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|
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:
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:
NWS La Crosse uses the following items in two storm survey kits:
Other items to consider:
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 -
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.
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.
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.
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:
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:
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:
|EF3||136-16 5 mph|
|EF5||Over 200 mph|
More detail on the Enhanced Fujita Scale can be found at these links:
Myth: Trees laying in different directions must mean tornado damage.
Myth: "Projectiles" automatically suggest a tornado.
Myth: A "roaring" sound suggests a tornado.
Myth: Damage = Tornado!
Myth: Uprooted trees are only caused by Severe Thunderstorms (gusts of 58 mph or higher).
Myth: Structures explode in a tornado due to lower pressure.
Myth: Twisting automatically means a tornado hit.
Myth: Tornadoes often skip.
Myth: "It struck without warning!"
Past Damage Surveys / Assessments
Local damage surveys and summaries can be found at the following link:
Service Assessments from the National Weather Service are found at: