Tornadoes are powerful and destructive weather phenomena. Accurately determining their intensity is necessary for historical record-keeping, scientific research, and improving building codes. After a tornado, meteorologists and engineers survey the destruction to classify the storm’s strength. This process allows for standardized comparison and provides data on wind speed and structural failure. While most tornadoes receive a numerical classification, a specific designation exists for storms where a conclusive rating cannot be determined.
The Enhanced Fujita Scale and Its Purpose
The standard system used to assess a tornado’s strength is the Enhanced Fujita (EF) Scale, which replaced the original Fujita scale in 2007. This update incorporated better engineering knowledge to correlate observed damage with estimated wind speeds. The scale ranges from EF0 (weakest damage) up to EF5 (most catastrophic destruction).
The National Weather Service (NWS) is the sole federal agency authorized to provide official tornado ratings. Personnel survey the damage path against 28 specific Damage Indicators (DIs), including various structures and vegetation. Defined Degrees of Damage (DoD) help estimate the range of wind speeds the tornado produced.
The final EF rating is based on the highest estimated wind speed along the damage path. The scale does not measure wind speed directly but provides estimates based on the damage caused. These estimates represent a three-second gust, making the EF Scale a damage-based classification system.
Defining the Unknown Designation
The designation EF-U stands for Enhanced Fujita-Unknown. This unique category differs significantly from numerical ratings (EF0 through EF5) because it is not a measure of the tornado’s strength. EF-U acknowledges that insufficient evidence exists to assign a definitive rating, meaning the true intensity remains undetermined due to a lack of surveyable damage.
Historically, tornadoes causing no damage were often classified as EF0, inaccurately inflating the count of low-end storms. The EF-U classification provides a more accurate climate record by distinguishing between a confirmed weak tornado and an unratable one. This rating reflects a procedural limitation, not a lack of confirmation of the storm’s existence.
The “U” category ensures a confirmed tornado is recorded without making an unsubstantiated claim about its wind speed. The NWS confirms the tornado’s presence through radar data, eyewitness video, or photographic evidence. However, without physical damage to a recognized Damage Indicator, a numerical rating cannot be assigned.
Circumstances Leading to an EF-U Rating
A tornado receives an EF-U rating when its path traverses an area lacking suitable structures or vegetation for accurate damage assessment. The rating process relies on finding a quantifiable sign of destruction, which is impossible if the tornado travels over open fields, remote wilderness, or large bodies of water. Thus, even if visible, there is no physical evidence to correlate with a specific wind speed range.
This scenario is common in the central and southern Plains States, which have a high frequency of tornadoes and vast expanses of sparsely populated land. Even a strong tornado can be rated EF-U if it touches down without encountering any of the 28 official Damage Indicators. Longer tracks over undeveloped territory can also result in this rating.
Obscured or Nonstandard Damage
Another circumstance leading to the Unknown designation is when damage evidence is obscured or destroyed before a survey can be conducted. Although NWS teams survey damage quickly, a delay can mean that important indicators, such as snapped tree limbs or shifted debris, have been moved or deteriorated. Furthermore, a tornado damaging only nonstandard indicators, like unique industrial equipment, may be classified as EF-U because the wind speed correlation is not established in the scale’s guidelines.