Severe Local Storm Damage Assessment

Todd Shea
Warning Coordination Meteorologist
National Weather Service La Crosse

Last Updated: 1/16/08

NOTE: Images used in this document are used for training purposes only. Copyright rules apply.

Introduction

Determining exactly what type of severe weather impacted an area can be a very challenging task. Usually staff members of the National Weather Service (NWS) or local Emergency Management officials survey damaged areas and try to determine exactly what happened when storms moved through. 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.

It is important to note up front 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, much of which is based on expereiences of the author. With nearly every storm type feature listed, you could answer "not always".

This document, which will be non-static and/or updated often, will present some concepts and misconceptions about Severe Local Storm Damage Assessment.

A large majority of content will be devoted to Straight Line Wind vs. Tornado damage.

Straight Line Winds

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

Damage can be tornado-like and is commonly mislabeled as a tornado strike 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.

  • 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.

  • 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).

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

    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.

    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.

    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.

    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.

    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:

    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:

  • NWS La Crosse Enhanced Fujita Scale page
  • Storm Prediction Center Enhanced Fujita web page

    • Conducting the Damage Survey

    Those conducting damage surveys should educate themselves on proper assessment techniques. Review previous damage assessments and build a database of experience to build upon. There are several references listed below to assist with training.

    When the decision has been made to conduct a damage survey, a team or teams are assembled to arrive on site as quickly as possible after the storm. This depends on several factors:

  • Time damage occurs - Is there enough time before sunset or will the survey need to be delayed until the following morning?

  • Available staffing - Besides routine duties, storm follow-up work also needs to be performed within the office.

  • Extent of damage - How large of geographic area needs to be surveyed? How many areas are there to survey?

  • Time allowance - How much time or resources should be spent in a particular area? This might affect how detailed the survey is.

  • Periodicity of Severe Weather - Is more severe weather expected the next day?

    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. In a major storm damage event, including a suspected F4 or stronger damaged area, a full assessment team may be deployed by the NWS. In most cases though damage assessment will be conducted locally.

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

    Other items to consider:

    Final Damage Assessments

    Local damage assessments 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

  • A Guide to F-Scale Damage Assessment

  • Enhanced Fujita Scale Damage Indicators


    • National Weather Service
    • La Crosse, WI Weather Forecast Office
    • N2788 County Road FA
    • LaCrosse, WI 54601
    • 608-784-7294
    • Page Author: ARX Webmaster
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    • Page last modified: January 16th 2008 8:48 PM
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