Severe Local Storm Damage Assessment


Prepared by: Todd Shea
Warning Coordination Meteorologist
National Weather Service
La Crosse, WI
  Late updated: September 14, 2012

Introduction The Survey Hail Flood Straight-line Wind Tornado EF-Scale Myths

Introduction

Assessing local storm damage can be a challenging task but is often needed to officially document what occurred when severe storms impact the area.  Usually staff members of the National Weather Service (NWS) or local Emergency Management officials survey damaged areas and try to determine exactly what happened.  In many ways, it is like a detective going in after the fact and using available clues and evidence to piece together what occurred.  Sometimes it is a relatively easy task.  Other damage is much more difficult to discern.

Why the need to document storm damage?  This information is used by state, county or local officials, along with insurance companies.  The National Weather Service (NWS) also uses the information to improve services and conduct research.  All known severe weather reports and associated damage are documented in an official publication called, "Storm Data" that is published monthly by the National Climatic Data Center.  You can also view local Storm Data entries by clicking here.

Surveys are usually conducted after a "significant" weather event that might include widespread damage, injuries, fatalities, large media interest, or when the type of storm is in question.  Some surveys are conducted only a few hours after the storms, while others are done days afterward depending on the scope of the event and needed assessment personnel.

It is important to note that every storm and associated damage area are different.  Rarely is there a "golden rule" or 100% certainity in the field of weather.  There are exceptions to nearly every rule.  Information in this document includes some common guidelines or clues in damage assessment based on experience and the advice of experts in the field of engineering and meteorology.

This document presents some concepts and misconceptions about Severe Local Storm Damage Assessment.


Conducting the Damage Survey

The need to conduct a damage survey is usually done as soon as possible after severe weather hits and is coordinated between the NWS and local officials (often Emergency Management).  The survey is performed by local experts and information tends to remain local.  If the event is more widespread or involves a large amount of fatalities and/or injuries, a national service assessment team might be formed by the NWS.  Follow-up related to a full assessment might take a week or more and involve experts from around the country.

Information typically collected from a local survey involves:

  • Location of damage (including width, length, path)
  • Type of damage
  • Intensity of damage (including peak wind speeds, rating)
  • Time damage occurred
  • Cause of damage
  • Public actions and observations

There are two main types of damage surveys: Ground or Aerial

Ground surveys are the most common and allow the flexibility to collect detailed facts.  Interviews can be conducted and detail to construction quality and type can be gathered.  The ground survey can also take a considerable amount of time so often the amount of detail collected is determined by the scope of the event and available time/resources.  A small, isolated damage spot might favor detailed examination while widespread or extensive damage might need to be summarized by a few, quick stops.

Aerial surveys are usually used for more widespread damage cases, typically arranged through the assistance of the Civil Air Patrol (CAP) or other partners.  While aerial surveys allow for quick determination of location and path, they often do not provide details available to ground survey crews.  An aerial survey can reveal more "secluded" damage, especially in steep terrain or harder to access areas.  Typically an aerial survey is conducted in addition to a ground survey.

Surveying a damage path can vary in technique.  If the damage path is known, the survey might start at the beginning.  If details are not known, the survey might start where the worst damage is known and proceed in both directions from there.  It is always recommended to examine ALL the evidence (taking in the 'big picture') before making determinations or final decisions.

There are numerous challenges with surveys, but determining exact times of the event can often prove difficult.  People often get emotional about the assigned tornado rating saying, "It had to be worst than that! - but remember the EF-scale rating is subjective in nature, and could be technically off by +1/-1 rating.  In addition, there is often pressure to determine what happened or what the rating is right away.  Care must be used to examine all evidence before concluding too quickly.

Two person teams are preferred over a solo survey to assist with map reading, note taking, workload, and damage assessment input. A staff member at the NWS office can also assist with coordination and pass along to the survey team(s) additional information that comes in.

Other background comments regarding damage surveys:

  • Damage clean up often begins right away - evidence is lost the longer you wait.
  • Having specific locations helps greatly.
  • "Gawkers" are everywhere.  Traffic jams will form within hours.
  • Those directly impacted will think the worst and are often emotional - respect is always used.
  • Most people interviewed are nice and willing to help out.
  • Safety comes first.  Need to dress properly and avoid dangerous areas, including unstable structures and ANY downed power lines.

NWS La Crosse uses the following items in two storm survey kits:

  • Cellular phone - used to coordinate with Emergency Management and Geotag photos.
  • Laptop w/GPS - To document survey path and note damage intensity/locations.
  • Digital camera (video footage is rarely used)
  • Detailed maps
  • Assorted pens/pencils
  • Pads of paper
  • Clip board
  • Compass
  • Tape measure
  • Walking distance meter (Rolatape)
  • Storm Damage Reference binder with examples, contacts, etc.
  • Paper clips
  • Extra batteries

Other items to consider:

  • Radar imagery from storm impact time
  • Proper Identification (try to coordinate ahead of time)
  • Proper clothing, especially footwear

Hail

Hail can cause extensive property or crop damage. Expenses from losses add up very quickly after a large hail storm, especially if the storm impacts a populated area. The National Weather Service (NWS) issues Severe Thunderstorm Warnings for storms when hail of 1 inch in diameter or larger is expected. (Nationally the hail criteria changed from 3/4" to 1" on January 5, 2010.)

During testing with numerous types of shingles (Marshall, T.P., Richard F. Herzog, and Steven R.Smith, 2002:, 21st Conference on Severe Local Storms, San Antonio, TX), no damage was noted at 3/4" while some 11 year old shingles were damaged with 1" hail. Nearly all types of shingles (even newer ones) showed damage at 1.25". It typically takes golf ball size hail (1.75") to dent cars.

When it comes to crops, a lot depends on crop maturity, how strong the associated winds were, and the overall persistence of the hail. Even pea size hail can cause damage.

Other hail references -

Floods

Flood surveys are not as common as wind or tornado surveys but are sometimes performed to learn more about impacts at certain levels, overall impact, and actions taken by the general public.

Flash flood surveys might focus on what caused the rapid rise in water, the surrounding terrain that may have enhanced the flood, and extent of damage.  In some cases the shear amount and intensity of rain may have led to damage while other cases involve debris blocking flowing water leading to quick floods.  Drainage design and engineering may also cause failures where "true" flooding never occurred.

Surveying larger river flooding is often performed best by the air given the extent of area to cover.  Ground surveys often focus on what type of impact occurred at various water levels.


Straight-line Winds

One of the most frequent severe weather events we experience is strong to severe thunderstorm wind gusts.  Rain cooled air advancing out of a thunderstorm can create significant damage - this is what is labeled as "Straight-line Wind".  The NWS considers thunderstorm related wind gusts severe when they reach or exceed 50 knots (58 mph).  Wind gusts can become life threatening around these speeds.  The NWS issues Severe Thunderstorm Warnings for this type of event.

Damage can be tornado-like and is commonly mis-diagnosed as a tornado immediately following the event.  This is especially true in the case of a Downburst, which is a strong downdraft resulting in an outward burst of damaging winds on or near the ground.  A Microburst is simply a small, concentrated downburst less than 2.5 miles across.

  • Debris from straight line wind damage is commonly blown in one general direction (for example, west to east). The debris will be blown down wind from the impact site.
storm damage image storm damage image
  • Damage patterns may be divergent in nature, especially in the case of a Downburst or Microbust. Often there will be an impact point with debris spread down wind in a fanned or divergent pattern.
  • Aerial surveys of downbursts often show pockets of wind damage, like foot prints, that may not be continuous.
storm damage image storm damage image storm damage image
  • Local terrain can change the direction of downed or thrown debris. Objects may be spread downstream in different directions or may be laying in a convergent pattern compared to a typical Downburst (divergent).
storm damage image
  • Local terrain can also dramatically increase the amount of damage observed because of funnel effects (more constricted air flow) or steeper angled impact.

Gustnadoes can occur with thunderstorm wind gusts as well. These are usually weak and short-lived ground-based rotations along the leading gust front. They are not associated with rotation from the thunderstorm above it. This is solely ground-based rotation that may be visible as a debris cloud or dust whirl. The damage pattern may look more rotational (tornado like?) compared to other downburst or straight line wind damage.


Tornado

A tornado is defined as a violently rotating column of air in contact with the ground. The condensation funnel may not always be visible from cloud to ground. Watch for signs of rotation in the cloud base (wall or funnel cloud) and debris on the ground.

Here are some tips for recognizing tornado related damage:

  • Tornado damage paths are usually narrow with evidence of rotation on a vertical axis. Evidence of rotation is key here. Look for surrounding clues that would suggest there was a rotational component.
storm damage image storm damage image
  • Damage or debris is often convergent in nature with a central axis or distinct path. Often the path is relatively easy to spot from the air.
storm damage image storm damage image tornado path in corn image
  • Debris (leaf litter, dirt, mud) is often sprayed on the sides of objects, especially the opposite sides from the storm path, in the case of a tornado. This might be a sign of rotational winds.
storm damage image storm damage image
  • Projectiles are often created in the case of a tornado. These may be sticking up out of the ground or stuck in nearby objects. You can often use these as tracers to the path of that particular object.
storm damage image storm damage image storm damage image
  • Debris might be twisted or laying in a twisted pattern. This is especially true in corn fields where rotation can easily be observed, especially from late June on when the corn is high enough to be impacted.  Corn is often a good tracer for tornado activity because it provides relative consistency from field to field.

Special care must be taken to fully review each damage indicator.  If there are poor construction standards or conflicting evidence from damage indicators in the same location, that location might not be a good source for properly determining an EF rating and/or wind speed.

Items that can complicate a tornado survey and rating include:

  • Forward speed of tornado
  • Landscape along track (vegetation, building)
  • Debris type
  • Vorticies

The Enhanced Fujita Scale

A tornado is ranked using the Enhanced Fujita Damage Scale (EF-scale), based on wind speeds and associated damage it produces. This is normally the worst damage observed in a tornado track and may constitute a very small percentage of total damage from the tornado. In other words a tornado may produce a lot of EF1 damage but peak at EF3 for a short segment. That particular tornado would be rated an EF3. Tornadoes are NOT rated by appearance.

The Enhanced Fujita Scale is subjective in nature and goes from EF0 to EF5. Even though every effort is made to assign correct ratings, they may be off by +1/-1 EF-scale rating. Numerous factors need to be taken into account when reviewing an EF-scale rating including:

  • Experience of surveyor or engineering knowledge
  • Detail of survey - time available
  • Age and quality of building contruction
  • Speed of tornado or duration of strongest wind
  • Impact of debris from nearby damage

EF-Rating Wind Speeds
EF0 65-85 mph
EF1 86-110 mph
EF2 111-135 mph
EF3 136-16 5 mph
EF4 166-200 mph
EF5 Over 200 mph

More detail on the Enhanced Fujita Scale can be found at these links:


Myths

Myth: Trees laying in different directions must mean tornado damage.

  • The direction trees are laying can be influenced by things like tree type, tree bark, size of tree, amount of foilage, wind load, root structure, and soil conditions.
  • Local terrain can influence the wind direction and change the way trees are blown down.
  • Branches or trunks can lay in different directions if there is tree rot causing weak or failure points.
storm damage image storm damage image

Myth: "Projectiles" automatically suggest a tornado.

  • While projectiles are common from a tornado, straight line wind (especially a downburst) can throw debris down wind and produce projectiles.

Myth: A "roaring" sound suggests a tornado.

  • Witnesses often describe a "roaring" or freight train sound with a tornado but that sound can also occur from straight line winds as they pass through trees, forests, or terrain. Those who have been struck by a tornado also describe a high-pitch whine or squeal sound as it hits.

Myth: Damage = Tornado!

  • As this document suggests, not all damage is tornado related. More times than not it is simply straight-line wind.
  • The severe thunderstorm that produces a tornado (usually a Supercell) often produces straight line wind damage as well. It is very common to have both types of damage related to the same parent thunderstorm in different areas.
storm damage image

Myth: Uprooted trees are only caused by Severe Thunderstorms (gusts of 58 mph or higher).

  • Root structure can make a big difference. Shallow roots may allow the tree to topple much easier.
  • Soil type and moisture content can also allow a tree to uproot with winds well below 58 mph.
  • Tree rot can cause branches or entire trees to be toppled with relatively weak wind speeds.
storm damage image storm damage image uprooted tree
storm damage image tree rot tree rot

Myth: Structures explode in a tornado due to lower pressure.

  • Structures are torn apart in a tornado from the wind or from the imapct of debris.

Myth: Twisting automatically means a tornado hit.

  • Not true. Houses, for example, are usually impacted by wind in one primary direction due to the size of the tornado. You need to examine the entire area impacted.

Myth: Tornadoes often skip.

  • Actually skipping tornadoes are rare. Usually one tornado will form and dissipate, and a different tornado will form at a later stage.

Myth: "It struck without warning!"

  • While true that not all storms are properly warned for, MOST severe weather is preceded by warnings.

Past Damage Surveys / Assessments

Local damage surveys and summaries can be found at the following link:

NWS La Crosse Storm Summaries

Service Assessments from the National Weather Service are found at:

http://www.nws.noaa.gov/om/assessments/index.shtml


Reference Material


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.