combined wind speeds (forward motion and speed of the whirling winds) exceeding 300 mph. The lifespan of a tornado rarely is longer than 30 minutes (FEMA 1997). Tornadoes can occur at any time of the day, with most occurring between 3:00 pm and 9:00 pm. In Pennsylvania, peak occurrence of tornadoes is May through July (NWS 2019).

Windstorms are generally defined with sustained wind speeds of 40 mph or greater lasting for one hour or longer, or winds of 58 mph or greater for any duration. Wind events can vary in spatial size from small, microscale events that take place over only a few hundred meters to large-scale synoptic wind events often associated with warm or cold fronts (PEMA 2018). There are eight different types of damaging winds: straight-line wind, downdraft, downburst, microburst, gust front, derecho, bow echo, and hook echo. Definitions of each type can be found on the NSSL website: https://www.nssl.noaa.gov/education/svrwx101/wind/types/

4.3.12.1 Location and Extent

Tornadoes

Tornadoes can occur throughout Westmoreland County though events are usually localized. However, severe thunderstorms that cover a larger geographic area may result in conditions favorable to the formation of numerous or long-lived tornadoes. Tornadoes can occur at any time during the day or night but are most frequent during late afternoon into early evening, which are the warmest hours of the day. These events are most likely to occur during the spring and early summer months of March through June when these conditions are prevalent (PEMA 2018).

Tornado movement is characterized in two ways: direction and speed of spinning winds and forward movement of the tornado, also known as the storm track. Most tornadoes have wind speeds of 110 mph or less, are approximately 250 feet across, and travel a few miles before dissipating. Some attain wind speeds of more than 300 mph, stretch more than a mile across, and stay on the ground for dozens of miles. Some tornadoes never touch the ground and are short-lived, while others may touch the ground several times (PEMA 2018).

While the extent of tornado damage is usually localized, extreme winds of this vortex can be among the most destructive on Earth when they move through populated, developed areas. Figure 4.3.12-1 shows the annual average number of tornadoes between 1985 and 2014 (Storm Prediction Center [SPC] 2019). The Commonwealth of Pennsylvania experienced an average of 15 tornado events annually.

https://www.nssl.noaa.gov/education/svrwx101/wind/types/



Figure 4.3.12-1. Annual Average Number of Tornadoes in the United States, 1985 to 2014

Source: SPC 2019

The NSSL used historical data to estimate the daily probability of tornado occurrences across the United States, without considering the magnitude of the tornado; refer to Figure 4.3.12-1. In Pennsylvania, it is estimated that the probability of a tornado occurring is 0.2 to 0.8 day per year. In Westmoreland County, it is estimated that the probability of a tornado occurring is 0.4 to 0.6 day per year (NSSL 2003).



Figure 4.3.12-2. Total Annual Threat of Tornado Events in the United States, 1980-1999

Source: NSSL 2003 Notes: The mean number of days per year with one or more events within 25 miles of a point is shown here. The fill interval for tornadoes

is 0.2, with the purple starting at 0.2 day. For the non-tornadic threats, the fill interval is 1, with the purple starting at 1. For the significant (violent) threats, it is 5 days per century (millennium). The oval indicates the general location of Westmoreland County.

Windstorms

All of Westmoreland County experiences straight-line winds and windstorms. Wind events can vary in spatial size from small microscale events that take place over only a few hundred meters to large-scale synoptic wind events often associated with warm or cold fronts (PEMA 2018). Effects from high winds can include downed trees and power lines and damaged roofs and windows. Table 4.3.12-1 lists wind classifications used by the NWS.

Table 4.3.12-1. NWS Wind Descriptions

Descriptive Term Sustained Wind Speed

(mph)

Strong, dangerous, or damaging ≥40 Very windy 30-40

Windy 20-30 Breezy, brisk, or blustery 15-25

None 5-15 or 10-20 Light, or light and variable wind 0-5

Source: NWS 2009 mph Miles per hour

4.3.12.2 Range of Magnitude

The magnitude of tornadoes and windstorms is largely dependent upon the most damaging aspects of each type. The magnitude of a tornado is categorized using the Enhanced Fujita Tornado Intensity Scale (EF Scale). This is the scale now used exclusively for determining tornado ratings by comparing wind speed and actual damage. Figure 4.3.12-3 illustrates the relationship between EF ratings, wind speed, and expected tornado damage.



Figure 4.3.12-3. Enhanced Fujita Tornado Intensity Scale Ratings, Wind Speeds, and Expected Damage

Source: NWS 2018

Tornado watches and warnings are issued by the local NWS office. A tornado watch is released when tornadoes are possible in an area. A tornado warning means a tornado has been sighted or indicated by weather radar. The current average lead time for tornado warnings is 13 minutes. Occasionally, tornadoes develop so rapidly that little, if any, advance warning is possible (NOAA 2011).

The following table provides the descriptions of winds and their associated sustained wind speed used by the NWS during wind-producing events.

Table 4.3.12-2. NWS Wind Descriptions

Descriptive Term Sustained Wind Speed

(mph)

Strong, dangerous, or damaging ≥40 Very windy 30-40

Windy 20-30 Breezy, brisk, or blustery 15-25

None 5-15 or 10-20 Light or light and variable wind 0-5

Source: NWS 2019



The NWS issues advisories and warnings for winds. Issuance is normally site-specific. High wind advisories, watches, and warnings are products issued by the NWS when wind speeds can pose a hazard or are life-threatening. The criterion for each of these varies from state to state. According to the NWS (2018), wind warnings and advisories for Pennsylvania are as follows:

High Wind Warnings are issued when sustained wind speeds of 40 mph or greater lasting for one hour or longer or for winds of 58 mph or greater for any duration or widespread damage are possible.

High Wind Watches are issued when there is the possibility that high wind warning criteria may be met at longer ranges (24 to 48 hours out).

Wind Advisories are issued when sustained winds of 31 to 39 mph are forecast for one hour or longer, or wind gusts of 46 to 57 mph for any duration.

The most severe tornado to hit Westmoreland County was an F4 on June 3, 1980. It was 33 yards wide and left a path 7.6 miles long. No deaths or injuries were reported, but damages were approximately $250 million (NCEI 2019).

4.3.12.3 Past Occurrence

Tornadoes have occurred throughout Pennsylvania. Western and southeastern sections of the Commonwealth have been struck more frequently. Table 4.3.12-3 documents the total number of tornadoes and windstorms that have occurred in Westmoreland County between 1950 and January 31, 2019 based on the NOAA-NCEI database and the Storm Prediction Center severe weather database.

Table 4.3.12-3. Tornado and Windstorm Events, 1950 to 2019

Hazard Type

Number of Occurrences

Between 1950 and 2019 Total Fatalities

Total Injuries

Total Property Damage

Total Crop Damage

Funnel Cloud 0 0 0 $0 $0

High Wind 5 0 0 $210,000 $0

Strong Wind 8 0 0 $232,000 $0

Thunderstorm Wind 658 3 9 $7 million $0

Tornado 36 1 47 $267.3 million $40,000

Total 707 4 56 $274.7 million $40,000

Source: NOAA-NCEI 2019; SPC 2019

Between 1954 and 2019, Pennsylvania was included in six FEMA declared tornado- and windstorm-related major disaster declarations (DR) or emergencies (EM) classified as one or a combination of the following hazards: high winds, flash floods, severe storms, tornadoes, and flooding. Generally, disasters cover a wide region of the state; therefore, many counties could have experienced various types of impacts. Of those declarations, Westmoreland County was included in one declaration (FEMA 2019).

Table 4.3.12-4. Tornado- and Windstorm-Related FEMA Declarations for Westmoreland County, 1954 to 2019

FEMA Declaration Number Date(s) of Event Incident Type Event Title

EM-3081 June 13, 1980 Tornado Severe Storms & Tornadoes

Source: FEMA 2019

The NOAA-NCEI Storm Events database records tornado and windstorm events. According to the database, there were 38 recorded tornadoes in Westmoreland County between 1950 and 2018. These tornadoes include eight F/EF0, 18 with an intensity of F/EF1, nine with an intensity of F/EF2, one with an intensity of F/EF3, one with an intensity of F/EF4, and one with an unidentified intensity. Westmoreland County’s worst tornado event



occurred on June 3, 1980 when an F4 tornado caused approximately $250 million in damage. Figure 4.3.12-4 illustrates the location of these tornado occurrences in Westmoreland County.

For this HMP update, known tornado and major windstorm events that have impacted Westmoreland County between 1950 and January 2019 are identified in Table 4.3.12-5. With documentation for Pennsylvania and Westmoreland County being extensive, not all sources have been identified or researched. Therefore, Table 4.3.12-5 might not include all events that have occurred in the County.



Figure 4.3.12-4. Westmoreland County Tornado History



Table 4.3.12-5. Tornado and Windstorm Events in Westmoreland County, 1950 to January 2019

Date(s) of Event Event Type

FEMA Declaration

Number (if applicable)

County Designated? Event Details

June 26, 1954 Tornado

(F2) N/A N/A Path width approximately 67 yards; $25,000 in property damages.

July 8, 1957 Tornado

(F1) N/A N/A Path width approximately 33 yards; $25,000 in property damages.


(F2) N/A N/A

Path length 2 miles; path width approximately 800 yards. $250,000 in property damages.

November 16, 1965

Tornado (F2)

N/A N/A Path width approximately 280 yards. $250,000 in property damages.

June 4, 1972 Lightning - Severe

Storm/Thunder Storm - Wind N/A N/A $125,000 in property damages


(F1) N/A N/A Path length 0.4 mile; path width approximately 130 yards.

June 30, 1974 Hail - Lightning - Severe

Storm/Thunder Storm - Wind N/A N/A $416,667 in property damages; $41,667 in crop damages

May 26, 1975 Tornado

(F1) N/A N/A Path length 0.8 mile; path width approximately 100 yards. No damages.

September 5, 1975 Tornado

(F1) N/A N/A

Path length 1 mile; path width approximately 83 yards. $2,500 in property damages.

June 1, 1976 Tornado N/A N/A Path length 1.5 miles; path width approximately 33 yards. $25,000 in property

damages.

July 11, 1976 Tornado DR-513 No Path length 11.8 miles; path width approximately 67 yards. $2.5 million in

property damages.


(F1) N/A N/A

Path length 0.5 mile; path width approximately 30 yards. $250,000 in property damages.

March 30, 1977 Tornado

(F1) N/A N/A Path length 1 mile; path width approximately 400 yards. No damages.

January 26, 1978 Wind - Winter Weather N/A N/A $2.6 million in property damages


(F2) N/A N/A

Path length 2.2 miles; path width approximately 20 yards. $250,000 in property damages.


(F4) EM-3081 Yes

Path length 7.6 miles; path width approximately 33 yards. $250 million in property damages.

July 16, 1980 Hail - Lightning - Wind N/A N/A $2.6 million in property damages


(F2) N/A N/A

Path length 13 miles; path width approximately 200 yards. $2.5 million in property damages.


(F2) N/A N/A

Path length 14 miles; path width approximately 200 yards. $2.5 million in property damages.




FEMA Declaration



June 29, 1987 Tornado N/A N/A Path length 0.5 mile; path width approximately 20 yards. $2,500 in property

damages.

June 30, 1987 Tornado N/A N/A Path length 0.5 mile; path width approximately 20 yards. $25,000 in property

damages.


(F1) N/A N/A Path length approximately 0.1 mile; path 40 yards wide. No damages.


(F1) N/A N/A Path length approximately 0.2 mile, path 40 yards wide. No damages.


(F1) N/A N/A Path length approximately 0.2 mile, path 40 yards wide. No damages.

April 9, 1991 Tornado

(F1) N/A N/A

Path length approximately 0.2 mile, path 50 yards wide. $25,000 in property damages.


(F1) N/A N/A

Path length 1.5 miles; path width approximately 50 yards. $25,000 in property damages.


(F1) N/A N/A

A tornado downed large trees from Jeannette to Greensburg, closing State Route 30. An apartment roof was blown off at Greensburg, leaving eight families

homeless. Other buildings sustained minor damage. Path length 1 mile; path width approximately 50 yards. $500,000 in property damages.

August 15, 1997 Tornado

(EF1) N/A N/A

A severe thunderstorm produced a tornado that produced substantial damage in a residential area east-northeast of Latrobe along the foothills of the Chestnut Ridge. The tornado first touched down 1 mile west of Derry. Path length 3.5

miles; path width 200 yards. $800,000 in property damages.

June 2, 1998 Tornado N/A N/A

A weak F0 tornado briefly touched down for approximately 5 minutes over extreme eastern Allegheny and western Westmoreland Counties. One roof was

blown off a structure. Otherwise, damage from this tornado was limited to downed trees. Path length 0.7 mile; path width approximately 50 yards. $15,000

in property damages.


(F1) DR-1219 No

An F1 tornado crossed over the Pennsylvania Turnpike one mile west of Exit 9 at Donegal. As it crossed the turnpike, it tipped over an eastbound tractor trailer,

injuring the driver. One mobile home was overturned. Otherwise, only minor structural damages occurred. Path length 8 miles; path width approximately 200

yards. $200,000 in property damages.

June 16, 1998 Severe Storm/Thunder Storm -

Wind N/A N/A $105,000 in property damages

June 16, 1998 Tornado N/A N/A A weak F0 tornado damaged shingles on one house and snapped/uprooted 30 to

40 trees. $10,000 in property damages.


Wind N/A N/A $1.1 million in property damages

June 30, 1998 Thunderstorm Winds N/A N/A Thunderstorm winds downed numerous trees and power lines. Major damage

was reported to around ten homes, with numerous other homes in the area




FEMA Declaration



receiving minor damage. A new concrete block was at a car dealership was knocked over. $1 million in property damages.

May 12, 2002 Thunderstorm Winds N/A N/A

A thunderstorm microburst passed across Westmoreland County, leaving a path of damage along its entire route. The greatest amount of damage occurred in the Irwin and North Huntington areas. However, damage also occurred in Jeanette,

Greensburg, and Latrobe, and Derry. Tree fell on car, injuring the driver and one passenger, killing the other passenger. $100,000 in property damages.

May 12, 2002 Severe Storm/Thunder Storm -


July 26, 2002 Thunderstorm Winds N/A N/A

Microburst hit portions of eastern Murrysville. The length of the damage path was about one quarter of a mile. The width was approximately 200 yards. The

strongest winds were estimated to be about 80 mph. 1 injury. $200,000 in property damages.

July 26, 2002 Severe Storm/Thunder Storm -


August 4, 2004 Tornado N/A N/A

An F-0 tornado touched down at Charter Oak, 3 miles east of Greensburg, in Unity Twp. It moved toward the southeast, uprooting several large trees, which crushed fencing at a nearby swimming pool. Path length 100 yards, path wide

30 yards, maximum wind 65 mph. $1,000 in property damages.

December 1, 2006 Tornado N/A N/A

A weak F1 tornado touched down in Greensburg at 11:40 a.m. about one quarter mile south of Greensburg Hospital. The tornado was only briefly on the ground for about 100 yards in Greensburg with F1 damage to trees, one house,

and an automobile. $75,000 in property damages.



June 28, 2008 Thunderstorm Winds N/A N/A $125,000 in property damages; trees and power lines down

June 29, 2008 Thunderstorm Winds N/A N/A One large tree fell on a moving vehicle driven by a 37-year-old male. $150,000

in property damages.



February 12, 2009 Wind N/A N/A $113,000 in property damages

April 20, 2009 Wind N/A N/A $100,000 in property damages

December 9, 2009 Wind N/A N/A $107,000 in property damages

April 16, 2010 Thunderstorm Winds N/A N/A Severe thunderstorms were scattered across eastern Ohio, the northern West

Virginia panhandle, and southwest Pennsylvania ahead of a cold front. $200,000 in property damages.




FEMA Declaration



April 16, 2010 Severe Storm/Thunder Storm -


April 16, 2010 Severe Storm/Thunder Storm -


March 23, 2011 Tornado

(EF2) N/A N/A

As a low-pressure system tracked along a warm front across Ohio and Pennsylvania severe thunderstorms developed just south of the front in Ohio

and progressed eastward. Large hail was reported with many of the storms. Path length 9 miles; path width approximately 300 yards. $4 million in property

damages, including high school building and athletic facility.

April 16, 2011 Wind N/A N/A $100,000 in property damages

August 19, 2011 Thunderstorm Winds N/A N/A $100,000 in property damages

August 19, 2011 Severe Storm/Thunder Storm -


June 1, 2012 Tornado N/A N/A

A strong squall line associated with a cold front crossed through western Pennsylvania in the afternoon on June 1. Significant damage to a camp and

conference center. Path length 5 miles; path width approximately 300 yards. $3 million in property damages; 9 million in total damage; 6 million was from a

church camp that was self-insured.

January 30, 2013 Thunderstorm Wind N/A N/A

A line of storms brought strong winds to western Pennsylvania and northern West Virginia. The highest wind gusts were measured at Arnold Palmer

Regional Airport in Latrobe, where gusts of 67 mph were recorded. County emergency management reported downed trees in the County. Approximately

$75,000 in damages was reported.

May 10, 2013 Thunderstorm Wind N/A N/A

Severe thunderstorms developed over Westmoreland County. The storms brought strong winds that downed trees and power lines in the Boroughs of Mount Pleasant and Ligonier. The storms caused approximately $80,000 in

damages.

August 22, 2017 Thunderstorm Wind N/A N/A

November 19, 2017

Tornado (EF1)

N/A N/A

A cold front produced isolated wind damage across southwest Pennsylvania. Most of the wind gusts recorded measured 50 mph or less. There was an area

that experienced straight-line wind damage and a confirmed tornado. A tornado damaged an area along Saltsburg Road from Plum (Allegheny County) to

Murraysville (Westmoreland County). It uprooted or snapped several large hardwood trees along the entire route. A vehicle was flipped in the parking lot

of a senior residential community center and air condensers on top of the building were removed. In Westmoreland County, damages totaled

approximately $25,000.




FEMA Declaration




(EF0) N/A N/A

Two short-track EF0 tornadoes were reported in Westmoreland County. One occurred in the Hempfield Township/Mount Pleasant area, mainly caused tree

damage and damage to a barn and outbuildings along Brinker Road. The second tornado occurred in Unity Township, near Arnold Palmer Regional Airport,

causing tree damage and damage to a barn roof. The County had approximately $25,000 in damages from this event.

October 2, 2018 Tornado (EF 1/2)

N/A N/A

A series of supercells and tornadoes were observed in western Pennsylvania, including Westmoreland County. An EF2 tornado was observed in Donegal

Township, where there was visible tree damage began on a hilltop west of Stairs Road. As the tornado moved down into the valley, tree damage was observed

near the intersection of Harr Road and Donegal Lake Road. A brief EF1 tornado was observed in Armbrust (Hempfield Township). It occurred near Ivy Lane

and Brinkerton Road to near Pattern Shop Road and Valley Green Road. Along Ivy Lane, several softwood tree trunks were snapped, and a few hardwood trees were uprooted. Farther north along Brinkerton Road, the tornado strengthened to its peak wind speed as it descended into a valley and carved a short, focused path of snapped and uprooted trees. Tree damage continued after the tornado

crossed Brinkerton Road, but wind speed weakened at this time. The last observed damage was noted near Pattern Shop Road and Valley Green Road as the tornado encountered a hillside. A few snapped limbs were noted at the end

of its path. Approximately $8,000 in damages were reported for this event. Source: FEMA 2019; NOAA-NCEI 2019; SPC 2019 FEMA Federal Emergency Management Agency NCDC National Climatic Data Center K Thousand ($) NOAA National Oceanic Atmospheric Administration PEMA Pennsylvania Emergency Management Agency M Million ($) TSTM Thunderstorm mph Miles per hour



4.3.12.4 Future Occurrence

Using events collected between 1950 and 2002, Figure 4.3.12-5 shows the number of total tornado events per square mile across Pennsylvania from the State Climatologist. The figure shows that a majority of Westmoreland County experienced a higher frequency of tornado events than the central and northern portions of the state.

Figure 4.3.12-5. Total Tornado Events Per Square Mile in Pennsylvania

Source: Pennsylvania State Climatologist 2016

Similar to tornadoes, the Pennsylvania State Climatologist used historical data between 1950 and 2002 to show the number of wind events per square mile in the Commonwealth. The figure shows that Westmoreland County experienced a higher frequency of events than a majority of the state.



Figure 4.3.12-6. Wind Events Per Square Mile in Pennsylvania

Source: Pennsylvania State HMP 2013

For the 2019 HMP update, the most up-to-date historic data was collected to calculate the probability of future occurrence of tornado and windstorm events for Westmoreland County. Information from NOAA-NCEI Storm Events database, the Pennsylvania State Climatologist, and the Storm Prediction Center were used to identify the number of tornado events that occurred between 1950 and December 2018. Table 4.3.12-6 presents the probability of future occurrence of tornado events in Westmoreland County.

Table 4.3.12-6. Probability of Future Tornado and Windstorm Events

Hazard Type

Number of Occurrences

Between 1950 and 2018

Recurrence Interval (in years)

(# Years/Number of Events)

Percent chance of occurrence in any given year

Tornadoes 38 1.82 55.07%

Strong Wind 8 8.63 11.59%

Thunderstorm Wind 658 0.10 100%

High Wind 5 13.80 7.25%

TOTAL 709 0.10 100%

Sources: NOAA-NCEI 2019



In Section 4.4, the hazards of concern identified for Westmoreland County are ranked according to relative risk. The probability of occurrence, or likelihood of the event, is one parameter used for ranking hazards. The probability of occurrence for severe tornado and windstorm events in Westmoreland County is considered highly likely (greater than 90 percent annual probability) as defined by the Risk Factor Methodology probability criteria (Section 4.4).

Westmoreland County experiences strong winds on a frequent basis, and when those winds occur, they can result in significant property damage, downed trees, and utility outages. It can be reasonably assumed that future tornadoes will be similar in nature to those that have affected Westmoreland County in the past. It is estimated that the County will continue to experience direct and indirect impacts of annual windstorms and tornadoes that may induce secondary hazards, such as infrastructure deterioration or failure; utility failures; power outages; water quality and supply concerns; and transportation delays, accidents, and inconveniences.

4.3.12.5 Vulnerability Assessment

To understand risk, a community must evaluate assets exposed and vulnerable within the identified hazard area. The following section discusses potential impacts of the tornado hazard on Westmoreland County, including:

Impacts on (1) life, health, and safety; (2) general building stock and critical facilities; (3) the economy; (4) the environment; and (5) future growth and development

Effect of climate change on vulnerability

Further data collections that will assist in understanding this hazard over time.

A qualitative assessment on potential impacts to life, health, and safety; buildings and critical facilities; and the economy are summarized below. Refer to Section 4.3.7 (Hurricane and Tropical Storms) for further details on estimated potential losses as a result of the 100- and 500-year mean return period wind events using HAZUS-MH v4.2.

Impact on Life, Health, and Safety

Impacts of a tornado or windstorm on life, health, and safety depend on several factors, including severity of the event and whether adequate warning time was provided to residents. All residents in Westmoreland County are exposed to the tornado hazard.

Residents may be displaced or require temporary to long-term sheltering. In addition, downed trees, damaged buildings, and debris carried by high winds can lead to injury or loss of life. Similar to other natural hazards, socially vulnerable populations are most susceptible based on a number of factors, including their physical and financial ability to react or respond during a hazard and locations and construction quality of their housing. Economically disadvantaged populations are more vulnerable because they are likely to evaluate their risk and make decisions based on the major economic impact on their family and may not have funds to evacuate. The population over the age of 65 is also more vulnerable and, physically, they may have more difficulty evacuating. The elderly are considered most vulnerable because they require extra time or outside assistance during evacuations and are more likely to seek or need medical attention that may not be available due to isolation during a storm event. Section 2 (County Profile) presents the statistical information regarding these populations in the County.

Impact on General Building Stock and Critical Facilities

While the chance of being affected by a tornado is small, the damage that results when a tornado occurs can be devastating. An EF4 tornado can carry wind velocities of 200 mph, resulting in a force of more than 100 pounds per square foot of surface area. This is a “wind load” that exceeds the design limits of most buildings.

The entire County’s building stock and critical facilities are exposed to the tornado hazard. Manufactured housing (i.e., mobiles homes) is particularly vulnerable to high winds and tornadoes. The U.S. Census Bureau defines manufactured homes as “movable dwellings, 8 feet or wider and 40 feet or more long, designed to be



towed on its own chassis, with transportation gear integral to the unit when it leaves the factory, and without need of a permanent foundation (US Census 2010).” They can include multiwides and expandable manufactured homes but exclude travel trailers, motor homes, and modular housing. Due to their light-weight and often unanchored design, manufactured housing is extremely vulnerable to high winds and will generally sustain the most damage.

Table 4.3.12-7 displays the number of manufactured housing units per municipality in Westmoreland County.

Table 4.3.12-7. Manufactured Housing Units per Municipality in Westmoreland County

Municipality Number of

Manufactured Homes

Municipality

Number of Manufactured

Homes

Adamsburg Borough 12 Mount Pleasant Township 565

Allegheny Township 190 Murrysville, Municipality of 344

Arnold, City of 12 New Alexandria Borough 78

Arona Borough 17 New Florence Borough 42

Avonmore Borough 75 New Kensington, City of 75

Bell Township 94 New Stanton Borough 86

Bolivar Borough 38 North Belle Vernon Borough 23

Cook Township 73 North Huntingdon Township 393

Delmont Borough 0 North Irwin Borough 0

Derry Borough 9 Oklahoma Borough 13

Derry Township 1,143 Penn Borough 0

Donegal Borough 20 Penn Township 245

Donegal Township 294 Rostraver Township 509

East Huntingdon Township 865 St. Clair Township 90

East Vandergrift Borough 0 Salem Township 939

Export Borough 26 Scottdale Borough 47

Fairfield Township 291 Seward Borough 31

Greensburg, City of 44 Sewickley Township 392

Hempfield Township 1,450 Smithton Borough 7

Hunker Borough 9 South Greensburg Borough 0

Hyde Park Borough 15 South Huntingdon Township 253

Irwin Borough 0 Southwest Greensburg Borough 0

Jeannette, City of 2 Sutersville Borough 22

Latrobe, City of 96 Trafford Borough 1

Laurel Mountain Borough 7 Unity Township 781

Ligonier Borough 16 Upper Burrell Township 75

Ligonier Township 359 Vandergrift Borough 0

Lower Burrell, City of 177 Washington Township 550

Loyalhanna Township 78 West Leechburg Borough 15

Madison Borough 37 West Newton Borough 11

Manor Borough 31 Youngstown Borough 13



Municipality Number of

Manufactured Homes

Municipality

Number of Manufactured

Homes

Monessen, City of 21 Youngwood Borough 53

Mount Pleasant Borough 10

Westmoreland County 11,164

Source: HAZUS-MH v4.2

According to the 2018 State HMP, there are wind speed zones developed for the design of tornado shelters; refer to Figure 4.3.12-7. As displayed, Westmoreland County is located in wind speed zones III and IV, meaning design wind speeds for shelters and critical facilities should withstand 3-second gusts up to 250 mph, regardless if the wind is from a tornado, hurricane, tropical storm, or windstorm event. It should be noted that these windspeeds represent the strongest anticipated throughout the Commonwealth and are not the normal wind speeds expected statewide (PEMA 2018).

Figure 4.3.12-7. Design Wind Speeds for Tornado Shelters

Source: PEMA 2018

Impact on Economy

Tornadoes also impact the economy, including loss of business function (e.g., tourism, recreation), damage to inventory, relocation costs, and wage loss and rental loss due to repair/replacement of buildings. Impacts on transportation lifelines affect both short-term (e.g., evacuation activities) and long-term (e.g., day-to-day commuting and goods transport) transportation needs. Utility infrastructure (power lines, gas lines, electrical systems) could sustain damage and impacts could result in loss of power, which can affect business operations and provision of heating or cooling to the population.



Impact on the Environment

Tornado events are typically localized; therefore, environmental impacts are rarely widespread. Severe damage to plant species is likely from both tornado and windstorm events. This includes uprooting or total destruction of trees, and increased threat to wildfire in areas of tree debris. Hazardous material facilities should meet design requirements for the wind zones identified in Figure 4.3.12-7 above (PEMA, 2018).

Future Growth and Development

As discussed in Section 2 (County Profile), areas targeted for future growth, development and re-development have been identified across Westmoreland County. Any areas of growth could be affected by the tornado and windstorm hazard because the entire County is exposed and potentially vulnerable to the wind hazard. Residential development, specifically manufactured homes, may be considered more vulnerable to the tornado hazard.

Effect of Climate Change on Vulnerability

An increase in storms will produce more wind events and may increase tornado activity. Additionally, an increase in temperature will provide more energy to produce storms that generate tornadoes (Climate Central 2016). With an increased likelihood of strong winds and tornado events, all of the County’s assets will experience additional risk for losses as a result of extreme wind events.

Additional Data and Next Steps

In time, HAZUS-MH will be released with modules that address straight-line wind and tornado events. As updated versions of HAZUS-MH are released, the County can run analyses for an overall picture of the wind damages and debris generated from tornado events. Over time, Westmoreland County can obtain additional data to support the analysis of this hazard. Data that will support the analysis would include additional detail on past hazard events and impacts, and an updated building inventory to include specific building information such as type of construction and details on protective features (for example, shutters and safe rooms).

section 4.3.12 - tornado, windstorm€¦ · tornadoes can occur at any time of the day, with most...

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