StormDamageFscale

Storm Damage Surveys and F-Scale Assessments: 

Storm Damage Surveys and F-Scale Assessments Brad Small - Senior Forecaster Jeff Johnson – Warning Coordination Meteorologist National Weather Service – Des Moines The views expressed are those of the authors, and do not necessarily represent those of the National Weather Service.

Topics: 

Topics Wind damage processes Tornado or Downburst? History and definition of F-scale Arriving at F-scale rating Bradgate, IA Emergency Managers filter damage reports, and are often the first line in interpreting damage. Information critical to determine what occurred and whether survey is needed. If so, how extensive. NWS Photo

Wind damage processes: 

Wind damage processes Wind forces, both in the vertical and horizontal, produce the vast majority of damage rather than pressure. This misunderstanding bred the “open window” myth. These forces act against the designed gravity-based load path FEMA

Slide4: 

Recall the “Bernoulli” effect Air has to flow faster above the object, with lower pressure causing lift. This same process works on walls and roofs of houses. Wind damage processes

Slide5: 

Weak point failures such as windows and garages compound the problem. This internal pressure aides the outside lift. Wind damage processes FEMA 342

Slide6: 

Garage doors are common weak points, and are often the first failure point. This can result in downstream projectile damage to structures which otherwise may not have occurred (Important F-scale considerations) Wind damage processes Walnut, IA May 8, 2004 NWS Omaha

Slide7: 

Wind damage processes Tornado and strong straight line winds in open areas may be stronger due to less friction, but with less debris. Storm in populated area may have lower winds but with more projectiles. Thus identical storm may produce more damage as it progresses through community due to increase in projectiles.

Tornado vs. Downburst: 

Tornado vs. Downburst Tornado damage is convergent. Most tornado damage on right flank.

Tornado vs. Downburst: 

Tornado vs. Downburst Best way to differentiate is an aerial survey Tornado damage is convergent. John McLaughlin – KCCI-TV Near Rolfe

Tornado vs. Downburst: 

Tornado vs. Downburst Downbursts exhibit straight or divergent damage patterns, and are often fairly widespread. Tornado damage could be embedded however.

Tornado vs. Downburst: 

Tornado vs. Downburst Downburst damage often linear or unidirectional with similar intensities.

Tornado vs. Downburst: 

Tornado vs. Downburst Differentiating between the two…. MYTH: Twisted trees or signs, or trees laying in different directions must mean tornado damage. FACT: No tree is perfectly symmetrical and will fall differently depending on wind direction, weight of canopy, and integrity. FACT: Signs may also twist because of unequal footings or posts. Could also blow around after initial break and appear twisted Tornado winds are essentially straight line to any one particular object. Tornado would have to have similar diameter to bring winds from several directions.

Tornado vs. Downburst: 

Tornado vs. Downburst Differentiating between the two…. MYTH: Witness reported a “roaring” sound with passage of storm. FACT: Strong wind produces the roaring sound, and can occur with either tornadoes or straight-line wind storms. MYTH: “My house was damaged. It must have been a tornado”. FACT: Straight line winds can produce F0 or F1 tornado damage.

F-scale: History and Definition: 

F-scale: History and Definition Developed in 1971 by University of Chicago professor Dr. T. Theodore Fujita Wind vs Damage Table, mainly correlated to wood frame houses (wind at standard anemometer height of 10 meters) Rating actually done by assessing damage. Windspeeds inferred by damage. Drawbacks Difficult to estimate in open areas or with lack of standard structures Wind speeds appear to be too strong in higher end of scale.

Fujita Scale: 

Fujita Scale F0 (40-72 mph) Some damage to chimneys and TV antennae; breaks twigs off trees, pushes over shallow-rooted trees; minor house damage (shingles, windows, siding, garage doors)

Fujita Scale: 

Fujita Scale F1 (73-112 mph, hurricane strength) Peels surfaces off roofs; windows broken or garages damaged; light trailer houses pushed over or overturned; some trees uprooted or snapped; moving cars pushed off road. NWS Photo

Fujita Scale: 

Fujita Scale F2 (113-157 mph) Roofs torn off frame houses leaving strong upright walls; weak buildings in rural areas demolished; trailer houses destroyed; large trees snapped or uprooted; railroad boxcars pushed over; light object missiles generated; cars blown off highway.

Fujita Scale: 

Fujita Scale F3 (158-206 mph) Roofs and some walls torn off frame homes; some rural buildings completely demolished; trains overturned; steel-framed hanger-warehouse type structures torn; cars lifted off the ground; most trees uprooted, snapped or leveled.

Fujita Scale: 

Fujita Scale F4 (207-260 mph) Whole frame houses leveled leaving piles of debris; steel structures badly damaged; trees debarked by small flying debris; cars and trains thrown some distance or rolled considerable distances; large missiles generated

Fujita Scale: 

Fujita Scale F5 (261-318 mph) Whole frame houses swept off foundations; steel-reinforced concrete structures badly damaged; automobile-sized missiles generated; incredible phenomena can occur

Arriving at F-scale: 

Arriving at F-scale Difficulties Identical tornadoes moving at different speeds produce different damage Tornado size cannot be used to rate damage Tornado hits nothing in open area Gaps in damage path Likely caused by weakening of near surface wind due to friction from structures and trees producing “tree top” level damage. Video does not support skipping or quick withdraw back into clouds.

Arriving at F-scale: 

Arriving at F-scale Damage to wood frame homes should be used as starting point, with trees and vehicles used as well. Structural integrity should be considered however. Shoddy construction or weak connections between foundation, walls, and roof can reduce the F-scale rating by 1 or more Orientation of weak points such as garage doors relative to wind can reduce F-scale rating (failure caused indirectly by weak point)

Arriving at F-scale: 

Arriving at F-scale Total destruction of mobile home occurs at F2. Absence of other damage cannot justify higher than F2. Check other heavy objects for movement or lack thereof. Vehicles moved or not moved. If so, check for markings to see if it was dragged, tumbled, or truly airborne. Check what wasn’t damaged. This often provides an upper bound. Pavement or vegetation scouring may support F2+, but correlation uncertain.

Arriving at Bradgate F-Scale: 

Arriving at Bradgate F-Scale F-scale Review (F0) Shingles lost, windows, siding, chimneys, garages damaged. Branches off trees, shallow ones uprooted. (F1) Small portions of roof off. Garage collapses. Trailer homes pushed over or overturned. Trees uprooted or snapped. (F2) Roofs torn off completely leaving walls. Mobile homes destroyed. Large trees snapped or uprooted. Railroad cars pushed over. (F3) Roofs and exterior walls torn off homes. Cars moved. Rural buildings demolished. Widespread trees uprooted or snapped. (F4) Whole frame houses destroyed leaving pile of debris. Steel structures badly damaged. Cars or trains moved some distance. Large missiles generated. (F5) Whole frame houses swept off foundation. Steel-reinforced concrete structures badly damaged. Car sized missiles.

Arriving at Bradgate F-Scale: 

Arriving at Bradgate F-Scale Keep in mind house damage, quality of construction, and other objects. F1? F2? F3? F4? NWS Photo

F-scale Quiz: 

F-scale Quiz F0 F1 F2 NWS Photo

Further References: 

Further References A Guide to F-Scale Damage Assessment. NOAA/NWS. http://www.crh.noaa.gov/dmx/prepare.shtml Severe Local Storm Damage Assessment (NWS La Crosse) http://www.crh.noaa.gov/arx/damage.php Building Performance Assessment Report. Midwest Tornadoes of May 3, 1999; Observations Recommendations and Technical Guidance. (FEMA 342) Handout (See myself or Jeff Johnson)

Thank you: 

Thank you Shannon Vitzthum WHO-TV/Iowa State Patrol Rolfe/Bradgate Tornadoes May 21, 2004